DIFFERENTIATION OF RECEPTOR-SITES FOR [NITRENDIPINE-H-3 IN CHICK HEARTS AND PHYSIOLOGICAL RELATION TO THE SLOW CA-2+ CHANNEL AND TO EXCITATION-CONTRACTION COUPLING

The properties of interaction of the Ca2+ channel antagonist [3H]nitrendipine were investigated in chick hearts at various stages of in ovo and postnatal development and in cultured cells. The Kd of the [3H]nitrendipine-receptor complex is between 0.4 nM and 0.5 nM for intact ventricle and cultured cells. [3H]Nitrendipine binding is antagonized by nitrendipine analogs. The order of efficacy of the different dihydropyridine molecules is nitrendipine > nimodipine > nifedipine > nisoldipine with Kd values ranging from 0.5-4 nM. Inhibition of [3H]nitrendipine binding by other antiarrhythmic molecules like amiodarone, F13004 and bepridil was observed. Half-maximum inhibitions (K0.5) were found for verapamil and D600 [methoxyverapamil], at concentrations between 0.23 and 0.26 .mu.M. The potency of organic Ca2+ blockers to depress by 50% the maximum amplitude of spontaneous beating of heart cells is closely related to K0.5 values obtained from [3H]nitrendipine binding experiments. Electrophysiological results indicate that the slow channel is insensitive to nitrendipine at the younger stage of development (3-day-old) whereas, in adult-like cells, nitrendipine (50 nM) abolished both slow action potential due to the slow Ca2+ channel and contraction. The maximum binding capacity for [3H]nitrendipine increased during development of the embryonic heart from 40 fmol/mg protein at day 3 to 100 fmol/mg protein at day 14, to stay relatively stable until day 18. Then the number of sites increases rapidly to reach a second plateau at 210 fmol/mg protein on day 4 after hatching. Treatment with 6-hydroxydopamine results in 35% increase in [3H]nitrendipine binding, whereas reserpine treatment is without effect. Developmental properties of nitrendipine-sensitive Ca2+ channels were compared with those of tetrodotoxin-sensitive Na+ channels and muscarinic receptors. Nitrendipine receptors evidently exist at the early stage of development (3-day-old-hearts) but they do not correspond to functional slow Ca2+ channels; in ovo development corresponds both to an increase of the number of [3H]nitrendipine receptors and to the transformation of silent Ca2+ channels into functional Ca2+ channels, and there is a regulation of the level nitrendipine-sensitive Ca2+ channels by innervation.