Effects of dihydropyridine calcium channel modulators in the heart: pharmacological and radioligand binding correlations

Bay k 8644 produced a concentration-dependent positive inotropic effect followed by a negative inotropic effect in isolated and intact cardiac preparations. Nimodipine in low concentrations produced slight positive inotropy and in higher concentrations, the usual negative inotropic action. Radioligand binding experiments revealed equilibrium dissociation constants that, taken together with the pharmacological data, suggest that dihydropyridines bind to receptor subtypes and have varying intrinsic activities.

Effects of Bay k 8644, a dihydropyridine analog, on [3H]nitrendipine binding to canine cardiac sarcolemma and the relationship to a positive inotropic effect

Equilibrium dissociation constants of Bay k 8644, a calcium agonist, and nitrendipine, a calcium antagonist, were determined in canine cardiac sarcolemma. The equilibrium dissociation constant for Bay k 8644 was compared to the concentration that produced a fifty percent increase, and the equilibrium dissociation constant for nitrendipine was compared to the concentration that produced a fifty percent decrease, in contractile force in canine heart trabecular muscle. Both saturation and inhibition binding data suggest that Bay k 8644 and nitrendipine bind to and compete for a high affinity dihydropyridine-binding site present in isolated cardiac sarcolemma preparations. The equilibrium dissociation constant (7-10 nM) and concentration that produced a fifty percent increase in contractile force in the canine trabecular muscle (30 +/- 8 nM) of Bay k 8644 were in a similar concentration range, but the equilibrium dissociation constant (0.29 +/- 0.025 nM) of nitrendipine binding was more than a thousand-fold lower than the concentration that produced a fifty percent decrease in contractile force in canine trabecular muscle (613 +/- 109 nM). These data suggest that binding of Bay k 8644 to high affinity binding sites is pharmacologically relevant, and is related to a positive inotropic effect.

Analysis of the inotropic action of ouabain in rat ventricles: two apparent ouabain inotropic responses

Two distinct inotropic effects of ouabain were observed in 68 of 82 right ventricular strips of rats with ED50s of 0.5 and 20 microM referred to as "low-dose" and "high-dose" effects, respectively. The other 14 strips showed monophasic dose-response curves, with an apparent ED50 of 0.3 microM; the inotropic response to ouabain or isoproterenol developed by these atypical strips did not exceed 50% over control. In the strips showing the biphasic inotropic response curves, the proportion of the "low-dose" effect varied from 6 to 89% of the maximum response. After treatment with 160 microM ouabain, followed by 60 min of washout, the maximum developed tension in response to ouabain was unchanged, but the "low-dose" effect was abolished or dramatically reduced. In those cases with a remaining "low-dose" response, the response was only 12% or less of the maximum and the ED50 was shifted from 0.3 to about 3 microM ouabain by the washout. The atypical strips, which showed only the "low-dose" effects during the first ouabain exposure, revealed after washout a consistent "high-dose" effect with an ED50 of about 12 microM ouabain. The data show that the two inotropic responses exist in all ventricular strips and that in some strips, after ouabain washout, a residue of the "low-dose" effect remains.

Diltiazem potentiates the negative inotropic action of nimodipine in heart

In Langendorff perfused rat hearts, nimodipine enhances coronary flow and inhibits contractility. The binding of [3H]nimodipine (160 Ci/mmol) to sarcolemma isolated from dog heart revealed a KD of 0.2 nM. d-cis-Diltiazem, but not 1-cis-diltiazem, a less active stereoisomer, stimulated [3H]nimodipine (0.17 nM) binding to sarcolemmal membranes (ED50 for diltiazem = 1.1 microM). In the presence of 10 microM d-cis-diltiazem, [3H]nimodipine binding sites were doubled, but there was no change in the apparent affinity. Perfused rat hearts were treated with 250 nM d-cis-diltiazem. The negative inotropic response to nimodipine was dramatically potentiated (I50, from 1.1 to 0.033 microM). The pharmacological and binding effects were observed only at 37 degrees C. It is possible that diltiazem in some way converts low affinity to high affinity sites.

Chronotropic, inotropic, and vasodilator actions of diltiazem, nifedipine, and verapamil. A comparative study of physiological responses and membrane receptor activity

The three major, chemically distinct calcium channel-blocking drugs, diltiazem, nifedipine, and verapamil, produce coronary vasodilation in the conscious dog. Coronary vascular resistance was reduced by 50% with an intravenous dose of 3 micrograms/kg nifedipine, 30 micrograms/kg verapamil, and 100 micrograms/kg diltiazem. In conscious dogs, nifedipine and verapamil increased heart rate, whereas diltiazem produced a smaller increase in heart rate. The rate of left ventricular pressure development in conscious dogs was unaffected by diltiazem, increased by nifedipine, and decreased by verapamil. Tachycardia was reversed to bradycardia and consistent negative inotropic effects were demonstrated by all three drugs only after combined autonomic blockade with atropine and propranolol in conscious dogs. In isolated dog coronary artery strips contracted ex vivo with 50 mM potassium chloride, the ID50 for relaxation was 0.01 microM for nifedipine, 0.02 microM for verapamil, and 0.30 microM for diltiazem. In isolated ex vivo hearts, all agents produced dose-dependent negative chronotropy with a 25% reduction in spontaneous heart rate achieved by 0.09 microM nifedipine, 0.20 microM verapamil, and 0.40 microM diltiazem. Similarly, the rate of force development in isolated myocardial strips was 50% depressed by nifedipine, 0.03 microM; verapamil, 0.10 microM; and diltiazem, 0.40 microM. On a membrane level, nifedipine, verapamil, and diltiazem interacted with a putative receptor or site associated with a calcium channel specifically labelled with [3H]nimodipine. The specific binding to cardiac sarcolemma was competitively inhibited by nifedipine, only partly inhibited by verapamil, and was stimulated by diltiazem. The effects of verapamil and diltiazem, but not the effect of nifedipine, occurred at pharmacologically active concentrations. Considerable nonspecific binding of dihydropyridines to sarcolemma may account, at least in part, for discrepancies between their dissociation constants on purified sarcolemma and their ED50 in pharmacological effects. Diltiazem and verapamil (1 microM) did not alter [3H]nimodipine nonspecific binding. These results strongly suggest that calcium channel-blocking drugs may have different sites of action.

Differential cardiovascular effects of calcium channel blocking agents: potential mechanisms

The three major calcium channel blocking agents, diltiazem, nifedipine and verapamil, inhibit calcium entry into excitable cells. Despite this apparent common action at the cell membrane, these drugs produce quantitative and frequently qualitative differences in cardiovascular variables (for example, heart rate, atrioventricular [A-V] conduction and myocardial inotropic state) when evaluated at equieffective vasodilator doses. All three drugs increase coronary blood flow in a dose-dependent fashion (nifedipine greater than diltiazem = verapamil), and produce a negative inotropic effect in vitro in isolated atria and ventricles, also in a dose-dependent manner (verapamil greater than nifedipine greater than diltiazem). However, in conscious dogs nifedipine increases, verapamil decreases and diltiazem has little effect on the inotropic state. A-V conduction is slowed by diltiazem and verapamil but not by nifedipine in anesthetized dogs and in conscious dogs as judged from the P-R interval in the electrocardiogram. Heart rate is slowed in pentobarbital-anesthetized animals but is accelerated in conscious dogs (nifedipine greater than verapamil greater than diltiazem). Nifedipine also appears to interfere significantly with the arterial baroreceptor reflex by an apparent vagolytic action that is less evident with diltiazem and verapamil. Diltiazem, and possibly verapamil and nifedipine as well, appears to retard myocardial damage that accompanies ischemia. The mechanisms and sites of action of these drugs are presumed to be at the cell membrane; however, intracellular sites may also be involved.

Effects of RMI 12330A, a new inhibitor of adenylate cyclase on myocardial function and subcellular activity

1 RMI 12330A, a lactam-imine, at concentrations of 10(-4) M and higher, inhibited basal as well as isoprenaline and NaF-stimulated adenylate cyclase activity of guinea-pig heart homogenates. However, RMI 12330A was a more potent inhibitor of histamine-stimulated adenylate cyclase (IC50 of 1.5 X 10(-5) M). 2 In the isolated work-performing heart of the guinea-pig, RMI 12330A (IC50 of 1.1 X 10(-6) M) depressed all cardiac functions: pressures developed, dP/dt, contractile force, dF/dt, work performance and stroke work. Left atrial pressure rose and the positive inotropic response to increasing heart rate (staircase) became negative. Histamine, isoprenaline and ouabain no longer caused positive inotropic effects. 3 Increasing the perfusate calcium concentration from 2.5 mM to 4.5 and 6.5 mM completely restored cardiac function after its depression by RMI 12330A. 4 RMI 12330A uncoupled mitochondrial oxidative phosphorylation; the classical uncoupler, dinitrophenol, had the same effects on cardiac dynamics as RMI 12330A. 5 RMI in high doses inhibited hydrolytic activity of Na+, K+-ATPase of crude and purified heart preparations (IC50 of 1.7 X 10(-4) M) and inhibited ouabain binding to the same enzymes (IC50 of 1.5 X 10(-4) M). 6 A lactam-imine analogue of RMI 12330A that had no effect on adenylate cyclase, was also without effect on any of the systems examined.

Enhanced carotid sinus baroreflex responses in dogs with chronic A-V block

Effects of carotid sinus pressure on arterial pressure, atrial rate, and ventricular rate were examined in anesthetized normal dogs and in dogs with chronic complete A-V block. Change in arterial pressure per mmHg change in sinus pressure was 0.8 plus or minus 0.2 mmHg for controls but increased (P is less than 0.001) to 1.6 plus or minus 0.1 mmHg in A-V blocked dogs. Arterial pressure was 140-145 mmHg at low sinus pressure in both groups, but at high sinus pressure arterial pressure was significantly lower in A-V blocked dogs (44 plus or minus 4 mmHg) than in controls (92 plus or minus 8 mmHg). These differences were virtually abolished after vagotomy. Heart rate increased in normal dogs as sinus pressure was increased before vagotomy, but decreased after vagotomy. In blocked dogs atrial and ventricular rates decreased at high sinus pressure either before or after vagotomy. The results show that reflex circulatory responses to changes in carotid sinus pressure are enhanced in dogs with A-V block. This enhancement may involve attenuation of buffering influences exerted from other baroreceptors whose afferents are in the vagus nerves.