THE LOW-AFFINITY DIHYDROPYRIDINE RECEPTOR AND NA+ CA2+ EXCHANGER ARE ASSOCIATED IN ADRENAL-MEDULLARY MITOCHONDRIA/

The effect of Ca2+ channel-acting drugs on bovine adrenal mitochondria Ca2+ movements was investigated. Mitochondrial Ca2+ uptake is perform ed by an energy-driven Ca2+ uniporter with a Km of 20.9 +/- 3.2 mu M a nd V-max of 148.1 +/- 7.2 nmol Ca-45(2+) min(-1) mg(-1). Ca2+ release is performed through an Na+/Ca2+ antiporter with a Km for Na+ of 4.2 /- 0.5 mM, a V-max of 7.5 +/- 0.4 nmol Ca-45(2+) min(-1) mg(-1), and a Hill coefficient of 1.4 +/- 0.2. Ca2+ efflux through the mitochondria l Na+/Ca2+ exchanger was inhibited by several dihydropyridines (nitren dipine, felodipine, nimodipine, (+)isradipine) and by the benzothiazep ine diltiazem with similar potencies, In contrast, neither CGP 28392, Bay-K-8644, amlodipine, nor verapamil had any effect on Ca2+ efflux. N itrendipine at 20 mu M modified neither the Km nor the Hill coefficien t for Na+, whereas the V-max was reduced to 2.9 nmol Ca-45(2+) min(-1) mg(-1), thus demonstrating noncompetitive modulation of the Na+/Ca2exchanger. None of the Ca2+ channel-acting drugs assayed at 100 mu M a ffected Ca2+ influx through the uniporter. Ca2+ channel blockers inhib ited the Na+/Ca2+ antiporter and displaced the specific binding of [H- 3]nitrendipine to intact mitochondria with Ki values similar to the IC (50)s obtained for the inhibition of the Ca2+ efflux. Ca2+ channel-act ing drugs that did not inhibit the Na+/Ca2+ exchanger (amlodipine, CGP 28392, Bay-K-9644, and verapamil, at concentrations of 100 mu M or hi gher) had no effect on [H-3]nitrendipine binding. These results sugges t that the adrenomedullary mitochondrial dihydropyridine receptor is a ssociated with the Na+/Ca2+ exchanger.