The negative inotropic potency of calcium antagonists, particularly nitrendipine and gallopamil, is highly dependent on stimulus frequency and transmembrane potential in isolated guinea-pig atria.
Transmembrane voltage and beat frequency are important determinants of the action of several organic calcium antagonists. This is well-known for the cationic amphiphilic calcium antagonists. We intended to assess the functional impact of these phenomena in cardiac muscle with special regard to dihydropyridines. Therefore, concentration-response curves were constructed in isolated guinea-pig left atria for the negative inotropic effect of various compounds. The dihydropyridines nifedipine, racemic nitrendipine, nisoldipine, and felodipine, and the enantiomers of isradipine were investigated at different stimulation frequencies (1 Hz, 2.5 Hz, 4.5 Hz), and at different extracellular K+ concentrations (2.7 mM, 5.4 mM, 10.8 mM). These drugs were compared with the cationic amphiphilic compounds gallopamil, verapamil and diltiazem. The potency of some dihydropyridines, particularly nitrendipine, could be modulated to a remarkable extent, covering several orders of magnitude. The potential-dependency of the drugs depended on stimulus frequency and ranged from less than a half to two orders of magnitude. At 2.5 Hz, the rank order of extent of potential-dependency was gallopamil greater than nitrendipine greater than diltiazem greater than verapamil = (+)-isradipine greater than (-)-isradipine greater than or equal to nisoldipine greater than or equal to felodipine = nifedipine. Based on data obtained from binding studies in intact atria and from patch-clamp measurements of calcium current blockade, a mathematical model was used which describes the observed potency changes.(ABSTRACT TRUNCATED AT 250 WORDS)
Herzig et al. (Tue,) studied this question.