BETA-ADRENERGIC MODULATION OF BA2-MUSCLE FIBERS( CURRENTS AND K+ CONTRACTURES IN FROG SLOW SKELETAL)

beta-Adrenergic modulation of the Ba2+ current (I-Ba) and K+ contractu re in slow skeletal muscle fibers of the frog (Rana pipiens) were inve stigated in intact fibers with the three-microelectrode voltage-clamp technique and isometric tension measurements. Application of epinephri ne (10(-6) to 10(-5) M) to the bath increased the amplitude of I-Ba. T his increase was blocked by the beta-antagonist propranolol (3 mu M), and a similar increase was observed with the beta-specific agonist iso proterenol (1 mu M). Thus the epinephrine effect was mediated mainly b y beta-adrenergic receptors. External application of permeable 8-bromo adenosine 3',5'-cyclic monophosphate (0.5 mM) increased the amplitude of both I-Ba and K+ contractures. The present results suggest that bet a-adrenergic modulation of I-Ba in slow skeletal muscle fibers could r eflect a modulation of Ca2+ channels via adenosine 3',5'-cyclic monoph osphate (cAMP). cAMP (0.5 mM) also potentiated the K+-evoked tension i n these slow fibers. The physiological contribution made by the modula tion of slow skeletal muscle Ca2+ channels to the increase in tension is still not completely understood.