Intrasynaptosomal free calcium concentration is increased by phorbol esters via a 1,4-dihydropyridine-sensitive (L-type) Ca2+ channel.

Incubation of non-depolarised fura-2-loaded rat cortical synaptosomes with 12-tetradecanoylphorbol-13-monoacetate (TPA) results in a dose-dependent increase in calcium concentration (to a maximum of 140%). It is dependent on extrasynaptosomal Ca2+, is partially blocked by 1 microM verapamil and effectively blocked by 100 microM verapamil (greater than or equal to 90%). Nifedipine (1 microM), nicardipine (1 microM) and omega-conotoxin fraction GVIA from Conus geographus (50 nM) (omega-CgTx) also cause blockade (greater than or equal to 90%) of the increase. The sensitivity of the TPA-induced increase in calcium concentration to omega-CgTx, nicardipine and nifedipine, but not to low concentrations of verapamil (1 microM), suggests that the TPA-induced rise in calcium concentration is mediated by increased Ca2+ influx through 1,4-dihydropyridine-sensitive Ca2+ channels. Incubation of synaptosomes with the inactive phorbol ester phorbol-13-monoacetate (TMA) does not result in any significant dose-dependent increase in calcium concentration. The data which are presented are consistent with (i) the proposal that phorbol ester-induced increases in calcium concentration are the result of Ca2+ influx through an L-type Ca2+ channel and (ii) the existence of functioning L-type Ca2+ channels on rat brain synaptosomes.