Positive inotropic effect of adrenaline on potassium contractures in tonic skeletal muscle fibres of the frog

K+ contractures were elicited in small bundles of tonic skeletal muscle fibres of the frog. Adrenaline (1 microM) increased the amplitude of K+ contractures in a [K+]o-dependent manner: maximal effects were produced between 20 and 60 mM [K+]o. In contrast, we found no effect of adrenaline on K+ contractures of twitch fibres. The potentiating effect of adrenaline depended on [Ca2+]o. Increasing [Ca2+]o from 1.8 to 10 mM doubled the positive inotropic effect of adrenaline. In a nominally Ca2+ free saline, adrenaline had no potentiating effect. The Ca2+ channel blockers nifedipine (20 microM) and Ni2+ (1.8 mM) reversibly reduced the amplitude of the tonic phase of K+ contractures and blocked the potentiation by adrenaline. The mechanical effects of adrenaline cannot be explained by changes in the membrane potential, as revealed by intracellular recordings at several [K+]o. It was concluded that the potentiating effect of adrenaline in tonic muscle fibres of the frog may be mediated through Ca2+ channels.