Abstract

The actions of the organic "Ca antagonists" verapamil and D-600 were tested on pinched-off presynaptic nerve terminals (synaptosomes) from rat brain, and on the frog neuromuscular junction. ⁴⁵Ca uptake was measured in control media, and in depolarizing media containing either 75 mM potassium or veratridine, an alkaloid that opens sodium channels. The extra uptake induced by depolarizing media appears to be mediated by voltage-sensitive Ca channels. Synaptosome depolarization was indirectly determined with the voltage-sensitive fluorescent dye, di-pentyl oxacarbocyanine. Verapamil or D-600 (100 µM) inhibited the K⁺-induced ⁴⁵Ca uptake by about two thirds, but had no effect on the K⁺-induced synaptosome depolarization; this inhibition of Ca uptake is, presumably, due to block of Ca-channels. Veratridine-induced ⁴⁵Ca influx was more than 80% inhibited by verapamil or D-600 (100 µM), and veratridine-induced depolarization was almost completely blocked. These observations indicate that Na channels as well as Ca channels are inhibited by verapamil and D-600. Recordings of miniature end-plate potentials were used to evaluate the actions of verapamil and D-600 at the frog neuromuscular junction, after miniature end-plate potential frequency had been made sensitive to changes in the bathing Ca concentration by raising the external K⁺. Miniature end-plate potential frequency was not affected by verapamil(40-50 µM) or D-600 (10 µM) but was significantly reduced by Mn²⁺ (0.2 mM), a known blocker of Ca channels. Although verapamil and D-600 appear to be very potent antagonists of Ca currents in heart and smooth muscle, we conclude that Ca channels in vertebrate neurons are much less sensitive to these drugs.