The blockade of Vmax of the atrioventricular action potential produced by the slow channel inhibitors verapamil and nifedipine

A method for estimation of drug affinity constants to the open conformational state of calcium channels

The affinity of D600 to calcium channels in the open state has been examined in isolated smooth muscle cells of the rabbit ear artery. Calcium channel currents were measured in high external barium solution by means of the patch-clamp technique. The current inhibition in various D600 concentrations (3-100 microM) on application of trains of short test pulses (20-80 ms) has been studied in nonmodified calcium channels and in cells where the calcium channels were modified by the agonist dihydropyridine (+) 202,791 (100 nM). The kinetics of the peak current decay has been analyzed with a mathematical model which is based on the experimental finding that D600 interacts primarily with calcium channels in the open conformational state. The model approach allows the estimation of drug affinity constants of D600 to the calcium channel in the open conformation. An association rate constant to the open conformational state of D600 of 6.16 x 10(4) M-1 s-1 was estimated. The association rate of the drug was not significantly changed after the calcium channels have been modified with 100 nM (+) 202,791. A method for correction of rate constants for possible drug trapping is discussed.

Mechanism of calcium channel block by D600 in single smooth muscle cells from rabbit ear artery

This study investigated the action of the calcium antagonist D600 on calcium channel currents recorded in high barium solution from single, enzymatically isolated smooth muscle cells from the rabbit ear artery using the whole cell configuration of the patch-clamp technique. D600 (1-100 microM) was applied by path perfusion or by a new technique that allowed a concentration jump during the current. Application of D600 at rest (holding potential, -60 mV) did not alter the peak inward current elicited on depolarization, but the activation of the channels led to a marked block and an acceleration of the current decay. The mechanism of block of calcium channels by D600 was studied by using pulse protocols with different pulse length and different interpulse intervals. The results were consistent with the hypothesis that D600 has a low affinity for the calcium channels in the resting state and that they have to pass to the open state before the drug affects the calcium channel current. A fast onset of the calcium channel block by D600 (time constant, 502 msec) could be shown by rapid application of D600 during the sustained current component of the barium inward current. However, experiments did not definitely distinguish whether binding occurred to the open or to the inactivated state (although there was some evidence of a long-lasting binding to an inactivated state).

Characterization of the effects of histamine on the transmembrane electrical activity of guinea-pig and rabbit SA- and AV-node cells

The effects of histamine on the transmembrane electrical activity of cells of small preparations (0.5 X 0.5 mm) of guinea-pig and rabbit sinoatrial- and atrioventricular-nodes were studied. Histamine at concentrations above 10(-7) mol/l increased the firing rate, the rate of diastolic depolarization, the maximum diastolic potential, the amplitude and the maximum rate of depolarization of the action potential of pacemaker cells of rabbit and guinea-pig sinoatrial cells and rabbit atrioventricular cells. These effects were antagonized by the H2-receptor blocker cimetidine (2.5 X 10(-6)mol/l) but they were not modified by the H1-receptor blocker chlorphenamine (2.5 and 5 X 10(-6)mol/l). Small preparations of guinea-pig atrioventricular node did not exhibit spontaneous activity, but it was induced by histamine and blocked by cimetidine. Histamine increased the maximum upstroke velocity of propagated action potential of cells of the central part of complete atrioventricular node in both species studied. These effects were blocked by cimetidine, but not by chlorphenamine. It is concluded that the increase in automaticity induced by histamine in guinea-pig and rabbit sinoatrial and atrioventricular nodes was due to stimulation of H2-receptors. Histamine did not depress electrical activity of atrioventricular node cells, but rather increased it. This effect was due to H2-receptor stimulation.

Frequency-dependent effects of calcium antagonists on atrioventricular conduction and refractoriness: demonstration and characterization in anesthetized dogs

Calcium-channel blockers are known to affect slow inward current in a frequency-dependent fashion. The purpose of these experiments was to study use-dependent effects of verapamil, diltiazem, and nifedipine on atrioventricular conduction in vivo. Loading and maintenance infusion techniques were developed to study each drug at a series of stable plasma concentrations in autonomically blocked dogs anesthetized with morphine and alpha-chloralose. All three agents produced changes in atrioventricular conduction and refractoriness that increased with increasing stimulation frequency. The time dependence of drug-induced changes in atrioventricular conduction was characterized both by varying the coupling of single test stimuli and by abruptly changing activation frequency. The time constants for onset of (tau on) and recovery from (tau off) block were typical for each drug, with nifedipine having a faster time constant (tau off = 0.36 +/- 0.12 sec) than verapamil (tau off = 3.2 +/- 1.0 sec, tau on = 28 +/- 8 sec) or diltiazem (tau off = 2.7 +/- 1.2 sec, tau on = 13 +/- 4 sec). The time constants for each drug were independent of concentration but the magnitude of time-dependent change increased with increasing drug concentration. We conclude that calcium-channel blockers have important frequency-dependent effects on atrioventricular conduction in vivo. This frequency dependence may result in selective depression of atrioventricular conduction in the presence of supraventricular tachyarrhythmias, with important potential implications for the clinical use of these agents.

Voltage-dependent block of calcium channel current in the calf cardiac Purkinje fiber by dihydropyridine calcium channel antagonists

We have investigated the mechanisms of blockade of calcium channel current by the dihydropyridines, e.g. nisoldipine, nitrendipine, and nicardipine. Membrane current was recorded in isolated calf Purkinje fibers using a two-microelectrode voltage-clamp technique, and voltage protocols were designed to identify voltage- and use-dependent block by these compounds systematically. Our results show that calcium channel blockade by dihydropyridine derivatives is strongly modulated by membrane potential. Block is more pronounced when current is measured from depolarized holding potentials, but in contrast to verapamil, this voltage-dependent block occurs in the absence of repetitive depolarizations. Use-dependent block by dihydropyridines is observed at pulse frequencies greater than 1 Hz. Our results suggest that dihydropyridines bind preferentially to the inactivated state of the calcium channel, and that the development of use-dependent block is related to the ionization constants of the compounds. Furthermore, binding is approximately one thousand times stronger to inactivated channels than to resting channels. This state-dependent difference in binding affinities may account for the previously reported contrast between electrophysiological and binding data for these compounds.

Comparison of the direct effects of nifedipine and verapamil on the electrical activity of the sinoatrial and atrioventricular nodes of the rabbit heart

We compared the effects of nifedipine and verapamil on the rabbit sinus and atrioventricular nodes. Both drugs slowed the rate of impulse initiation by sinus node cells. Verapamil exerted a greater negative chronotropic effect at low concentrations, but at higher concentrations verapamil and nifedipine were equipotent. Nifedipine also reduced the amplitude of sinus node action potentials and the Vmax of phase O, effects which are identical to those previously described for verapamil. Both drugs slowed AV nodal conduction and prolonged refractory periods, but verapamil was more potent than nifedipine. Nifedipine reduced the amplitude of AV nodal action potentials and Vmax of phase O the same as verapamil. Nifedipine and verapamil, therefore, have nearly identical direct effects on the nodes. The failure of nifedipine to depress AV nodal conduction in situ and abolish reentrant AV nodal tachycardia is probably a result of the decreased sensitivity of the AV node to nifedipine compared to verapamil.

Interaction between calcium and slow channel blocking drugs on atrial rate

The relationship between extracellular calcium concentration and the chronotropic effect of prenylamine, verapamil and nifedipine was studied in isolated spontaneously beating rat atria. The three slow channel blocking drugs produced a concentration-dependent decrease in atrial rate, though with different relative potencies. The order of potency for decreasing atrial rate, independently of the calcium level (1.0, 3.0, 6.0 or 9.0 mmol/l) was: verapamil greater than nifedipine greater than prenylamine. Increasing calcium from 1.0 to 6.0 and 9.0 mmol/l increased atrial rate from 251 +/- beats . min-1 to 265 +/- 6 beats . min-1 and 285 +/- 9 beats . min-1 (mean +/- 1 standard error) respectively (P less than 0.05). Despite their positive chronotropic effect high calcium levels failed to reverse the negative chronotropic effect of the slow channel blockers. Furthermore, the negative chronotropic effect of both verapamil and nifedipine was enhanced at high calcium levels. Raising calcium from 1.0 to 6.0 mmol/l in the presence of verapamil (1 X 10(-7) mol/l) or nifedipine (3 X 10(-7) mol/l) increased 2-fold the negative chronotropic effect of the calcium channel blockers. In addition, the concentration-effect curves for verapamil and nifedipine shifted to the left by 0.50 +/- 0.14 and 0.50 +/- 0.16 log units, respectively, when calcium increased from 1.0 to 6.0 mmol/l. The data show that increasing calcium may produce positive or negative chronotropic effects depending on whether or not the calcium channels are blocked. This paradoxical effect of calcium ions can be produced either by opposite chronotropic effects on automatic cells or by shifting the pacemaker activity to a group of cells which respond in a different way to an increment of calcium.

Characterization of [3H]nifedipine binding sites in rabbit myocardium

A specific, high affinity (KD 1.8 nM) binding site for the calcium entry blocking drug [3H]nifedipine was identified in homogenates of rabbit myocardium. [3H]Nifedipine binding was rapid (t1/2 3 min) and reversible (t1/2 11 min). Calcium entry blockers with different chemical structures competed with [3H]nifedipine binding in the potency order: nifedipine much greater than D600 = verapamil greater than tiapamil greater than cinnarizine = prenylamine. Diltiazem and perhexiline did not significantly inhibit [3H]nifedipine binding. The potencies of these drugs to inhibit binding were similar to their abilities to depress contractions of the isolated rabbit papillary muscle. The stereoselectivity of D600 and verapamil ((-)-much greater than (+)-isomers) as inhibitors of papillary muscle contractions was not apparent in [3H]nifedipine competition experiments. The slopes of the concentration-inhibition curves for D600 and verapamil were significantly less than for nifedipine. It is concluded that [3H]nifedipine may be labelling part of the myocardial Ca2+ channel, and that verapamil-like substances and nifedipine differ in their mode of interaction with this binding site.

Differential blockade of ATP, noradrenaline and electrically evoked contractions of the rat vas deferens by nifedipine

Nifedipine has recently been found to block electrically induced contractions of the prostatic portion of the vas deferens. The present results show that nifedipine blocks contractile responses to ATP in the parallel with the nerve mediated response, whereas the rapid phasic contractile response to noradrenaline is unaffected. The slower, tonic contractions to noradrenaline are blocked most easily, when the nerve-mediated responses are unchanged. These findings support the view that neurally released ATP may contribute to contractile responses of the vas.