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Cardiovascular Research 1997 34(2):289-298; doi:10.1016/S0008-6363(97)00022-9
© 1997 by European Society of Cardiology
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Copyright © 1997, European Society of Cardiology

Insulin improves heart function and metabolism during non-ischemic cardiogenic shock in awake canines

Jeffrey A Kline*, Richard M Raymond, Elena D. Leonova, Thomas C Williams and John A Watts

Department of Emergency Medicine, Carolinas Medical Center, Charlotte NC 28232, USA

* Corresponding author. Tel. +1 704 355-7092; Fax +1 704 355-7047.

Received 19 September 1996; accepted 9 December 1996


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objectives: This study was undertaken to examine in-situ heart function and metabolism during insulin treatment of verapamil-induced cardiogenic shock in awake canines. Methods: Twenty mongrel canines were instrumented to monitor myocardial substrate uptakes (glucose, lactate, free fatty acids, oxygen [MVO2]), as well as left ventricular (LV) end-systolic elastance (Emax), LV efficiency (LV minute work/MVO2), and Tau. Shock was induced by graded intraportal verapamil infusion followed by randomized assignment to one of 4 treatment groups: saline control (3.0 ml/kg/min, n=5), epinephrine (5 µg/kg/min, n=5), glucagon (10 µg/kg/min, n=5) or insulin (1000 mU/min, n=5) with dextrose to clamp arterial [glucose] ±10% of basal concentrations. Results: Insulin treatment significantly increased Emax (34±3 vs. 17±3 mmHg/mm, saline control), and shortened Tau (9±3 ms) compared to saline control (42±5 ms), epinephrine (20±4 ms) and glucagon (35±8 ms). With insulin treatment, mechanical efficiency increased to 20 097±2070 vs. 12 424±1615 mmHg·mm/ml O2/100 g in controls. Simultaneously, insulin increased myocardial lactate uptake (35±2 vs. 17±4 µmol/min/100 g, saline control), but did not increase glucose uptake. Epinephrine and glucagon decreased mechanical efficiency compared to saline controls, coincident with increased myocardial fatty acid consumption, but without increasing lactate uptake. One dog died early with glucagon treatment before the first death in the saline-treated group. Conclusions: Insulin improves systolic and diastolic heart function during aerobic shock and accelerates in-vivo myocardial lactate oxidation.

KEYWORDS Metabolism; Insulin; Glucagon; Epinephrine; Verapamil; Shock; Contractility; Dog, anesthetized


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
Clinical and laboratory studies of insulin as a treatment for acute heart failure have demonstrated conflicting results regarding heart function, metabolism, and survival. Whether insulin benefits or worsens heart function during distress may depend upon the method of insult. The majority of studies employing insulin therapy have induced heart failure with ischemia. Insulin has been shown to worsen heart function when administered prior to or during induction of severe ischemia in isolated rat hearts [1–3]. However, with anoxia and normal myocardial perfusion, or in ischemia where even minimal perfusion is maintained, insulin improved heart function [2, 3]. Clinical studies, conducted prior to the advent of coronary recanalization techniques, showed that insulin–glucose–potassium treatment did not decrease the incidence of cardiogenic shock after acute myocardial infarction [4, 5]. More recent clinical trials have shown positive effects of insulin treatment, especially when used with techniques of reperfusion. In diabetic patients with myocardial infarction treated by streptokinase, intensive insulin therapy improved long-term survival by 29% [6]. Insulin–glucose therapy improved heart function and metabolism when used to treat cardiogenic shock after aorto-coronary bypass surgery [7–9]. Insulin–glucose treatment also improved heart function in other stress conditions, including endotoxin shock, experimental brain death, and halothane-induced cardiodepression [10–12]. All of these stress conditions impair insulin-mediated myocardial glucose uptake, and cause myocardial dysfunction without evidence of ischemia [10, 11, 13, 14]. Thus it appears that insulin produces optimal effects in non-ischemic (or post-ischemic) cardiodepression, especially when associated with myocardial insulin resistance.

Verapamil in high concentrations induces acute myocardial failure associated with myocardial insulin resistance and clinical findings similar to diabetic ketoacidosis [15–19]. Verapamil also induces a myocardial metabolic switch whereby the heart becomes more dependent upon carbohydrate usage rather than fatty acid usage [18]. Preliminary observations from our laboratory have demonstrated that high-dose-insulin euglycemia treatment improves systolic heart function during verapamil-induced shock in anesthetized canines [20].

The hypothesis of the present study was that high-dose insulin–glucose therapy would stimulate carbohydrate usage and thereby improve myocardial mechanical efficiency. Verapamil was chosen to produce heart failure in this study for two reasons: (1) verapamil does not cause myocardial ischemia, and (2) because of its increasing clinical significance as a cause of refractory cardiogenic shock [21–25]. In the United States, the incidence of calcium channel blocker overdose has increased by 150% per year for the past decade, necessitating hospitalization for nearly 6000 patients in 1994 [21, 26, 27]. As the clinical utility of calcium channel antagonism continues to increase, it follows that toxic exposures to these drugs will increase as well.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
All methods were approved by the Carolinas Medical Center Animal Care and Use Committee in accordance with NIH guidelines.

2.1 Instrumentation
Experiments were performed in mongrel hounds of either gender weighing 21–25 kg. Two weeks prior to experimentation, dogs were trained to rest quietly in a harness in the experimental environment. Immediately prior to instrumentation, animals were injected with 1 g intravenous cefazolin, anesthetized with 20 mg/kg i.v. sodium pentobarbital and 1 mg SQ atropine, followed by endotracheal intubation and pressure-limited ventilation. Surgical anesthesia was maintained with 0.5% isoflurane. A left thoracotomy was performed and the pericardium was incised longitudinally with care to avoid the phrenic nerve. To measure arterial blood pressure, a silastic catheter was advanced through a controlled puncture in the descending aorta and secured with redundant continuous suture. The circumflex artery was then dissected to allow placement of a pulse-Doppler flow probe (Triton Technology, San Diego, CA) to estimate coronary blood flow. Via right atrial appendage incision, a silastic catheter filled with 1000 U/ml heparin in 0.9% NaCl was placed and sutured in the coronary sinus to collect venous effluent for myocardial substrate uptake determinations. Two additional silastic catheters were placed in the right atrial appendage for drug infusion. To measure instantaneous left ventricular (LV) pressure and its first derivative, a Konigsberg micromanometer was placed in the LV via apex incision, and secured with a purse string suture. Paired piezoelectric crystals (20 MHz, Triton Technology, San Diego, CA) were diametrically placed across the LV minor axis to estimate ventricular shortening. To produce partial aortic occlusions for the purpose of determining maximum elastance at end-systole (Emax), a circumferential balloon occluder was placed around the aortic arch (In Vivo Metric, Healdsburg, CA). Via left subcostal incision, splenectomy was performed and a silastic catheter was advanced through the splenic vein and its tip positioned in the portal vein to infuse verapamil. All leads and catheters were then externalized near the dorsal midline, a suction thoracostomy tube placed to evacuate pneumothorax, and incisions closed in standard fashion. At least a 7-day postoperative interval was allowed prior to any experiment. Twenty dogs were successfully instrumented for full data collection.

2.2 Measurements and calculations
Maximum elastance at end-systole (Emax) was determined by simultaneously measuring ventricular diameter and instantaneous LV pressure during partial aortic occlusion as previously described [20]. Emax was iteratively determined according to the equation Emax=P(t)/[V(t)–V0], where P(t)/V(t) represents the maximal left ventricular pressure/volume ratio iteratively determined between peak –dP/dt and end-diastole; V0 represents the extrapolated ventricular volume at zero pressure. These determinations were made post-hoc by a software analysis system (Calcview) which allows for editing erratic loops. For each animal, myocardial stroke work was determined as the sum of the area inside the left ventricular pressure–dimension loops for 1 min. This value was computed from left ventricular pressure (LVP) and myocardial axis diameter (D) according to the power integral:


Formula

Myocardial mechanical efficiency was calculated by dividing minute stroke work by the standardized myocardial minute oxygen consumption [28]. Tau was measured as the time required for left ventricular pressure to decay 50% from the point of end-systole to the first deflection of left ventricular filling. Arterial blood pressure was determined by connecting the fluid-filled arterial catheter to a Statham transducer. Hemodynamic data were processed by Gould amplifiers, recorded on-line with an AstromedR chart recorder and archived on the Po-Ne-MahR digitizing system for later analysis. Myocardial substrate uptakes were estimated as the arterio-coronary sinus concentration difference multiplied by the circumflex artery blood flow standardized to 100 g myocardium. For myocardial substrate uptake measurements, two arterial and two coronary sinus samples (duplicate paired samples) were obtained and analyzed for each time point. These samples were used to calculate two uptake measurements which were averaged to yield one number for each time point (basal control, after induction of shock, and after 90 min treatment). To standardize circumflex flow and myocardial substrate uptakes, myocardium supplied by the circumflex was stained via post-mortem intracoronary methylene blue injection. The stained area was excised and weighed. Substrate uptakes for each animal were divided by this number and multiplied times 100.

Plasma insulin concentrations were determined by a commercially available radioimmunoassay (International Chemical Nuclear, Costa Mesa, CA). Glutathione-treated plasma was assayed for total catecholamine concentration using radioimmunoassay (Amersham, UK). Total non-esterified free fatty acids and triglycerides were quantified in heparin-free plasma samples using absorption spectrophotometry (Wako Chemicals, Richmond, VA). Glycerol concentrations were measured using enzymatic-UV absorption techniques [29]. Verapamil and norverapamil were extracted from acidified plasma in heptane using previously described techniques; total plasma concentrations were determined by using HPLC [30]. Recovery of verapamil and norverapamil from canine plasma was adjusted according to the free fraction of each drug available for chromatography. These fractions, (0.87 for verapamil, and 0.46 for norverapamil) were derived from standard techniques used to quantify free drug in plasma. Blood pH, pCO2, pO2, and total O2 concentrations were determined on a Radiometer ABL 520 blood gas analyzer. Whole blood glucose and lactate concentrations were determined with a Yellow Springs Instruments 2300 Stat analyzer.

2.3 Experimental protocol
Experiments were performed on fasting animals trained to rest quietly in a harness. No chemical sedation was used.

2.3.1 Control insulin-euglycemia studies (n=8)
The purpose of this group was to document cardiac mechanical responses to high-dose insulin infusion in otherwise healthy dogs. After basal measurements, recombinant insulin (Novolin, Bagsvaerd, Denmark) was infused in increasing amounts at 1000 mU/min for at least 1 h. Arterial glucose concentration was maintained ±10% of basal by infusing 50% dextrose. Measurements were made when arterial blood glucose was stabilized with a steady state of dextrose infusion. Total infusion rate of fluids was held constant at 3.0 ml/kg/h. Afterwards, animals were monitored to prevent hypoglycemia and at least 72 h was allowed before proceeding with further study.

2.3.2 Verapamil-induced shock
After basal control measurements, (±) verapamil HCl (Sigma Chemical Co, St Louis, MO, dissolved in 0.9% NaCl, pH 5.6), was infused into the portal vein to induce toxicity. Graded verapamil toxicity was achieved by increasing the infusion rate as follows: 0.04 mg/kg/min (hour 1), 0.08 mg/kg/min (hour 2), 0.1 mg/kg/min (end of hour 3, shock). Verapamil infusion was continued during the treatment phase at 0.2 mg/kg/min.

2.3.3 Treatment
After 3 h of verapamil infusion, animals were randomized to one of 4 treatments:

1. Saline control (n=5). 3 ml/kg/min 0.9% NaCl.
2. Epinephrine (n=5) (Epinephrine HCl, Abbott Laboratories, Chicago, IL), 100 µg/ml in 0.9% NaCl, pH 5.6, and light-shielded, was infused initially at 5.0 µg/kg/min and then titrated to a maximum of 10.0 µg/kg/min in attempt to maintain left ventricular pressure at basal levels.
3. Glucagon (n=5) (Eli Lilly, Indianapolis, IN) 500 µg/ml in 0.9% NaCl was first bolus-injected (0.2 mg/kg), then infused at 10 µg/kg/min.
4. Insulin–glucose (n=5): 1000 mU/min calcium-free insulin was infused, together with 50% dextrose to clamp arterial [glucose] ±10% basal concentrations.

Total infusion volume was maintained at 3.0 ml/kg/h in all treatment groups. Treatment and toxicity was continued until death for all animals.

2.4 Statistical analysis
Data were transferred from the Po-Ne-Mah data acquisition system to the SuperAnova statistical package. For myocardial substrate uptake measurements at each reported time point, data were averaged from 2 paired arterial and coronary sinus samples; for hemodynamic measurements, data were averaged from 10–15 ventricular contractions (except for stroke work data, which were measured for 1 min) means±s.e. are reported. Data were tested for homoscedasticity prior to hypothesis testing. Non-homogeneous data were appropriately transformed. A paired t-test was used to compare data between basal control and shock measurements. Treatment effects are compared using one-way repeated-measures ANOVA with Duncan's new Multiple Range post-hoc test. The amount of verapamil required to cause death (LD100) in each treatment group is presented as the overall index of treatment success. LD100 data are compared among treatment groups using the log-rank statistic. P<0.05 was used to reject the null hypothesis.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
3.1 Basal control and induction of shock
Hemodynamic and metabolic measurements observed at basal control (prior to verapamil infusion), and during shock (after 3 h of verapamil infusion, before the treatment interventions) are presented in Table 1Table 2. Because there were no significant differences observed for any variable among the individual treatment groups before the initiation of treatment interventions, the data from all 20 animals are combined to describe the model of shock. Verapamil shock was defined as the onset of sinus arrest on ECG. This definition resulted in a constellation of hemodynamic and metabolic deficits.


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  		Table 1 Hemodynamic data during basal and shock conditions (n=20)

 

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  		Table 2 Myocardial substrate consumption during basal and shock conditions (n=20, all values expressed per 100 g wet myocardial weight)

 
The onset of verapamil shock resulted in a rapid reduction in mean arterial blood pressure (mean change –25±5 mmHg) together with a doubling of mean arterial lactate concentrations immediately after sinus arrest. This pattern was observed in every animal in the last half of the third hour of verapamil infusion. Indices of cardiac function were significantly depressed at the end of the shock period, with values of Emax, +dP/dt, –dP/dt and LV work that were 66–68% of the values observed in the basal control state (Table 1). In addition, Tau increased from 28±2 to 37±5 (P<0.05, paired t-test), a 28% increase, while heart rate decreased by 19% (Table 1). Significant changes in myocardial substrate usage were also observed during verapamil shock (Table 2). There was a significant reduction in free fatty acid (20% of basal) and oxygen consumption (64% of basal), while glucose and lactate consumption increased (5.7- and 3-fold, respectively). In addition, the arterio-coronary sinus pH gradient (arterial pH – coronary sinus pH) decreased significantly (0.04±0.01, basal control, vs. 0.01±0.01, shock; P<0.05, paired t-test). The decrease in external cardiac work and indices of contractile function indicated that verapamil induced heart failure; however, the increase in myocardial lactate uptake and the decrease in MVO2 with no change in coronary perfusion indicate that verapamil shock occurred under non-ischemic conditions. Endogenous catecholamine concentrations increased during the induction of shock from 371±62 to 7081±511 pg/ml (P<0.05, paired t-test).

3.2 Treatment effects
Fig. 1 illustrates pertinent indexes of systolic and diastolic heart function after 90 min treatment of verapamil shock with saline, epinephrine, glucagon, or insulin. Table 3 also shows changes in overall hemodynamic parameters with the treatments. Insulin was the only treatment that significantly increased left ventricular efficiency, Emax, +dP/dt, and –dP/dt, and also decreased Tau compared to saline control (Fig. 1). The decrease in Tau (79%, compared to saline control) was not entirely heart-rate-dependent since heart rate only increased by 30% with insulin treatment. Insulin treatment also significantly improved LV work and systolic blood pressure, but did not change coronary perfusion (Table 3). Glucagon treatment produced highly variable inotropic responses between dogs. Treatment with glucagon produced a lethal ventricular fibrillation after 15 min of treatment in one animal, thus the results of the 4 surviving animals are presented in Fig. 1 and Table 3. Statistically significant improvements were observed in +dP/dt and heart rate with glucagon treatment; however, other indices of contractile function or overall hemodynamics were not significantly improved. Treatment of verapamil shock with epinephrine significantly shortened Tau compared to saline control; however, there was no improvement in other indices of systolic function, heart rate or arterial blood pressure compared to saline control.


Figure 1
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  		Fig. 1 Myocardial mechanical function after 90 min treatment of verapamil-induced cardiogenic shock. Values are means±s.e.; n=4, glucagon; n=5 all other groups. *P<0.05, vs. saline control; **P<0.05 vs all other treatments, repeated measures ANOVA, Duncan's new multiple range post-hoc test.

 

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  		Table 3 Hemodynamic data after 90 min treatment (n=4 glucagon; n=5 all other groups)

 
The results indicate that treatment with insulin in the verapamil shock state improves cardiac systolic and diastolic function as well as systemic hemodynamic parameters. To determine if these changes are present only in the shock condition or if they occur in the control state, animals were treated with hyperinsulinemia-euglycemia without verapamil-induced shock. In healthy dogs, insulin produced no significant change in any index of systolic or diastolic heart function (Table 4).


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  		Table 4 Hemodynamic data in healthy dogs (n=8) under basal conditions (basal) and after 60 min of 1000 mU/min insulin infusion with arterial glucose maintained (hyperinsulinemia-euglycemia)

 
Myocardial substrate consumption was determined during the treatment of verapamil-induced shock (Table 5). Treatment with saline resulted in a profile of substrate usage that was similar to the shock state without treatment (Table 2). Thus, in the absence of inotropic support, the hearts continued to preferentially use lactate and glucose with markedly attenuated fatty acid uptake. Although lactate uptake remained positive, the arterio-coronary sinus pH gradient remained decreased significantly versus. basal control conditions (0.02±0.01 vs. 0.04±0.01, P<0.05, paired t-test). Epinephrine increased fatty acid uptake, and appeared to reduce glucose usage compared with saline treatment. However, because of fluctuations in arterial glucose with epinephrine treatment, it is possible that myocardial glucose uptake was measured under non-steady-state conditions. Nonetheless, when compared to saline treatment, epinephrine treatment tended to shift myocardial lactate and proton exchange shifted significantly toward a net release of these substrates. Treatment of verapamil shock with glucagon caused increased myocardial consumption of both free fatty acids and oxygen compared with saline treatment. There was no significant change in lactate exchange or pH gradient with glucagon treatment. Treatment of verapamil shock with insulin resulted in a significant increase in lactate uptake and significantly shifted the arterio-coronary sinus pH gradient to a more positive value (0.11±0.02, P<0.05 vs saline control). Insulin decreased the arterial plasma glucose concentration and did not increase either coronary artery perfusion (Table 3) or myocardial glucose extraction (4±1% saline vs 3±2% insulin); therefore, insulin treatment did not increase myocardial glucose uptake. Unlike the epinephrine and glucagon treatments, myocardial fatty acid consumption remained near zero with insulin treatment.


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  		Table 5 Myocardial substrate consumption after 90 min of treatment of verapamil-induced cardiogenic shock (n=4 glucagon, n=5 all other groups, all values expressed per 100 g wet myocardial weight)

 
The data in Table 5, when compared to data in Fig. 1a, indicate that epinephrine and glucagon treatment stimulated fatty acid uptake while significantly reducing left ventricular mechanical efficiency. In contrast, treatment with insulin improved myocardial mechanical efficiency and decreased fatty acid uptake.

3.3 Arterial metabolic data and verapamil concentrations
Arterial metabolic measurements were made during the treatment of verapamil shock with saline, epinephrine, glucagon or insulin (Table 6). Treatment of verapamil shock with insulin caused a significant increase in arterial insulin levels and a significant decrease in arterial glucose and lactate concentrations compared to saline control treatment. The insulin treatment group showed no significant change in catecholamine concentrations compared with saline treatment. Treatment of verapamil shock with glucagon also caused a significant decrease in arterial lactate concentrations, but, in contrast with insulin, there was a significant increase in catecholamine concentration with glucagon treatment. Treatment with epinephrine produced a significant increase in measured catecholamines, but did not change arterial concentrations of any other substrate compared with saline control treatment.


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  		Table 6 Arterial substrate, insulin, catecholamine and verapamil concentrations during treatment of verapamil-induced cardiogenic shock (n=4 glucagon, n=5 all other groups)

 
Changes in arterial pH were observed with the treatments. Epinephrine produced significantly greater acidemia (pH 7.24±0.06) and hyperventilation (pCO2 13±2 torr) compared with the saline control treatment (pH 7.32±0.04, pCO2 20±4 torr, P<0.05, ANOVA). In contrast, arterial pH was not altered by either insulin or glucagon treatments.

Arterial verapamil concentrations were not significantly different among the saline, epinephrine and insulin treatment groups. Glucagon treatment significantly reduced the plasma verapamil concentration compared with the saline control treatment. Norverapamil, the primary metabolite of verapamil, did not change significantly in any of the treatment groups, suggesting that hepatic demethylation of verapamil could not account for the decrease in verapamil concentration observed in the glucagon treatment group. Death from verapamil-induced shock was associated with pulseless electrical activity in 19 of the 20 canines. The mean lethal dose of verapamil for canines treated with saline was 43±10 mg/kg. Only insulin treatment produced a significant increase in the lethal dose of verapamil (85±12 mg/kg) compared to saline (Fig. 2, P<0.05, log-rank statistic). Glucagon and epinephrine treatments did not significantly change the lethal dose of verapamil. Survival duration with each treatment was: 149±28 min, saline controls; 125±34 min, epinephrine; 208±45, glucagon; and 360±51 min, insulin–glucose. These data show the mean time until death with escalating verapamil toxicity with each treatment. Measurements at the 90 min treatment time point were thus collected during severe cardiogenic shock, but prior to significant attrition from death (only one animal in the glucagon group expired).


Figure 2
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  		Fig. 2 Mean fatal dose (LD100 mg/kg) of intraportal verapamil for each treatment. See Fig. 1 for legend information.

 

   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
The present studies show that insulin-euglycemia produced superior improvements in left ventricular systolic and diastolic function during aerobic cardiogenic shock from verapamil overdose in awake canines. Both epinephrine and glucagon increased myocardial fatty acid usage, but at a cost of decreased mechanical efficiency. Insulin–glucose improved left ventricular mechanical efficiency while promoting myocardial lactate consumption. The inotropic improvements observed with insulin–glucose also improved overall survival during verapamil-induced cardiogenic shock. In contrast, one of the 5 dogs treated with glucagon expired prior to 90 min of treatment, prior to the first death among saline-treated dogs. These findings support the role of insulin–glucose treatment for non-ischemic cardiodepression, and show a contrast between the mechanism of improvement in heart function with insulin–glucose treatment compared to more conventional positive inotropes. It is clear from the available data that insulin–glucose did not improve function by increasing catecholamine concentration, or by increasing coronary flow, rather via actions upon myocardial metabolism.

The present findings indicate that left ventricular mechanical efficiency and diastolic function vary with the use of different inotropic agents to treat non-ischemic heart failure. It is generally accepted that an increase in myocardial uptake of fatty acids will decrease mechanical efficiency [31, 32]. During treatment for verapamil shock, an inverse relationship was observed between fatty acid uptake and left ventricular efficiency: epinephrine and glucagon increased fatty acid consumption with apparent oxygen wasting, whereas insulin–glucose increased mechanical efficiency while fatty acid consumption remained near zero. Epinephrine treatment may have actually been harmful since myocardial lactate exchange was shifted more negatively (compared to saline treatment), implying the existence of functional ischemia. It was therefore not surprising that mechanical efficiency was lower with epinephrine than with other treatments. In contrast, insulin–glucose stimulated myocardial lactate uptake while improving global heart function. Insulin–glucose treatment, however, did not improve any parameter of heart performance in the absence of shock. Taken together, these findings suggest that insulin–glucose can push the aerobic heart toward greater mechanical efficiency in aerobic shock conditions by augmenting myocardial lactate metabolism.

It is well known that shock shifts myocardial metabolism from predominate fatty acid oxidation to preferential carbohydrate-oxidation [33]. Insulin–glucose may facilitate lactate disposal and help translate this obligatory metabolic switch into improved mechanical efficiency. While the healthy heart normally oxidizes fatty acids to provide 90% of the energy for ATP biosynthesis, under shock conditions, fatty acids are thermodynamically inefficient, and more difficult than lactate to catabolize [34]. The canine heart will thus preferentially utilize lactate compared to other substrates [35, 36]. Because verapamil shock produced hyperlactacidemia, and narrowed the arterio-coronary sinus pH gradient, it is likely that myocardial lactate content was elevated as well. An elevated myocardial lactate content can stall glycolysis and decrease heart systolic and diastolic function [2, 3, 34, 37]. Insulin–glucose treatment decreased plasma lactate concentrations, but increased myocardial lactate and proton consumption while producing no increase in glucose uptake. These findings suggest that insulin–glucose improved myocardial lactate clearance without a significant increase in glycolytic flux. An insulin-mediated increase in myocardial pyruvate dehydrogenase (PDH) activity would explain this selective increase in lactate uptake. Insulin has been shown to increase in-vivo myocardial PDH activity in concentrations above 1000 µU/ml [38]. However, pharmacologic activation of PDH does not cause a positive inotropic effect in the normal heart [39]. Thus, activation of PDH would also help explain how insulin–glucose could improve myocardial contractility during shock, yet produce no change in myocardial contractility in healthy dogs. Studies are presently underway in our laboratory to examine this hypothesis.

This hypothesis can be extended to explain the ability of insulin–glucose to improve left ventricular relaxation during shock, but to have no such effect in healthy dogs. In the current context, it is ironic to note that verapamil is used to treat diastolic dysfunction in humans. Again, it appears that insulin–glucose removed a restraining effect of shock upon left ventricular diastolic function. Insulin–glucose could have helped clear the cytosol of lactic acid and other glycolytic by-products which can impair calcium handling and cause diastolic dysfunction [2, 40]. There is also some evidence that insulin–glucose treatment may improve the function of membrane calcium pumps in heart cells with diastolic dysfunction by increasing availability of ATP from glycolysis [41].

The present findings support the strategy of improving metabolic efficiency as the first step to resuscitating heart function during aerobic shock. Catecholamines are typically chosen as cardiotonic agents to treat myocardial depression during aerobic shock. The rationale that is usually cited for the choice of these agents centers on their gross hemodynamic effects on cardiac output and systemic vascular resistance [42]. However, clinicians are becoming more aware of the role of drugs which first improve efficiency of myocardial energy production. Catecholamines may not satisfy this aspect of treatment inasmuch as they tend to decrease myocardial carbohydrate usage and increase turnover of free fatty acids [43]. The present findings are consistent with clinical studies that employed dichloroacetate to improve heart function in non-ischemic conditions. Dichloroacetate produces insulin-like effects, primarily by increasing pyruvate dehydrogenase activity, causing the balance of myocardial metabolism to shift toward predominant carbohydrate oxidation. Bersin and Wolfe demonstrated that dichloroacetate reversed myocardial lactate efflux and increased mechanical efficiency, while dobutamine showed the opposite effects in humans with cardiomyopathy [44]. Using radiolabelled substrates, McVeigh and Lopaschuk have elegantly demonstrated in isolated rat hearts that insulin and dichloroacetate improve post-ischemic heart function primarily by stimulating carbohydrate oxidation through the tricarboxylate acid cycle rather than by stimulating glycolysis [45]. In fact, Lopaschuk et al. have shown that insulin stimulation during myocardial ischemia impairs functional recovery during reperfusion, because of disproportionate glycolytic stimulation compared to net pyruvate oxidation [46].

Insulin–glucose treatment has shown highly variable results in both clinical and experimental studies of myocardial ischemia [4, 47–49]. In-vivo studies show more reliable beneficial effects of insulin on heart function in non-ischemic stress models. One potential mechanistic explanation for this difference can be found by examining changes in lactate consumption. Where insulin increased function in the non-ischemic models, myocardial lactate consumption increased significantly [7, 10–12, 17, 50]. Where insulin has failed to improve heart function in ischemic cardiodepression, insulin either did not increase lactate uptake in vivo, or was associated with an increased myocardial lactate content in isolated hearts [3, 47, 48]. These studies and the present findings are concordant with the concept that the heart responds most favorably to insulin stimulation during global, aerobic cardiodepression, when it is capable of balanced, synchronous increase in carbohydrate oxidation and glycolytic flux.

The present findings also leave open the possibility for other explanations for the improved mechanical efficiency observed with insulin–glucose treatment in addition to lactate oxidation. Russell and Taegtmeyer have shown that the addition of lactate as a competing substrate to acetoacetate significantly improved function in isolated, working rat hearts [51]. In a subsequent study, the same authors demonstrated that lactate is readily carboxylated to malate (i.e., anaplerosis), which can enter the citric acid cycle directly, when pyruvate dehydrogenase is relatively inactive [52]. In the present study, because MVO2 did not increase substantially with insulin–glucose treatment, some fraction of lactate may have undergone anaplerosis and converted to other substances without being oxidized. Drake et al. have shown previously that the dog heart can increase myocardial lactate consumption two-fold without an increase in oxygen consumption, an observation attributed to the fact that the dog heart will oxidize lactate preferentially to (and instead of) all other carbon sources [35]. Insulin–glucose treatment can also reverse processes that cause oxygen wastage. Fatty acids are recognized to uncouple mitochondria, and insulin–glucose probably decreased myocardial fatty acid content. However, verapamil had already significantly decreased fatty acid uptake, raising the question as to whether insulin–glucose modified endogenous triglyceride usage. Hearts are known to utilize endogenous triglycerides preferentially to exogenous carbohydrate [34]. In the present study, after 90 min treatment, insulin–glucose did significantly decrease glycerol release, suggesting that insulin–glucose inhibited use of fatty acids from intracellular triglyceride hydrolysis. The notion that insulin–glucose treatment could improve mitochondrial coupling has been shown previously in isolated hearts perfused with insulin in fat-free buffers [53, 54]. Insulin is also known to decrease cellular purine efflux during myocardial hypoxia, an effect which could afford an oxygen-sparing effect in shock [55]. Finally, insulin–glucose may reduce oxygen expenditure in the form of enthalpy produced by catecholamine stimulation [56].

Time for primary review 25 days.


   Acknowledgements
 
This study was supported by an Emergency Medicine Foundation Career Development Award.


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 

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