Blockade by cAMP of native sodium channels of adult rat skeletal muscle fibers

Although the skeletal muscle sodium channel is a good substrate for cAMP-de pendent protein kinase (PKA), no functional consequence was observed for th is channel expressed in heterologous systems. Therefore, we investigated th e effect of 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT -cAMP), a membrane-permeable cAMP analog, on the native sodium channels of freshly dissociated rat skeletal muscle fibers by means of the cell-attache d patch-clamp technique. Externally applied CPT-cAMP (0.5 mM) reduced peak ensemble average currents by similar to 75% with no change in kinetics. Sin gle-channel conductance and normalized activation curves were unchanged by CPT-cAMP. In contrast, steady-state inactivation curves showed a reduction of the maximal available current and a negative shift of the half-inactivat ion potential. Similar effects were observed with dibutyryl adenosine 3',5' -cyclic monophosphate but not with cAMP, which does not easily permeate the cell membrane. Incubation of fibers for 1 h with 10 mu M H-89, a PKA inhib itor, did not prevent the effect of CPT-cAMP. Finally, the beta-adrenorecep tor agonist isoproterenol mimicked CPT-cAMP when applied at 0.5 mM but had no effect at 0.1 mM. These results indicate that cAMP inhibits native skele tal muscle sodium channels by acting within the fiber, independently of PKA activation.