Differences in actions of organic and inorganic Ca-antagonists on the Ca current of Helix neurone

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Differences in actions of organic and inorganic Ca-antagonists on the Ca current were investigated in snail (Helix aspersa) neurones perfused intracellularly and voltage-clamped by a suction pipette technique. Organic Ca-antagonists used were diltiazem, verapamil, nifedipine, nicardipine and KB-944; and inorganic Ca-antagonists used were cobalt, manganese and cadmiun. Results are summarized as follows: 1) The organic Ca-antagonists showed a time-dependent inhibition on the current amplitude during a depolarizing pulse lasting for 1 to 2 sec and a use-dependent depressant effect on the peak amplitude of the current. Double-pulse experiments showed that the organic Ca-antagonists also accelerated the current inactivation. Inorganic Ca-antagonists did not exhibit the accelerating actions on the current inactivation. 2) Diltiazem and verapamil showed almost equivalent inhibitory effects on the divalent cation currents carried by Ba, Ca and Mn ions which were permeant through the Ca channel. On the other hand, an inorganic Ca-antagonist such as cobalt ions greatly suppressed the peak amplitude of the Ba current, but slightly suppressed that of the Mn current. These results suggested that the site of action of organic Ca-antagonists partly differed from those of inorganic Ca-antagonists.