Phosphatidylinositol hydrolysis is involved in production of Ca(2+)-dependent currents, but not non-selective cation currents, by muscarine in chromaffin cells.

Whether phosphatidylinositol hydrolysis and a subsequent Ca2+ mobilization are responsible for muscarine-induced transient outward currents (IO) and non-selective cation currents (INS) in the guinea-pig chromaffin cell was investigated using the perforated patch method. IO, but not INS, failed to be reproduced in Ca(2+)-free solution and was markedly reduced by prior exposure to caffeine under Ca(2+)-free conditions or by addition to normal solution of cyclopiazonic acid (CPA), a Ca2+ ATPase inhibitor. Application of CPA in Ca(2+)-free solution, however, suppressed INS by about 50% in 73% of the cells tested. Bath application of 1.5 mM neomycin, a phospholipase C inhibitor, induced the time-dependent decline of IO with near abolition at 20 min or less, whereas it produced a time-independent decrease of INS and an inwardly rectifying K+ current. INS in the presence or absence of neomycin was well fitted to rectangular hyperbolas with the same ED50 of 2.17 microM, but with a 33% smaller maximum amplitude in the former, indicating a non-competitive inhibition by neomycin. We conclude that, while phosphatidylinositol hydrolysis mediates the production of IO, it does not mediate that of INS by muscarine.