PHORBOL ESTER-INDUCED M-CURRENT SUPPRESSION IN BULLFROG SYMPATHETIC-GANGLION CELLS - INSENSITIVITY TO KINASE INHIBITORS

1 The effects of 1-oleoyl-2-acetyl-sn-glycerol (OAG), phorbol 12-myristate 13-acetate (PMA), 4-alpha-phorbol and muscarine on B-neurones from bull-frog sympathetic ganglion were studied by means of whole-cell patch-clamp recording. With the exception of 4-alpha-phorbol, all of these agonists reduced the steady-state outward current recorded at -30 mV as a result of suppression of a voltage-dependent, non-inactivating K+-current, the M-current, (I-M). 2 Of the cells tested, 34% displayed bonafide responses to OAG (20 mu M). The chance of recording a response was not decreased when the protein kinase inhibitor, 1-(5-isoquinolinyisulphonyl)-2-methylpiperazine (H-7; 50 or 75 mu M) was included simultaneously in the extracellular solution and in the pipette solution. 3 The presence of 50 mu M H-7 on both sides of the membrane or 500 nM staurosporine in the pipette solution did not prevent responses to brief (1 -2 min) or prolonged (>20 min) applications of PMA. 4 Brief (1-2 min) extracellular application of H-7 (300 mu M) suppressed I-M by about 29%. 5 The most likely explanation of these data is that PMA and OAG modulate I-M via a mechanism that is independent of protein kinase C (PKC). The availability of such a mechanism poses new questions as to the mechanism of muscarine-induced I-M suppression.