CHLORIDE CHANNEL REGULATION IN SKELETAL MUSCLE OF NORMAL AND MIOTONIC GOATS

External intercostal muscle biopsies from normal and congenitally myotonic goats were studied in vitro at 30°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-β-phorbol-12,13-dibutyrate, (0.1-2.0 μM) 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 μM). The 'inactive' 4-α-phorbol-12,13,didecanoate had no effect at 50μM, whereas the 'active' 4-β isomer blocked 41% G(cl) at 1 μM. 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