Intramembrane charge movements in frog skeletal muscle in strongly hypertonic solutions

Intramembrane charge movements were studied in intact, voltage-clamped frog (Rana temporaria) skeletal muscle fibers in external solutions made increasingly hypertonic by addition of sucrose. The marked dependence of membrane capacitance on test potential persisted with increases in extracellular sucrose concentration between 350 and 500 mM. Charge movements continued to show distinguishable early monotonic (q beta) decays and the strongly voltage-dependent delayed (q gamma) charging phases reported on earlier occasions. In contrast, a further increase to 600 mM sucrose abolished the most steeply voltage-sensitive part of the membrane capacitance. It left a more gradual variation with potential that closely resembled the function that resulted when q gamma charge was abolished by tetracaine in the presence of 500 mM sucrose. Charging transients were now simple monotonic (q beta) decays and lacked delayed (q gamma) transients. Furthermore, tetracaine (2 mM) altered neither the kinetic nor the steady-state features of the charge left in 600 mM sucrose. However, Ca2+ current activation in the same fibers persisted through such tonicity increases under identical conditions of temperature, external solution, and holding voltage. Tonicity changes thus accomplish an independent separation of q gamma and q beta charge as defined hitherto through their tetracaine sensitivity. Their effects on q gamma charge correlate with earlier observations of osmotic conditions on delta[Ca2+] signals (1987. J. Physiol. (Lond.) 383:615-627.) and the parallel effects of other agents on excitation-contraction coupling and q gamma charge. In contrast, they suggest that Ca2+ current activation does not require q gamma charge transfer whether by itself or as part of the excitation-contraction coupling process.