REGULATION OF RESTING IONIC CONDUCTANCES IN FROG SKELETAL-MUSCLE

The membrane electrical properties and resting ionic conductances of f rog semitendinosus muscle fibres were studied in vitro at 25-degrees-C with the two-microelectrode cable technique, in the presence of an ac tivator or inhibitor of protein kinase C (PKC) or in the presence of a n activator of adenylate cyclase. The PKC activator, 4beta-phorbol 12, 13-dibutyrate (4beta-PDB), reduced chloride conductance (G(Cl)) at con centrations greater than 1 muM and did not affect potassium conductanc e (G(K)). At 150 muM, the maximum concentration of 4beta-PDB tested, G (Cl) was reduced by 42%. The ''inactive'' phorbol ester 4alpha-phorbol 12,13-dibutyrate did not affect G(Cl) or G(K). The inhibitory effect of 4beta-PDB on G(Cl) was prevented by pretreatment of the muscle prep aration with the PKC inhibitor staurosporine. The adenylate cyclase ac tivator forskolin (1.5-8 muM) significantly increased the G(K) Of the fibres, without affecting G(Cl). Thus, we conclude that frog skeletal muscle G(Cl), unlike rat muscle G(Cl), is relatively insensitive to ac tivators of PKC. Moreover, in frog muscle, protein kinase A is a likel y modulator of G(K), but not G(Cl).