Effects of sodium substitutes on transient inward current and tension in guinea‐pig and ferret papillary muscle.

Publisher Website
Google Scholar
Abstract
Ouabain-treated guinea pig and ferret papillary muscles were used to study transient inward current (Iti), after-contractions and tonic tension development during voltage-clamp pulses. Li, sucrose and choline were used isosmotically as Na substitutes to evaluate the effect of altering the Na equilibrium potential. Outward Iti was not detected at any potential up to +30 mV in normal or Na-depleted solutions. Reduction of Na had a biphasic effect on Iti, intially increasing it and then reducing it at all clamp potentials from -50 to +20 mV. After-contractions were initially increased and, in sufficiently Na-depleted solutions, decreased by reduction of extracellular Na. The peak in the after-contraction always occurred later than the increase in Iti and frequently coincided with the maximum suppression of the current. Complete suppression of after-contractions was not often achieved and always required more complete Na replacement than Iti suppression. Tonic tension responses were reduced by Na replacement, usually in synchrony with the reduction of Iti. The responses of Iti to Na replacement are consistent with a model of electrogenic Na-Ca exchange over the potential range positive to -50 mV. The responses deviate from the predictions of the model at more negative potentials. Oscillatory changes in internal free Ca concentration underlie both Iti and after-contractions.