Robert P. Steffen

Subclasses of cyclic AMP-specific phosphodiesterase in left ventricular muscle and their involvement in regulating myocardial contractility.

Ventricular muscle contains a low Km, cyclic AMP-specific form of phosphodiesterase (PDE III), which is believed to represent the site of action for several of new cardiotonic agents including Imazodan (CI-914), amrinone, cilostamide, and enoximone. However, species differences in the inotropic response to these agents have raised questions about the relationship between PDE III inhibition and cardiotonic activity. The present study demonstrates that these differences can be accounted for by the presence of two subclasses of PDE in in ventricular muscle and variations in the intracellular localization of these two enyzmes. For these experiments, PDE III was initially isolated from canine, guinea pig, and rat left ventricular muscle. The results demonstrate that canine left ventricular muscle contains two functional subclasses of PDE III: an imazodan-sensitive form, which is membrane bound, and an imazodan-insensitive form, which is soluble. Although only weakly inhibited by imazodan, this latter enzyme is potently inhibited by the selective PDE III inhibitors, Ro 20–1724 and rolipram. Guinea pig ventricular muscle also contains the imazodan-sensitive subclass of PDE III. Unlike canine left ventricle, however, this enzyme is soluble in the guinea pig. No membrane-bound subclass of PDE III was observed in the guinea pig. Rat left ventricle possesses only the soluble form of PDE III, which apparently represents a mixture of the imazodan-sensitive and imazodan-insensitive subclasses of PDE III. Measurements of in vivo contractility in these three species showed that imazodan exerts a potent positive inotropic effect only in the dog, in which the imazodan-sensitive subclass of PDE III is membrane bound. In addition, a strong correlation was observed between in vitro inhibition of the membrane-bound, imazodan-sensitive PDE III from canine ventricular muscle and the in vivo positive inotropic response to imazodan, amrinone, and related cardiotonic agents in the dog. Inhibitors of the imazodan-insensitive subclass of PDE III did not exert any pronounced inotropic effect in the dog. These results demonstrate that functional subclasses of PDE III exist in ventricular muscle and suggest that species differences in the positive inotropic response to imazodan and related cardiotonics can be attributed to the proper intracellular localization of the imazodan-sensitive subclass of PDE III.

PD 116,948, a highly selective A1 adenosine receptor antagonist.

(R)-N-(1-Methyl-2-phenylethyl) adenosine (R-PIA), an adenosine receptor agonist has both negative chronotropic activity and coronary vasodilator activity. These actions of R-PIA are proposed to be mediated by subtypes (A1 and A2) of adenosine receptors. PD 116,948 is a xanthine derivative which is a highly selective A1 adenosine receptor ligand. In this study PD 116,948 selectively antagonized the negative chronotropic activity of R-PIA in the isolated rat heart. These results are consistent with, and add further support to the hypothesis that adenosine receptor agonists mediate their negative chronotropic activity via A1 receptors and their vasodilator activity via A2 receptors.

Correlation of adenosine receptor affinities and cardiovascular activity.

Binding affinities of 28 adenosine analogs at A1 adenosine receptors (rat whole brain membranes, [3H]N6-cyclohexyladenosine, CHA), and at A2 adenosine receptors (rat striatal membranes, [3H]NECA) were compared to their EC25 values for decreasing heart rate and increasing coronary flow in the isolated rat heart. Heart rate (an A1 response) correlated with A1 binding affinity (r2 = 0.71, p less than 0.0001) but not with A2 binding affinity (r2 = 0.007, n.s.); conversely, coronary flow (an A2 response) correlated with A2 binding affinity (r2 = 0.83, p less than 0.0001) but not with A1 binding affinity (r2 = 0.05, n.s.). These results confirm that the brain A1 and A2 receptors, studied by binding methods, bear close similarities to their respective counterparts in the heart, studied by means of functional responses.

The effect of the cardiotonic imazodan (CI-914) on myocardial and peripheral hemodynamics in the anesthetized dog

Summary: The purpose of this study was to evaluate the effect of the new cardiotonic, imazodan (CI-914), on myocardial hemodynamics and oxygen consumption, and peripheral blood flow distribution. Organ blood flow was measured by the radiolabeled-microsphere-reference-withdrawal technique and myocardial oxygen consumption calculated from arterial and coronary sinus O2 content and blood flow. At inotropic levels, CI-914 decreases mean arterial pressure with a minimal increase in heart rate. CI-914 decreases coronary vascular resistance and increases supply to demand ratio, indicating an active coronary vasodilation. CI-914 does not alter peripheral blood flow distribution, thus suggesting equivalent organ vasodilation. These data suggest that CI-914 may be useful in the treatment of congestive heart failure because of its positive inotropic, coronary vasodilator, and peripheral vasodilator properties.

Chapter 9 Agents for the Treatment of Congestive Heart Failure

Publisher Summary Insufficient cardiac output initiates compensatory processes involving increased activity of the adrenergic, renin-angiotensin-aldosterone,, prostaglandin, bradykinin, and vasopressin systems. These compensatory mechanisms further decrease peripheral circulation, leading to congestive heart failure (CHF). Even though progress has been made in understanding this disease and developing new therapies, overall mortality has not declined and many unanswered questions remain. The etiology, pathophysiology, and drug treatment of CHF have been the subject of a number of recent reviews. Clearly, the number and diversity of new agents and mechanisms now available or under development is likely to impact favorably on future therapy for CHF. The increase in survival demonstrated in the V-HeFT study is a milestone and other agents, including angiotensin converting enzyme (ACE) inhibitors, are undergoing similar trials. The potential role of the phosphodiesterase (PDE) inhibitor cardiotonics is likely be defined in the near future following the publication of multicenter trials, involving milrinone and enoximone. As a result of this progress, CHF may be better controlled and overall survival may improve. Nonetheless there remains a challenge to discover drug therapies that not only ameliorate symptoms but have a substantive effect on the disease process and progression.

Cardiovascular pharmacology of the vasodilator cardiotonic agent, 349U85

Summary: 349U85 is a chemically novel, nonglycoside, noncatecholamine cardiotonic-vasodilator agent with a unique cardiovascular profile in vitro and in vivo. 349U85 and milrinone, 10-6to3 x 10-5 M each, produce concentration-dependent increases in tension development of 33–60% and 37–60%, respectively, with corresponding 5–18% and 17–55% increases in contractile rate, respectively, in guinea pig spontaneously beating isolated paired atria. In anesthetized dogs, 349U85 at 0.03–1.0 mg/kg i.v. produces dose-dependent increases in left ventricular contractility (dPIdt) of 12–159%, decreases in total peripheral resistance of 11–38%, and increases in heart rate of 3–26%. Milrinone, Cl-914, and enoximone produce comparable increases in dP/dt and decreases in peripheral resistance yet increase the heart rate a maximum of 71, 49, and 41%, respectively. Intra-arterial injection of 349U85 into the vascularly isolated hindlimb of anesthetized dogs produces dose-dependent direct vasodilation. The inotropic effect of 349U85, following a single intravenous dose, is sustained in excess of 4 h while comparable initial inotropic effects of milrinone and enoximone are sustained less than 1 and 2.5 h, respectively. 349U85 effectively reverses acute cardiac depression in anesthetized dogs with a duration exceeding that of milrinone. In conscious dogs, 349U85, at 0.1–1.0 mg/kg p o., produces a dose-dependent positive inotropic effect (15–73%) with no significant effect on heart rate. Following a single oral dose of 349U85, the inotropic effect is sustained in excess of 6 h. Results of these studies indicate that 349U85 is a potent, long-lasting positive inotropic and vasodilator agent with minimal heart rate effect in vitro and in vivo and is different from a number of reference inodilators.