Stimulation rate modulates effects of the dihydropyridine CGP 28 392 on cardiac calcium-dependent action potentials

Identification of cardiotonic sodium channel activators by potassium depolarization in isolated guinea-pig atria

The inotropic actions of various drugs known to increase force of contraction in isolated mammalian cardiac muscle were investigated in electrically driven (1 Hz) guinea-pig left atria under both normal [K+]o (4.7 mM) and high [K+]o (22 mM). Under normal [K+]o a concentration-dependent increase in force of contraction could be confirmed with the beta-adrenoceptor agonist, isoprenaline, the cyclase activator, forskolin, the inhibitors of the cyclic AMP-phosphodiesterase (PDE), amrinone, IBMX, and OPC 8212, the Na+ channel activators, DPI 201-106, SDZ 210-921, veratridine, and ATX II, the Na(+)-ionophore monensin, the inhibitor of Na+/K(+)-ATPase, ouabain, and the Ca2+ channel activators, Bay K 8644, CGP 28 H 392, and SDZ 202-791. Partial depolarization of the muscle preparations by increasing [K+]o in the organ bath to 22 mM completely abolished the positive inotropic action of the Na+ channel-activating drugs. In contrast, the effects of the other compounds were still present, although changes in the maximal force development were observed. The efficacy of the PDE inhibitors amrinone and IBMX were slightly increased; the maximal effects of isoprenaline, monensin, forskolin, and OPC 8212 were unchanged; the effect of ouabain decreased to about half maximal values; while the efficacy of the Ca2+ channel activators were either unchanged (CGP 28 392) or decreased (Bay K 8644 and SDZ 202-791). The results suggest that inactivation of cardiac fast Na+ channels by partially depolarizing isolated, electrically driven atria is a suitable model to distinguish between cardiotonic agents acting through activation of Na+ channels and those with other mechanisms of action.

Voltage- and use-dependent modulation of cardiac calcium channels by the dihydropyridine (+)-202-791

The modulation of L-type voltage sensitive calcium channels in isolated guinea pig ventricular myocytes by the dihydropyridine (+)-202-791 was examined with the whole-cell voltage-clamp technique with 1.8 mM Ba or Ca as the charge carrier. Striking voltage- and use-dependent effects of the dihydropyridine calcium channel "agonist" (+)-202-791 were revealed. From a holding potential of -60 mV, depolarizing test pulses in the presence of (+)-202-791 demonstrated a concentration-dependent (EC50, 177 nM) increase in the measured peak inward barium current compared to control. In contrast, more depolarized holding potentials (greater than or equal to -30 mV) (+)-202-791 caused a biphasic effect on the peak inward current resulting in a transient enhancement followed by a steady-state block. A saturable, concentration-dependent hyperpolarizing shift in the voltage dependence of current inactivation was observed in the presence of (+)-202-791 with an EC50 of 10.2 nM. The voltage dependence of current activation was also shifted in the hyperpolarizing direction in the presence of (+)-202-791. A use-dependent relative block by (+)-202-791 was observed after repetitive depolarizing test pulses at a frequency of 2 Hz. Thus, the single enantiomer (+)-202-791 can result in either an increase in the whole cell calcium channel current (favored by hyperpolarized holding potentials and low rates of stimulation) or block of calcium channel current (favored by depolarized holding potentials and high rates of stimulation). Various combinations of (-)-202-791, a reported calcium channel antagonist, and (+)-202-791 resulted in intermediate effects on voltage sensitive calcium or barium currents compared with the presence of either enantiomer alone, and no clear cooperative interactions between the enantiomers were observed in contrast to a previous single channel study (Kokuban S, Prod'ham B, Becker C, Porzig H, Reuter H: Studies on Ca channels in intact cardiac cells: Voltage-dependent effects and cooperative interaction of dihydropyridine enantiomers. Mol Pharmacol 1986;30:571-584). The results are discussed in relation to the possible presence of multiple dihydropyridine receptors associated with the voltage sensitive calcium channel.

Influence of endothelium on the response to calcium agonists, calcium, potassium and noradrenaline in rat aorta

We have studied the influence of endothelium on BAY K 8644, CGP 28392, potassium, noradrenaline and calcium induced responses in isolated thoracic aorta from rats. In aortic preparations with intact endothelium BAY K 8466 and CGP 28392 (10(-9)-10(-5) M) had a concentration dependent contractile effect, but BAY K 8644 was more effective in contracting the vessels than CGP 28932. Removal of endothelium increased the contractile response to both substances without affecting the difference in contractile effect between the two calcium agonists. In high concentrations (10(-4) M) BAY K 8644 and CGP 28392 had an equal relaxing effect on potassium precontracted preparations, which was unaffected by removal of endothelium. Removal of endothelium caused a shift to the left of the potassium and noradrenaline concentration response curves without any change in maximal contractile response. The contractile response to calcium in BAY K 8644 and potassium activated vessels was unaffected by removal of endothelium. We conclude that endothelium derived factor(s) depress the contractile response to calcium agonists, potassium, and noradrenaline but has no influence on the relaxant effect of BAY K 8644 and CGP 28392 in rat aorta. The influence of endothelium on the contractile effect on BAY K 8644 and potassium seems not to be due to a direct effect on calcium influx but rather to an influence on the membrane potential of the smooth muscle cells.

Multiple calcium channels and neuronal function

Recent investigations have demonstrated that neurons have a number of different types of calcium channels, each with their own unique properties and pharmacology. These calcium channels may be important in the control of different aspects of nerve activity. Some of the possibilities can now be discussed.