M-CURRENT SUPPRESSION BY AGONIST AND PHORBOL ESTER IN BULLFROG SYMPATHETIC NEURONS

Activation of protein kinase C (PKC) by phorbol esters is known to sup press M-current. 4-beta-Phorbol 12,13-dibutyrate (PDBu) irreversibly s uppressed M-current in a concentration-dependent manner (K-i 38 nM). I nhibitors of PKC, the pseudo-substrate peptide PKCI (19-31), staurospo rine and 1-(5-isoquinolinylsulfonyl)2-methylpiperazine (H7) antagonize d PDBu-mediated suppression of M-current. Suppression of M-current by muscarine and luteinizing hormone-releasing hormone (LHRH) was unaffec ted by PKCI (19-31) and H7, but was antagonized by staurosporine. The balance of data suggests that suppression of M-current by agonists is probably not mediated by activation of PKC. Addition and subsequent re moval of PDBu to M-current suppressed by muscarine prevented the actio n of PDBu, while closing M-channels by voltage or blocking by barium d id not. This suggests that M-channel closure by muscarine protects tho se channels from the effects of PDBu. Partial suppression of M-current by low concentrations of muscarine antagonized the response to PDBu, with the magnitude of suppression equivalent to that seen with PDBu al one. It is suggested that two interconvertable populations of M-channe ls exist, one that is sensitive to both agonist and PDBu and another t hat can only be suppressed by agonist.