CHLORIDE CHANNEL REGULATION IN THE SKELETAL-MUSCLE OF NORMAL AND MYOTONIC GOATS

External intercostal muscle biopsies from normal and congenitally myotonic goats were studied in vitro at 30-degrees-C using a two-microelectrode square-pulse cable analysis assisted by computer. The resting chloride conductance (G(cl)) was estimated from the difference between the mean membrane conductance in chloride-containing and chloride-free bathing media. The protein kinase C (PKC) activator, 4-beta-phorbol-12,13-dibutyrate, (0.1-2.0-mu-M) blocks a maximum of 76% of G(cl) in normal goat fibers and induces myotonic hyperexcitability similar to that of congenitally myotonic goat fibers. The G(cl) block was partially antagonized by pretreatment with the PKC inhibitor, staurosporine (10-mu-M). The "inactive" 4-alpha-phorbol-12,13,didecanoate had no effect at 50-mu-M, whereas the "active" 4-beta isomer blocked 41% G(cl) at 1-mu-M. The nearly absent G(cl) of congenitally myotonic goat fibers was not restored by treatment with high concentrations of the PKC inhibitors staurosporine, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7), or tetrahydropapaveralone (THP). Also, forskolin and cholera toxin, which may increase cyclic adenosine monophosphate (cAMP) levels, or the R(+) clofibric acid enantiomers and taurine, which increase G(cl) in normal fibers, were also unable to restore G(cl) in myotonic goat fibers. The data suggest that PKC may be a chloride channel regulator in normal goat skeletal muscle fibers, however the molecular defect of congenitally myotonic fibers does not appear to be due to excessive activity of PKC.