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Cardiovascular Research 1998 37(3):676-683; doi:10.1016/S0008-6363(97)00251-4
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Copyright © 1998, European Society of Cardiology

Guanylyl cyclase inhibition reduces contractility and decreases cGMP and cAMP in isolated rat hearts

Richard E Klabundea,*, James Tseb and Harvey R Weissb

aDeborah Research Institute, 20 Pine Mill Road, Brown Mills, NJ 08015, USA
bUMDNJ-Robert Wood Johnson Medical School, Piscataway, NJ 08855, USA

* Corresponding author. Tel. (+1-609) 893 1016; Fax (+1-609) 893 2441.

Received 28 April 1997; accepted 12 September 1997


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objective: Several studies have shown that increasing cGMP in the heart reduces contractility, however, decreasing basal cGMP levels have also been shown in some studies to reduce contractility. This study was designed to evaluate the hypothesis that decreasing basal levels of cGMP decreases ventricular contractility, and that this depressed function is associated with a decrease in cAMP. Methods: Using paced, constant flow, buffer-perfused rat hearts, we determined the effects of intracoronary infusions of the guanylyl cyclase inhibitor, LY83583 (10–5 M), on ventricular function, oxygen consumption, and ventricular content of cGMP and cAMP. These experiments were conducted in the absence and presence of isoproterenol (ISO) to increase baseline left ventricular developed pressure (LVDP) and cAMP. Results: LY83583, infused for 25 min, decreased LVDP by 44±3 (SE), 77±20 and 120±38 mmHg, in control, 10–9 M, and 10–8 M ISO-stimulated hearts, respectively. Regardless of the level of ISO stimulation, LY83583 reduced LVDP to the same sub-basal level. Oxygen consumption also decreased, but proportionately less than LVDP. ISO increased cAMP without changing cGMP. LY83583 decreased cGMP by about 25% at all levels of ISO, and decreased cAMP by 22% in the 10–8 M ISO-stimulated group. Conclusion: Guanylyl cyclase inhibition by LY83583 decreased cGMP, cAMP and ventricular contractility. However, LY83583 depression of contractility was not always associated with a reduction in cAMP, suggesting that LY83583 can depress contractility by both cAMP-dependent and independent mechanisms.

KEYWORDS Myocardial; Cyclic nucleotides; Nitric oxide; Oxygen consumption; LY-83583; Rat


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
The heart, like many other tissues, converts L-arginine to nitric oxide (NO) by the action of constitutive nitric oxide synthase (cNOS) [1]. This NO then binds to, and activates, guanylyl cyclase which leads to the formation of cGMP. Increased levels of cGMP, which occur when NO production is enhanced or when NO donor compounds are administered to the heart or isolated myocytes, depress myocardial contractility [1–5]. Furthermore, there are in vitro studies (isolated myocytes) [6]and in vivo studies [7, 8]demonstrating that NOS inhibition and reduced cGMP levels enhance inotropic responses to beta-adrenoceptor stimulation. These studies suggest that basal levels of NO and cGMP inhibit inotropic responses to beta-adrenoceptor agonists.

The negative inotropic actions of NO and cGMP suggest that reducing basal levels of NO and cGMP should increase contractility; however, this does not always occur. NOS inhibition in isolated, constant flow, buffer-perfused rat hearts causes a reduction in developed pressure that is associated with a fall in both cGMP and cAMP, but only when the inotropic state is first elevated by isoproterenol (ISO) [9]. This study, along with others which have used isolated myocytes [10], suggests that a fall in cGMP can reduce cAMP via the cGMP-inhibited isoform of cAMP-dependent phosphodiesterase, and this reduction in cAMP causes a fall in contractility. There is increasing evidence that NO can be both cardiostimulatory and depressant, depending upon its concentration and the concentration of cGMP. Studies in isolated rat and guinea pig myocytes [10, 11]and in cat papillary muscles [12]have shown biphasic responses to NO and cGMP, with low concentrations increasing contractility and high concentrations decreasing contractility.

The purpose of this study was to evaluate the role of constitutively formed, endogenous NO on ventricular contractility, oxygen consumption, and cyclic nucleotide concentrations in isolated, constant flow, buffer-perfused rat hearts. Instead of using a NOS inhibitor to block NO production and reduce cGMP, we used a guanylyl cyclase inhibitor (LY83583) to reduce endogenous cGMP. The study was designed to test the hypothesis that decreasing basal levels of cGMP decreases isoproterenol-stimulated ventricular contractility, and that this depressed function is associated with a decrease in cAMP.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
2.1 Isolated heart preparation
The care and use of animals conformed to the Guide for the Care and Use of Laboratory Animals (NIH Publication No. 85-23, revised 1985) and were approved by the Animal Care Committee of the Deborah Research Institute. Sprague-Dawley rats (male, 350–550 g) were anesthetized with pentobarbital sodium (65 mg/kg, ip). Heparin (1000 U) was injected into the tail vein. When the animal obtained a deep surgical plane of anesthesia, the heart was rapidly removed from the thorax and immediately placed into ice-cold normal saline. The aortic stump was cannulated, and the coronary vasculature flushed with 1–2 ml of cold Krebs–Henseleit buffer.

The heart was mounted on the Langendorff apparatus, and retrograde aortic perfusion was initiated with a 37°C, pH 7.4, Krebs–Henseleit solution containing NaCl, 118.0 (mM); CaCl2, 2.0; KCl, 4.7; NaHCO3, 24.9; Mg2SO4, 1.17; KH2PO4, 1.2; glucose, 11.1; insulin, 5 IU/liter. The buffer was equilibrated with 95% O2 and 5% CO2. The perfusion flow rate (coronary flow) was set to achieve a coronary perfusion pressure of 85 mmHg. A drain tube was inserted through the mitral valve orifice into the left ventricle, and pushed through the apex. The right atrium was closed with a hemaclip and the pulmonary artery cannulated to obtain samples of coronary effluent which were assayed for pO2. A balloon-tipped catheter (#4 balloon, Radnotti Glass Technology, Monrovia, CA), connected to a microliter syringe and pressure transducer, was inserted into the left ventricle via an opening in the left atrium. Balloon volume was inflated with water to 70 µl, which has been shown previously to result in an optimal ventricular preload in this isolated heart model [13]. An in-line flow probe (Model 2N, Transonic Systems, Ithaca, NY) was interposed in the perfusion apparatus to measure coronary flow. Coronary perfusion pressure was measured just above the aortic cannula using a TXX-R disposable transducer. An injection port just proximal to the aortic cannula was used for infusion of isoproterenol (ISO) and LY83583. The infusion rate of ISO and LY83583 was set at 2% of coronary flow; therefore, infusing a 0.05 or 0.5 µM stock concentration of isoproterenol resulted in a coronary concentration of 10–9 or 10–8 M, respectively. The stock solution LY83583 (50x10–5 M), infused at 2% of coronary flow, resulted in a coronary perfusate concentration of 10–5 M. Electrodes were attached to the ventricles and the heart paced at 312 beats/min (1 volt, 30 ms pulse duration). At the end of the experiment, the heart was frozen by rapidly removing the left ventricular balloon, cutting the heart at the atrioventricular ring, and dropping the ventricles into liquid nitrogen. This method allows freezing to proceed simultaneously from both the inside and outside of the ventricles, and enables subsequent separation of the right from the left ventricle so that only left ventricular tissue is assayed. The heart samples were stored at –80°C until assayed for cyclic nucleotides.

2.2 Protocol
Following a 30 min stabilization period, a baseline recording was made. ISO infusion (10–9 or 10–8 M), or vehicle was started at 0 min. This infusion continued throughout the experiment which lasted 30 min. During the first 5 min of ISO infusion, coronary flow was increased to maintain the coronary perfusion pressure at 85 mmHg. Once set, the coronary flow was not changed again during the protocol. Therefore, subsequent infusion of LY83583 or vehicle was conducted under constant flow conditions. Infusion of LY83583 or vehicle was begun 5 min after ISO infusion was initiated and continued until the hearts were frozen at 30 min (25 min infusion of LY83583). This study design resulted in 6 treatment groups, with 6 hearts/group.

2.3 Functional measurements and biochemical assays
All functional data (left ventricular pressure, coronary flow and coronary perfusion pressure) were recorded on a MacLab workstation. Left ventricular developed pressure (LVDP) and heart rate were derived from the left ventricular pressure recording. LVDP was the difference between maximal and minimal left ventricular pressure. Coronary vascular resistance was calculated as coronary perfusion pressure divided by coronary flow. The data at specific times were obtained by averaging the values over a 10 s period. Myocardial oxygen consumption (MVO2) was calculated as the product of the arterial-venous pO2 difference and the coronary flow, multiplied by the Bunsen solubility coefficient of oxygen (24 µl O2·ml–1·atm–1 at 37°C) [14].

Cyclic nucleotides were assayed as follows. The frozen ventricles were warmed to 0°C and the left ventricle was cut into a circumflex and left anterior descending artery perfusion field. These left ventricular regions were randomized for the assays and homogenized in ethanol using a Brinkmann Polytron placed in an ice bath. The homogenate was centrifuged at 30 000 g for 15 min in a Sorvall RC-5B centrifuge. The supernatant was recovered. The pellet was resuspended in 1 ml of 2:1 ethanol:water and centrifuged as before. The combined supernatants were evaporated to dryness in a 60°C bath under a stream of nitrogen gas. The final residue was dissolved in 1.5 ml of assay buffer (0.05 M sodium acetate, pH 5.8, containing sodium azide). cGMP levels were determined using radioimmunoassay (Amersham). This assay measures the competitive binding of Formula -cGMP to a cGMP specific antibody. After construction of a standard curve, cGMP levels were determined directly from the counts in pmol/g of tissue wet weight. The other half of the ventricle was warmed to 0°C and homogenized in 3 ml of ethanol in a Brinkmann Polytron in an ice bath. The homogenate was centrifuged at 30 000 g for 20 min in a Sorvall RC-5B centrifuge. The supernate was decanted and evaporated to dryness in a 55°C bath under a stream of nitrogen gas. The pellet was then resuspended in one ml of 2:1 ethanol:water and centrifuged again. The supernatant was evaporated to dryness. The second supernate was added to the first and the pellet discarded. The residue was dissolved in 0.5 ml of TRIS/EDTA buffer and the supernate was assayed. cAMP levels were determined using a competitive binding assay measuring the displacement of 3H-cAMP from a binding protein. Samples (200 µl) were placed in scintillation vials with 10 ml of scintillation fluid and counted on a scintillation counter. After construction of a standard curve, cAMP levels were determined directly from the counts by a linear regression to obtain values in pmol/g of tissue wet weight.

2.4 Statistical analysis
Data from the MacLab were entered into an Excel spreadsheet for generating descriptive statistics. Differences in control values among treatment groups were evaluated using one-way analysis of variance (InStat®, GraphPad Software, San Diego, CA). A significant treatment effect of LY83583 on LVDP and MVO2 compared to vehicle was determined by expressing the data at a given time point as a change from the pre-infusion (4 min) value, then conducting a Student's t-test between the vehicle and LY83583 groups using these data. The effects of ISO on cAMP and cGMP were evaluated by one-way analysis of variance (ANOVA) followed by a Tukey-Kramer multiple comparison test. The effects of LY83583 on cAMP and cAMP were evaluated using the Student's t-test. Statistical significance was determined at the 0.05 level. All data are presented as mean±SE.

2.5 Drugs
LY83583 (6-anilino-5,8-quinolinequinone) was purchased from Calbiochem-Novabiochem International, La Jolla, CA. Isoproterenol (Elkins-Sinn, Cherry Hill, NJ) was diluted into 0.9% sodium chloride solution and 5 mg of LY83583 were dissolved into ethanol (0.5 ml) and distilled water (39.5 ml). The LY83583 vehicle was 1.25% ethanol, which resulted in an ethanol concentration of 0.025% in the buffer perfusing the heart.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
3.1 Functional data
The baseline cardiac function data among the 6 treatment groups were not significantly different (Table 1). LVDP in the absence of ISO averaged from 79 to 103 mmHg among the groups. Mean coronary flows among the groups ranged from 14.1 to 19.0 ml/min, and coronary perfusion pressures ranged from 83.7 to 86.4 mmHg. Average baseline MVO2 values ranged from 172 to 207 µl O2•ml–1. Arterial and venous pO2 values were approximately 600 and 200 mmHg, respectively, in the unstimulated hearts, and not significantly different among groups. Heart rates and coronary vascular resistances were also similar among the groups.


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  		Table 1 Baseline cardiac and coronary function

 
ISO infusions produced significant increases in LVDP. Four min after beginning 10–9 M infusion of ISO, LVDP increased approximately 20 mmHg (Fig. 1, Panel B). During infusion of 10–9 M ISO, coronary flow was manually increased within the first few min by approximately 1 ml/min to maintain coronary perfusion pressure at the control value. Once coronary flow was adjusted, it was kept constant throughout the remainder of the protocol. This concentration of ISO also increased MVO2 by about 26 µl O2•ml–1•atm–1 (Fig. 2, Panel B). The LVDP/MVO2 ratio, however, was unchanged by 10–9 M ISO. ISO at a concentration of 10–8 M increased LVDP by about 80 mmHg, a 100% increase (Fig. 1, Panel C). Coronary flow was increased by approximately 7 ml/min to maintain the baseline coronary perfusion pressure while 10–8 M ISO was infused. As with the lower concentration of ISO, coronary flow, after this initial adjustment, was held constant for the remainder of the protocol. Venous pO2 fell from a baseline of about 200 mmHg to about 150 mmHg during infusion of 10–8 M ISO. This higher concentration of ISO also increased MVO2 by about 118 µl O2•ml–1, a 66% increase (Fig. 2, Panel C). Heart rate remained unchanged during ISO infusion except in a few hearts which escaped the pacing rate of 312 beats/min.


Figure 1
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  		Fig. 1 Effects of LY83583 on left ventricular developed pressure (LVDP) in the absence and presence of ISO. Panel A (top), no ISO; Panel B (middle), 10–9 M ISO; Panel C (bottom), 10–8 M ISO. Vertical bars represent S.E. * indicates significant difference (p<0.05) from vehicle when data are expressed as change from pre-LY83583 infusion (4 min).

 

Figure 2
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  		Fig. 2 Effects of LY83583 on myocardial oxygen consumption (MVO2) in the absence and presence of ISO. Panel A (top), no ISO; Panel B (middle), 10–9 M ISO; Panel C (bottom), 10–8 M ISO. Vertical bars represent S.E. * indicates significant difference (p<0.05) from vehicle when data are expressed as change from pre-LY83583 infusion (4 min).

 
The effects of 10–5 M LY83583 on LVDP in the absence and presence of ISO infusion are shown in Fig. 1. After 25 min of LY83583 infusion into hearts treated with vehicle, 10–9 or 10–8 M ISO, LVDP decreased by 44±3, 77±20, and 120±38 mmHg, respectively (Fig. 1). The magnitude of the decrease in LVP during LY83583 infusion was related to the LVDP prior to infusion. Hearts which were stimulated by 10–8 M ISO showed the greatest decline in absolute LVDP during LY83583 infusion. When the data were expressed as percent change, the fall in LVDP was similar (30–40%) in the control and ISO-treated hearts (Fig. 3, Panel A). Twenty-five min after starting LY83583 infusion, the LVDP was 64±6, 68±9, and 65±15 mmHg in control, 10–9 and 10–8 M ISO groups. This reduced level of LVDP was approximately 25% lower than the initial baseline LVDP for all groups. Therefore, irrespective of the level of prior inotropic stimulation, LY83583 decreased LVDP to the same sub-basal level. Control hearts, receiving vehicle or ISO without LY83583, showed no significant change in LVDP over the time course represented by LY83583 infusion (Fig. 1).


Figure 3
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  		Fig. 3 Effects of LY83583 on percent change in LVDP and MVO2 in the absence and presence of ISO. Vertical bars represent S.E. * indicates p<0.05 compared to respective vehicle control using Students t-test.

 
LY83583 also decreased MVO2 by about 25–30%, but only in hearts first stimulated by ISO to elevate basal contractility and MVO2 (Figs. 2 and 3Go). Vehicle controls showed no reduction in MVO2 over time.

LY83583 had no significant effect on coronary vascular resistance; however, vehicle-treated hearts showed a significant increase in coronary vascular resistance at 15 min (23% increase; p<0.05). Therefore, relative to its vehicle control, LY83583 decreased coronary vascular resistance. This effect of LY83583 was only observed in hearts not stimulated by ISO. Stimulating this heart preparation with ISO leads to maximal coronary vasodilation so that the vasodilating effect of drugs cannot be observed. The coronary flow was held constant throughout the infusion of LY83583 in all groups.

3.2 Cyclic nucleotide assays
Cardiac cGMP concentrations were approximately 4 pmol/g tissue and were not altered by ISO infusion (Fig. 4). Infusion of LY83583 decreased cGMP concentrations by about 25%. The decreases in cGMP content caused by LY83583 were similar in hearts treated with vehicle, 10–9 or 10–8 M ISO. Control concentrations of cAMP (195±19 pmol/g) were increased to 283±18 and 377±12 pmol/g in the presence of 10–9 M and 10–8 M ISO, respectively (Fig. 4). LY83583 caused a significant reduction in cAMP (from 377±12 to 311±31 pmol/g), but only in hearts stimulated by 10–8 M ISO.


Figure 4
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  		Fig. 4 Effects of LY83583 on ventricular concentrations of cGMP and cAMP in the absence and presence of ISO. Vertical bars represent S.E. * indicates p<0.05 compared to respective vehicle-treated group (Students t-test). + Indicates p<0.05 compared to respective Saline group (ANOVA followed by Tukey–Kramer test). ++ Indicates p<0.05 compared to 1 nM ISO (ANOVA followed by Tukey–Kramer test).

 

   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
The purpose of this study was to determine the effects of inhibition of guanylyl cyclase on ventricular function, oxygen consumption, and cyclic nucleotide concentrations in isolated rat hearts perfused under constant flow conditions. Based upon an earlier study which used a NOS inhibitor in a similar model [9], we hypothesized that decreasing basal levels of cGMP would decrease ISO-stimulated ventricular contractility, and that this depressed function would be associated with a decrease in cAMP. We found that the guanylyl cyclase inhibitor, LY83583, decreased cGMP. Furthermore, LY83583 decreased cAMP, but only in hearts in which cAMP was first elevated by ISO. LY83583 decreased basal and ISO-stimulated ventricular pressure development regardless of the change in cAMP.

Several studies have shown that increased levels of cGMP are associated with depressed myocardial contractility when NO production is enhanced or when NO donor compounds are administered to the heart or isolated myocytes [1–5]. Conversely, decreasing cGMP with intracoronary infusion of methylene blue (guanylyl cyclase inhibitor) increases basal contractility in anesthetized dogs [15]. Studies conducted on isolated myocytes and in vivo have demonstrated that NOS inhibition enhances beta-adrenoceptor-mediated positive inotropy [16, 17], suggesting that basal levels of NO and cGMP inhibit inotropic responses to beta-adrenoceptor agonists. The attenuation of ISO-induced positive inotropy appears to involve activation of cGMP-dependent protein kinase [11]and inhibition of inward Ca2+ currents [11, 18]. NO may also decrease myofilament responsiveness to Ca2+ [19].

There is also evidence that decreasing NO and cGMP decreases contractility. In isolated, ISO-stimulated rat hearts, NOS inhibition decreases cGMP and decreases contractility [9]. In dogs, intracoronary infusion of NOS inhibitors decreases regional segment shortening [20], and decreases inotropic responses to ISO [21].

The reasons for the disparate results regarding the inotropic effects of endogenous levels of NO and cGMP are not readily apparent; however, there is increasing evidence from in vitro experiments that low levels of NO can be cardiostimulatory, while high levels are cardiodepressant [10–12]. It has been postulated that decreased cGMP reduces cAMP via the cGMP-inhibited isoform of cAMP-dependent phosphodiesterase [9, 10]. Therefore, low levels of NO may be cardiostimulatory through the cAMP signal transduction pathway, while high levels of NO may be cardiodepressant through the cGMP-protein kinase pathway. These studies [10–12], which evaluated a wide range of NO concentrations, may help to explain why exogenously applied NO generally causes cardiodepression, while NOS inhibitors and guanylyl cyclase inhibitors can sometimes reduce contractility. Application of a NO donor, whether it be to isolated myocytes or infused into coronary arteries, may result in cellular concentrations of NO that far exceed those normally generated by cNOS (typically in the nM range) [22].

Whether a reduction in basal cGMP by a NOS or guanylyl cyclase inhibitor decreases or increases contractility may depend upon the level of constitutively formed NO within the myocyte. For example, if the experimental model has high levels of constitutively formed NO, then these inhibitors might cause cardiac stimulation because the basal level of NO may be high enough to be cardiodepressant. This would be consistent with results from studies where myocytes, which were first exposed to endotoxin or cytokines to increase NO production, showed positive inotropic responses to NOS or guanylyl cyclase inhibition [2, 4, 19, 23]. If endogenous NO levels are low, then NOS or guanylyl cyclase inhibition might lead to reduced contractility by a mechanism that does not involved cGMP-dependent protein kinase, such as cGMP modulation of cAMP-dependent phosphodiesterase [9, 10].

Our results are consistent, in part, with the idea that a decrease in basal cGMP can decrease contractility through a reduction in cAMP. This mechanism may be involved in the hearts first stimulated with ISO to increase basal levels of cAMP. However, this interaction between cGMP and cAMP cannot explain our finding that in the absence of ISO stimulation, LY83583 decreased contractility without reducing cAMP. These present results also differ from the earlier study [9]using the same rat heart model where NOS inhibition in the absence of ISO decreased cGMP without changing contractility or cAMP. We do not have an explanation for this finding, although there are several possibilities. First, cyclic nucleotide concentrations measured in whole tissue extracts can miss significant changes which might be occurring in different cellular compartments, therefore the absence of a measurable change in cAMP does not necessarily mean that cAMP did not change. Assuming, however, that the cAMP measurements do provide an accurate assessment of changes in intracellular cAMP, then a second possibility should be considered, that is, LY83583 has some unidentified mechanism of action which reduces contractility under basal conditions.

It is possible that LY83583 depressed contractility, regardless of the prevailing level of ISO stimulation, by a mechanism that is totally independent of changes in cyclic nucleotides and their respective signal transduction pathways. This explanation would be consistent with the results showing that LY83583 decreased LVDP to the same sub-basal level in the absence and presence of ISO. However, non-specific cardiac depression by LY83583 has not been reported by others, and this compound has been shown to stimulate isolated myocytes [24]. LY83583 is known to be a guanylyl cyclase inhibitor, although its mechanism of action probably involves the formation of superoxide anions [25, 26]. Evidence for this is based upon studies showing that the inhibitory actions of LY83583 on guanylyl cyclase are blocked by free radical scavengers such as superoxide dismutase, catalase, or dimethylsulfoxide. Regardless of how LY83583 exerts its pharmacological effect, it clearly inhibits the activity of guanylyl cyclase and the formation of cGMP, and blocks the effects of exogenous NO [25, 26]. However, superoxide anion generation by LY83583 might directly inhibit contractility in addition to inhibition of guanylyl cyclase.

Our results showed that LY83583 decreased MVO2 when there was a background of inotropic stimulation by ISO. A recent study in conscious dogs demonstrated that intracoronary infusion of NOS inhibitors decreases MVO2 at a given pressure-rate product [20]. The reduction in MVO2 may simply be the consequence of reduced oxygen demand resulting from less ventricular pressure generation. It is well known that high levels of NO lead to decreases in oxygen consumption [27–29]by decreasing mitochondrial respiration [28, 29]and creatine kinase activity [30]. These studies suggest that a reduction in constitutive levels of NO or cGMP should increase oxygen consumption. This might be the case in some studies [31, 32], however, the reduction in oxygen demand by the decrease in contractility might have overridden this metabolic effect of NO and resulted in the net reduction in MVO2 in our study, and in the study by Sherman et al. [20].

In summary, our results show that the guanylyl cyclase inhibitor LY83583 decreases contractility in isolated rat hearts perfused under constant flow conditions. The decrease in contractility is associated with a reduction in cGMP, and in the case of ISO-stimulated hearts, with a reduction also in cAMP. This latter finding is consistent with the hypothesis that cGMP-inhibited, cAMP dependent phosphodiesterase is a pathway for cGMP-mediated positive inotropy. However, LY83583 was also able to reduce basal contractility in the absence of any change in cAMP suggesting that this compound has negative inotropic properties of unknown mechanism.

Time for primary review 32 days.


   Acknowledgements
 
The authors wish to thank Connie Daloisio for her expert technical assistance. This study was funded by Deborah Hospital Foundation, Grant #DRG-94-09.


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

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