Differentiation of receptor sites for 3Hnitrendipine in chick hearts and physiological relation to the slow Ca2+ channel and to excitation‐contraction coupling

The properties of interaction of the Ca2+ channel antagonist 3Hnitrendipine have been investigated in chick hearts at various stages of in ovo and post-natal development and in cultured cells. The dissociation constant of the 3Hnitrendipine-receptor complex is between 0.4 nM and 0.5 nM for intact ventricle and cultured cells. 3HNitrendipine binding is antagonized by nitrendipine analogs. The order of efficacy of the different dihydropyridine molecules is nitrendipine greater than nimodipine greater than nifedipine greater than nisoldipine with Kd values ranging from 0.5 to 4 nM. Inhibition of 3Hnitrendipine binding by other antiarrhythmic molecules like amiodarone, F13004 and bepridil was observed. Half-maximum inhibitions (K0.5) were found for verapamil and D600 at concentrations between 0.23 and 0.26 microM. 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 3Hnitrendipine 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 3Hnitrendipine is found to increase 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 3Hnitrendipine binding, whereas reserpine treatment is without effect. Developmental properties of nitrendipine-sensitive Ca2+ channels have been compared with those of tetrodotoxin-sensitive Na+ channels and muscarinic receptors. These results indicate that nitrendipine receptors exist at the early stage of development (3-day-old-hearts) but that they do not correspond to functional slow Ca2+ channels, that in ovo development corresponds both to an increase of the number of 3Hnitrendipine receptors and to the transformation of silent Ca2+ channels into functional Ca2+ channels, and that there is a regulation of the level nitrendipine-sensitive Ca2+ channels by innervation.