Inhibition of Na+-Ca2+ exchange by calcium antagonists in rat brain microsomal membranes.

Na+-Ca2+ exchange rates were studied in native and/or pronase pretreated rat brain microsomal membranes in the presence of calcium channel antagonists verapamil, nimodipine and nifedipine. In native membranes all the substances used inhibited Na+-Ca2+ exchange. A relatively stronger inhibition was observed in membranes pretreated with pronase. The values of Ki for nimodipine and nifedipine did not change and it fell to about one half for verapamil. Lineweaver-Burk's plots have revealed that the verapamil inhibition in native membranes as well as in pronase pretreated ones was of a non-competitive type; Km for calcium oscillated around 15 mumol.l-1. It is suggested that the inhibition strength depends on the access of inhibitors to the membrane binding sites as well as on the solubility of inhibitors in membrane lipids.