Abstract

Local anesthetics were used to probe differences in the binding of [3H]nitrendipine to dihydropyridine calcium antagonist binding sites on rat brain and cardiac membranes. Local anesthetics inhibited [3H]nitrendipine binding to brain and cardiac membranes with the rank order of potency, dibucaine = proadifen much greater than tetracaine greater than meproadifen greater than RAC-109 (S) greater than RAC-109 (R) greater than benzocaine. Lidocaine, procaine, piperocaine and bupivacaine produced either a small potentiation or inhibition of [3H]nitrendipine binding. Dibucaine inhibited [3H]nitrendipine binding to brain membranes (IC50, 4.9 +/- 0.5 microM) by increasing the Kd, whereas in cardiac membranes (IC50, 8.5 +/- 0.9 microM) it both increased the Kd and decreased the maximum binding site capacity of [3H]nitrendipine. The potency of dibucaine to inhibit [3H]nitrendipine binding was reduced in both tissues by monovalent (Li+ greater than Na+ = K+ = Rb+; EC50, 40-50 mM) and divalent (Ca++, Mg++ and Mn++; EC50, 10-50 microM) cations. These cations reduced the effect of dibucaine on the Kd of [3H]nitrendipine in brain and on the maximum binding site capacity of [3H]nitrendipine in cardiac membranes. Inhibition of [3H]nitrendipine binding by dibucaine was best described by high (2 microM) and low (50 microM) affinity sites. The apparent affinities of these sites, but not the fractional occupancies, were similar in brain and cardiac membranes. Na+ modulated the occupancies of these sites in brain, but not in cardiac membranes, whereas Ca++ inhibited occupancy of the high affinity site in both tissues. The effects of Li+ were similar to those of Ca++. These findings indicate that brain and cardiac dihydropyridine calcium antagonist binding sites are coupled to different allosteric effectors or exist in a different membrane environment.