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Cardiovascular Research 1999 42(1):232-239; doi:10.1016/S0008-6363(98)00325-3
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Copyright © 1999, European Society of Cardiology

Nitric oxide does not modulate the increases in blood flow, O2 consumption, or contractility during CaCl2 administration in canine hearts

George J Crystala,b,c,* and Xiping Zhoua,b

aDepartment of Anesthesiology, Illinois Masonic Medical Center, Chicago, IL 60657, USA
bDepartment of Anesthesiology, University of Illinois College of Medicine, Chicago, IL 60680, USA
cDepartment of Physiology and Biophysics, University of Illinois College of Medicine, Chicago, IL 60680, USA

* Corresponding author. Tel.: +1-773-296-5375; fax: +1-773-296-5362.

Received 29 June 1998; accepted 12 October 1998


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objective: Endothelium-derived nitric oxide (EDNO) has been shown to have vascular, metabolic, and contractile effects in the heart. We evaluated these effects during intracoronary (i.c.) administration of CaCl2 in dogs. Methods: The left anterior descending coronary artery of nine anesthetized, open-chest dogs was perfused at controlled pressure (80 mm Hg) with arterial blood. Coronary blood flow (CBF) was measured with a Doppler transducer and segmental shortening (SS) with ultrasonic crystals. Myocardial oxygen consumption (MVO2) and oxygen extraction (EO2) were calculated. Responses were assessed during i.c. infusions of CaCl2 (5, 10, 15 mg min–1) before and after administration of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 300 µg min–1 for 15 min, i.c.). Results: Before L-NAME, CaCl2 caused dose-dependent, proportional increases in SS and MVO2. Although CBF also increased, these responses were less than proportional to those in MVO2, and thus EO2 increased. L-NAME did not alter the cardiac effects of CaCl2. Conclusions: (1) CaCl2 had direct inotropic and coronary vasoconstricting effects. (2) The vasoconstricting effect impaired coupling of CBF to the augmented metabolic demands by local vasodilating mechanisms. (3) EDNO did not modulate the increases in CBF, MVO2, or SS during administration of CaCl2.

KEYWORDS Coronary hemodynamics; Inotropic drugs; Cardiac performance; Dogs; NG-nitro-L-arginine methyl ester


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
Nitric oxide (NO) is produced in the vascular endothelium from the amino acid L-arginine in a reaction catalyzed by NO synthase (NOS) and triggered by rises in intracellular Ca++ concentration [1]. Endothelium-derived NO (EDNO) diffuses to the underlying vascular smooth muscle, where it stimulates guanylate cyclase and production of cyclic guanosine 3'-5'-monophosphate (cGMP), which has vasorelaxing effects [1]. EDNO is released tonically in the coronary circulation, and its release is accelerated by substances, e.g., acetylcholine, which operate via specific receptors on the endothelial cell, and by mechanical factors, such as shear stress [2]. EDNO has been shown to play an important role in the reductions in coronary vasomotor tone elicited by a variety of physiological stimuli, including hypoxia [3], reduced perfusion pressure (autoregulation) [4], transient occlusion, i.e., reactive hyperemia [5], hypercapnia [6], and the increased cardiac work associated with tachycardia and increased wall tension [7]. Few studies have assessed the role of EDNO in the myocardial functional hyperemia accompanying inotropic stimulation, and they were limited to agents that operate via the β-adrenergic receptor-cyclic adenosine 3'-5'-monophosphate (cAMP) pathway, i.e., isoproterenol and dobutamine [8–10].

Calcium is an inotropic agent that acts independently of the β-adrenergic receptor-cAMP pathway [11]. In a recent study [12], we assessed the coronary vascular effects accompanying CaCl2-induced augmentations in contractility in normal canine hearts. Intracoronary (i.c.) infusions of CaCl2 were associated with increases in coronary blood flow, although these increases were less than proportional to those in myocardial oxygen consumption. This suggested that a direct constricting effect for calcium partially antagonized the metabolic vasodilation associated with increased cardiac work [13]. The signal-transduction pathway underlying the coronary vasodilation during administration of CaCl2 remains to be clarified. A role for the NO-cGMP pathway is suggested by the ability of Ca++ concentration to modulate NOS activity in the endothelial cell (see above).

The present study tested the hypothesis that EDNO modulates the myocardial functional hyperemia during administration of CaCl2. This was evaluated by comparing the CaCl2-induced changes in coronary blood flow before and after treatment with the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME). Recent in vitro studies have suggested that EDNO may reduce myocardial oxygen consumption directly via inhibition of mitochondrial enzymes [14]and indirectly via a negative inotropic effect [15]. If NO has this influence in the intact heart during exposure to CaCl2, the increases in myocardial oxygen consumption may be accentuated following NOS inhibition, thus providing an enhanced metabolic stimulus for coronary vasodilation. To distinguish between direct and metabolism-mediated effects of L-NAME on the myocardial functional hyperemic responses, the inotropically-induced changes in coronary blood flow were assessed in the context of the concomitant changes in myocardial oxygen consumption and segmental shortening.

The study was performed using an extracorporeal system to perfuse selectively the left anterior descending coronary artery (LAD) in in situ canine hearts. This approach facilitated the use of selective i.c. infusions of drugs, which minimized their systemic effects.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
2.1 Canine preparation
The investigation conforms with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1985). Experiments were performed on nine healthy mongrel dogs of either sex (weight, 20.4–22.8 kg). Anesthesia was induced with intravenous bolus injection of thiopental 15 mg kg–1, and maintained by continuous intravenous infusion of fentanyl and midazolam at rates of 12 µg kg–1 h–1 and 0.6 mg kg–1 h–1, respectively. After tracheal intubation and left thoracotomy in the fourth intercostal space, the lungs were mechanically ventilated with room air enriched with oxygen to maintain arterial PO2 greater than 200 mm Hg. The tidal volume and respiratory rate were adjusted to maintain arterial PCO2 and pH at physiological levels (PCO2; 38±2 mm Hg; pH; 7.39±0.01). PO2, PCO2, and pH of arterial and venous blood samples (see below) were measured electrometrically (model 413, Instrumentation Laboratories, Lexington, MS). Muscle paralysis was achieved with an intravenous injection of vecuronium bromide, 0.1 mg kg–1 with supplements at 0.05 mg kg–1 h–1. Body temperature was maintained at 38°C with a heating pad. Lactated Ringer’s solution was administered continuously at a rate of 5 ml kg–1 h–1 intravenously to compensate for evaporative fluid losses. Heparin (400 U/kg with supplementation) was used for anticoagulation.

The LAD was isolated for cannulation approximately 2 cm from its origin. A thin-wall stainless-steel cannula (2.5 mm inside diameter) was introduced into the isolated segment of the artery, so that it could be perfused selectively by an extracorporeal perfusion system [6, 16]. The perfusion system contained a pressurized reservoir, which was supplied with blood from the left femoral artery.

The tubing connecting the reservoir to the LAD was equipped with (1) a heat exchanger to maintain the temperature of coronary arterial blood at 38°C, (2) a Doppler flow transducer (Transonics Systems, Ithaca, NY) to measure coronary blood flow, (3) ports for collecting samples of coronary arterial blood and for infusing drugs, and (4) a mixing chamber for drugs infused into the perfusion tubing. Coronary perfusion pressure was measured through a small-diameter tube positioned at the outlet of the perfusion cannula.

Measurements of aortic pressure, heart rate, left atrial pressure, left ventricular +dP/dtmax, and left ventricular –dP/dtmin were obtained using standard methods [6, 16]. A continuous record of hemodynamic variables was obtained on a physiological recorder (model 2800S, Gould, Cleveland, Ohio).

2.2 Experimental measurements
2.2.1 Myocardial segmental shortening
Measurements of myocardial segmental length in the LAD bed were obtained with a pair of ultrasonic crystals [16]. Changes in distance between the crystals were recorded from measurements of the ultrasonic transit time between the crystals (Triton Technology, San Diego, CA). The end-diastolic and end-systolic lengths (EDL, ESL) were identified by the beginning of the rapid increase in left ventricular pressure just before isovolumetric contraction and –dP/dt min, respectively. Segmental shortening (SS; in %) was calculated from the formula:

Formula

2.2.2 Myocardial oxygen consumption
Measurements of myocardial oxygen consumption were obtained in the LAD perfusion territory. The anterior interventricular vein was cannulated at the same level as the LAD cannula for collection of regional coronary venous effluent [17]. The venous cannula was allowed to drain freely into a beaker to prevent venous stagnation and interstitial edema. This venous blood was returned intermittently to the dog to maintain isovolemic conditions. At specified times in the study, 1-ml blood samples were collected from the coronary venous cannula under mineral oil to maintain anaerobic conditions. These venous samples were paired with 1-ml arterial samples from the perfusion tubing, so that the coronary arteriovenous difference for oxygen could be determined. Hemoglobin concentration and percent hemoglobin oxygen saturation of the blood samples were measured with a CO-Oximeter (model 482, Instrumentation Laboratories, Lexington, MS), and used to calculate oxygen bound to hemoglobin assuming an oxygen carrying capacity for hemoglobin of 1.39 ml O2/g. The oxygen dissolved in the blood was computed (O2 dissolved=0.003 ml O2 per 100 ml of blood per mm Hg) and added to the bound component to compute total oxygen content. Myocardial oxygen consumption (in ml min–1 100 g–1) was computed using the Fick equation, i.e., from the product of the coronary arteriovenous oxygen difference and coronary blood flow. The myocardial oxygen extraction (in %) was calculated by dividing the arteriovenous oxygen difference by the arterial oxygen content. These values are a reflection of the relationship between myocardial oxygen consumption and coronary blood flow [13], and they served as an index of the effectiveness of metabolic vasodilation during CaCl2. The oxygen cost of inotropic stimulation was quantified by dividing the inotropically-induced percentage increase in myocardial oxygen consumption by the inotropically-induced percentage increase in segmental shortening. The resultant value was termed the oxygen cost ratio.

2.2.3 Coronary arterial ionized calcium concentration
Ionized calcium concentration ([Ca++]) in the LAD blood during infusion of CaCl2 was estimated by calculating the [Ca++] added to the blood (by dividing the infusion rate for calcium by the steady state value for coronary blood flow), and adding it to the control value for [Ca++]. This approach was validated in our previous study [12]. The average value for measured [Ca++] under control conditions in our previous study (1.22±0.06 mmol l–1 was used in the calculations of [Ca++] [12].

2.3 Experimental protocols
After >45 min for recovery from surgical preparation, control measurements for coronary blood flow, segmental shortening, oxygen consumption and oxygen extraction were obtained. Then CaCl2 was infused into the LAD in a graded fashion (5, 10, 15 mg min–1). Measurements were obtained when steady state conditions were achieved at each drug infusion rate (as indicated by stable increases in coronary blood flow and segmental shortening), which was within 2–3 min after varying the rate of infusion. The dose range for CaCl2 was selected because it had been shown previously to span a significant portion of the dose–response curve [18]. Isotonic saline was used to dilute CaCl2 to 5.0 mg ml–1, which resulted in infusion rates over the range 1.0 to 3.0 ml min–1. Preliminary studies demonstrated that infusion of the saline vehicle alone at these low flow rates had no effect on coronary blood flow or segmental shortening. After a final dose of CaCl2, the infusion pump was stopped and at least 20 min was allowed for recovery. Then L-NAME was infused at a rate of 300 µg min–1 i.c. for 15 min to inhibit production of NO from the vascular endothelium [6, 10, 16]. Ten min were allowed before the graded infusions of CaCl2 were repeated. Efficacy of L-NAME was evaluated using i.c. infusions of the endothelium-dependent vasodilator acetylcholine (ACh; 20 µg min–1), and of the endothelium-independent vasodilator sodium nitroprusside (SNP; 80 µg min–1). The doses for ACh and SNP were the highest that could be used without causing aortic hypotension [6, 10, 16]. Vasodilator reserve was assessed with a maximally-dilating infusion of adenosine (8 mg min–1 i.c. [6, 10, 16]). A coronary perfusion pressure of 80 mm Hg was used throughout the study.

At the termination of each experiment, 5 ml of Evans blue dye (10 mg ml–1 saline) was injected into the LAD to identify its perfusion territory. After the heart was stopped with a 10-ml bolus injection of KCl (80 mg ml–1 saline) into the left ventricular cavity, it was removed and trimmed. The dyed tissue was excised and weighed so that coronary blood flow could be expressed on a per 100-g basis. The average weight of the LAD perfusion territory was 32±2 g.

2.4 Statistical analyses
A two-way analysis of variance for repeated measurements was used to assess the dose-dependent effects of CaCl2 before and after L-NAME [19]. Post hoc comparisons were made using the Student’s t-test with the Bonferroni correction [19]. Additional statistical analyses were performed using the paired version of the Student’s t-test [19]. Data are presented as mean±standard error. A P<0.05 was considered significant throughout this study.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
Fig. 1 presents the effects of CaCl2 on coronary blood flow, myocardial oxygen consumption, oxygen extraction, and segmental shortening before and following L-NAME. CaCl2 caused dose-dependent increases in coronary blood flow, which at constant coronary perfusion pressure, mirrored the induced decreases in coronary vascular resistance. These increases in coronary blood flow were less than proportional to those in myocardial oxygen consumption, with the result that oxygen extraction increased. CaCl2 caused dose-dependent increases in segmental shortening, which were proportional to those in myocardial oxygen consumption; thus, oxygen cost ratio remained equal to unity (Table 1). L-NAME had no independent effect on the local cardiac variables. There was no interaction between the effects of CaCl2 and L-NAME.


Figure 1
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  		Fig. 1 Dose-related effects of intracoronary CaCl2 on coronary blood flow (CBF), and myocardial oxygen consumption (MVO2), oxygen extraction (EO2), and segmental shortening (SS), before and after L-NAME. Findings were similar before and after L-NAME. Values are mean±SE. P<0.05. * vs. control; {dagger} vs. 5.0; {ddagger} vs. 10.

 

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  		Table 1 Values for the O2 cost ratio during graded intracoronary infusions of CaCl2 before and after L-NAME

 
Table 2 shows that the values for calculated [Ca++] varied directly with the rate of infusion of CaCl2, and that they were not different before and following L-NAME.


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  		Table 2 Calculated values for ionized calcium (mmol l–1) in coronary blood supply

 
L-NAME itself had no effect on the baseline values for cardiac and systemic hemodynamic variables (Table 3); however, it blunted the increases in coronary blood flow by ACh (178±41% vs. 39±6%), although it had no effect on the increases in coronary blood flow by SNP (127±17% vs. 125±14%) (Fig. 2). Adenosine caused nearly five-fold increases in coronary blood flow (116±15 to 564±56 ml min–1 100 g–1).


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  		Table 3 Baseline values for cardiac and systemic hemodynamic parameters before and following L-NAME

 

Figure 2
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  		Fig. 2 Effects of acetylcholine (ACh) and sodium nitroprusside (SNP) on coronary blood flow before and after L-NAME. L-NAME blunted the increases in coronary blood flow by ACh but not by SNP. Values are mean±SE. * vs. Before L-NAME.

 
Table 4 shows the effect of the i.c. infusions of CaCl2 before L-NAME on systemic hemodynamic and global cardiac variables. CaCl2 had no effect on these variables, with the exception that it increased left ventricular +dP/dtmax. The responses were similar following L-NAME (data not shown).


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  		Table 4 The intracoronary infusions of CaCl2 before L-NAME had no effect on systemic hemodynamic parameters, with the exception that they increased left ventricular dP/dtmax. Findings were similar following L-NAME

 

   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
4.1 Critique of methods
The baseline values for oxygen extraction in the cannulated LAD bed were lower than those usually found in anesthetized dogs with an intact coronary circulation [20]. This suggests vasodilation in the control preparation probably because of dilators released from blood cells within the extracorporeal circuit [21]. Nevertheless, vascular responsiveness to both an endothelium-dependent vasodilator (ACh) and an endothelium-independent vasodilator (SNP) was pronounced (Fig. 2). Moreover, vasodilator reserve was appreciable, as shown by the nearly five-fold increases in coronary blood flow during adenosine infusion. These latter findings indicated that the increases in coronary blood flow during CaCl2 administration were not limited by the vasodilator reserve of the preparation.

The ability of changes in segmental shortening to reflect changes in myocardial contractility is limited by variations in heart rate and in the loading conditions of the heart [22]. The constant values for heart rate and in indices of afterload (aortic pressure) and preload (left atrial pressure) during the i.c. infusions of CaCl2 and L-NAME suggest that this methodologic limitation did not apply to the present study.

Our baseline values for myocardial oxygen consumption were typical of those obtained by us in this canine preparation [6, 10, 12, 16], but they were lower than those reported by other investigators also studying open-chest, anesthetized dogs [7, 23]. This discrepancy may be due to the lower, more physiological values, for heart rate when fentanyl, rather than pentobarbital sodium, is used for general anesthesia. Fentanyl reduces heart rate by stimulation of vagal preganglionic neurons in the medulla [24], whereas pentobarbital sodium increases heart rate via a vagolytic action [25]. Our baseline values for segmental shortening were well within the range of those previously found in similar canine preparations [26, 27].

[Ca++] in the coronary arterial blood during infusion of CaCl2 was calculated by dividing the infusion rate for calcium by the steady state value for coronary blood flow, and assuming a constant control value for [Ca++], i.e., that no recirculation of the infused calcium occurred. This assumption was based on our previous finding indicating only negligible increases in [Ca++] within the aortic blood during a high-dose (15 mg min–1) infusion of CaCl2 into the LAD [12]. The strong correlation between our calculated values for [Ca++] and those measured directly [12]validated our approach for estimating [Ca++] in the coronary arterial blood.

Intravenous infusions of NOS inhibitors cause increases in systemic vascular resistance (and concomitant increases in arterial pressure and left ventricular afterload) [1, 4], suggesting that tonic release of NO may play an important role in modulating basal vascular tone in the peripheral circulation. The use of i.c. infusions of L-NAME in the present study avoided its systemic effects, which simplified interpretation of the findings. Our i.c. dose for L-NAME was adopted from previous studies [6, 10, 16], which showed that it produced a 70–80% attenuation of the ACh-induced increases in coronary blood flow without effects on systemic circulatory variables or on the increases in coronary blood flow caused by SNP. These findings were confirmed in the present study.

The residual ACh-induced increase in coronary blood flow following L-NAME raises the possibility that our dose for L-NAME was not adequate for a complete blockade of the NO-cGMP pathway. Although we cannot definitively rule this out, it seems unlikely since previous investigators using an i.c. dose for L-NAME four-times our dose (and sufficient to cause increases in arterial pressure) reported similar results [8]. Indeed, the failure of NOS inhibitors, including L-NAME, to completely abolish ACh-induced coronary vasodilation is a consistent finding in the literature [5–10, 16, 23], and it is likely explained by the ability of ACh to cause release of a hyperpolarizing factor, in addition to NO, from the vascular endothelium [28].

4.2 Role of nitric oxide in cardiac effects of calcium chloride
Coronary blood flow is normally matched to the prevailing myocardial oxygen demand by local adjustments in coronary vasomotor tone mediated by metabolic control mechanisms [13]. This local control of coronary blood flow tends to maintain coronary venous PO2, and therefore oxygen extraction, constant. The increases in oxygen extraction during administration of CaCl2 in the present study reflected increases in coronary blood flow that were less than proportional to the induced augmentations in myocardial oxygen consumption (secondary to calcium’s positive inotropic effect). These findings are consistent with a direct vasoconstrictor effect for calcium, which was capable of partially antagonizing, but not completely overriding, metabolic vasodilation. These findings in the basal preparation confirm findings from our previous study [12].

Our previous study in the same canine model using dobutamine provides a reference for assessing the effects of CaCl2 [29]. Dobutamine caused increases in myocardial contractility that were associated with proportional increases in myocardial oxygen consumption and coronary blood flow with the result that oxygen extraction remained constant. These findings implied that metabolic control of coronary blood flow remained intact during dobutamine infusion, and thus that this agent had no direct vasomotor effect on coronary resistance vessels [13]. They also ruled out the possibility that the increase in oxygen extraction (and the apparent direct vasoconstrictor effect) during CaCl2 was an obligatory response to inotropic stimulation in our canine cardiac preparation.

It has been established that endothelial NO synthase (so-called constitutive NO synthase) is Ca++-dependent, and that production of EDNO is modulated by intracellular Ca++ concentration [1]. Yet, we showed that L-NAME did not blunt the increases in coronary blood flow or alter their relationship to myocardial oxygen consumption, i.e., the increases in oxygen extraction, during CaCl2, thus suggesting that EDNO did not contribute to the coronary vasodilating response. We can propose two potential explanations for this finding: (1) The increases in Ca++ concentration in the plasma were not reflected within the vascular endothelium. (2) The ability of Ca++ to modulate EDNO production in the coronary resistance vessels, like that in isolated aortic segments [30], does not apply to concentrations above the physiological range. Of course, we cannot rule out the possibility that EDNO was indeed involved in the coronary vasodilating response during CaCl2, but that an alternate vasodilating factor, e.g., an augmented adenosine release, compensated for its inhibition.

The failure of NOS inhibition to reduce baseline coronary blood flow has been demonstrated previously [5, 6, 8–10, 16, 31], and it has been explained by the emergence of an alternate metabolic mechanism, which preserves myocardial oxygen supply/demand balance when the tonic influence of EDNO is blocked. In support of this view, Kostic and Schrader [32]reported augmented release of adenosine from isolated guinea pigs hearts following administration of L-NAME.

Investigators using a variety of isolated cardiac preparations and experimental approaches have demonstrated that the NO has the capability to reduce myocardial contractility via an increase in cGMP, and, in turn, a stimulation of cGMP-dependent protein kinase [15], although this effect appears limited to concentrations of NO that greatly exceed physiological values [33]. Interestingly, Kojda et al. [34]in a recent study in isolated ventricular myocytes, showed that NO, in low concentrations, may paradoxically increase myocardial contractility via its ability to inhibit cAMP phosphodiesterase, leading to increased levels of cAMP, and stimulation of cAMP-dependent protein kinase. Previous in vivo studies have consistently shown no influence of EDNO on contractility in non-stimulated myocardium [16, 23, 31], while some [15], but not others [10], have demonstrated a negative inotropic effect in myocardium stimulated with a β-receptor agonist. The present study extends these latter observations to myocardium stimulated with CaCl2. The lack of influence of EDNO on myocardial contractility in vivo has been attributed to the ability of hemoglobin and myoglobin to bind NO, thus restricting its access to the cardiomyocytes [16].

Our findings pertain strictly to NO synthesized in the coronary endothelium via the constitutive NOS pathway. Another form of NOS (so-called inducible NOS) has been identified in a variety of cell types, e.g., neutrophils, vascular smooth muscle, and cardiac myocytes, following induction by immunological stimuli, such as cytokines and endotoxin [1]. NO production is more extended and at much higher concentrations via the inducible NOS pathway [1]. Evidence for expression and activity of inducible NOS has been obtained in myocardial samples from patients with dilated cardiopathy, ischemic heart disease, valvular heart disease, and cardiac allograft rejection [35–37]. Studies conducted in various isolated tissues, including cardiac muscle slices and hepatocytes, have demonstrated that the elevated NO concentrations associated with the inducible NOS pathway may have a direct depressive effect on tissue oxygen use [14, 38, 39]. This effect has been attributed to the binding of NO to the heme moiety of cytochrome enzymes in the mitochondrial electron-transport chain.

Recent studies have sought to determine whether this inhibitory metabolic effect extends to physiological-relevant concentrations of NO in the normal heart in vivo [5–7, 10, 16, 23, 31, 40, 41]. To date, the preponderance of evidence has indicated otherwise. Neither NOS inhibitors, NO donors, nor ACh affected basal myocardial oxygen consumption [5–7, 16, 23, 31, 40, 41]. Furthermore, NOS inhibitors did not alter the increase in myocardial oxygen consumption caused by atrial pacing, aortic constriction, isoproterenol, or dobutamine [7, 10]. The present findings extend these latter observations to the increase in myocardial oxygen consumption caused by CaCl2. They indicate that CaCl2 administration caused proportional changes in segmental shortening and myocardial oxygen consumption whether or not tissue NO concentrations were decreased with L-NAME. This implies that basally-released EDNO had no direct effect on the rate of myocardial oxidative metabolism.

Bernstein et al. [41]recently reported that an intravenous infusion of an NOS inhibitor increased myocardial oxygen consumption at various levels of cardiac work in exercising, chronically instrumented dogs. The apparent discrepancy between these findings and those from the present study may be attributable to methodological differences, including the absence or presence of general anesthesia, the route and dose of the NOS inhibitor, and the levels of myocardial oxygen consumption studied. Another factor may be the difficulty in distinguishing changes in myocardial oxygen consumption due to direct effects on the mitochondria from those due to variations in the hemodynamic determinants of cardiac workload, e.g., the increases in aortic pressure, during exercise in the study of Bernstein et al. [41]. Our use of i.c. infusions of CaCl2 to alter the level of cardiac work precluded this potential pitfall.

We conclude that: (1) CaCl2 had a vasoconstricting effect and a positive inotropic effect in the normal heart in vivo. This vasoconstricting effect impaired coupling of coronary blood flow to the augmented myocardial oxygen demand by metabolic vascular control mechanisms. (2) EDNO did not modulate the increases in coronary blood flow, myocardial oxygen consumption, or myocardial contractility during administration of CaCl2.

From a clinical point of view, the present findings suggest that CaCl2 may be risky as an inotropic agent following termination of cardiopulmonary bypass, when the generation of metabolic vasodilators, e.g., adenosine, may be decreased due to a diminished cardiac responsiveness. This could unmask the coronary vasoconstricting action of CaCl2, which could precipitate myocardial ischemia. Our findings also imply that the myocardial functional hyperemia during CaCl2 should be normal in the patient in which endothelial function is impaired because of chronic disease, e.g., atherosclerosis [42], or acute insult, e.g., ischemia and reperfusion [43].

Time for primary review 28 days.


   Acknowledgements
 
The authors thank Derrick L. Harris, B.S., for his expert technical assistance.


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

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