Sarcotubular anomalous rectification of frog sartorius muscle.

The membrane site responsible for anomalous rectification was determined in frog sartorius muscle fibers. The total current-voltage relation of glycerol-treated fibers which represents mainly the properties of the sarcolemma was linear for membrane potentials between about -90 and -50 mV. Thus moderate depolarization-induced anomalous rectification in intact fibers represents a property of the sarcotubular system. The absence of slow hyperpolarization in glycerol-treated fibers was caused by the abolition of early conductance increase, and the sarcotubular system is responsible for the inward rectifier. Picrotoxin selectively inhibited both moderate depolarization-induced anomalous rectification and hyperpolarization-induced early conductance increase. This suggests that the same component in the sarcotubular system is responsible for these conductance changes. The inhibition with picrotoxin of moderate depolarization-induced anomalous rectification suggests the possibility that it is caused by an electrogenic effect rather than a decrease in K conductance. A sarcolemmal hyperpolarization-activated slow conductance increase was revealed.