Cation Regulation in the Smooth Muscle of Frog Stomach

Cation composition of frog smooth muscle cells was investigated. Fresh stomach muscle rings resembled skeletal muscle, but marked Na gain and K loss followed immersion. Mean Na (49.8-79.7 mM/kg tissue) and K (61.8-80.1 mM/kg tissue) varied between batches, but were stable for long periods in vitro. Exchange of 6-30 mM Na/kg tissue with (22)Na was extremely slow and distinct. Extracellular water was estimated from sucrose-(14)C uptake. Calculated exchangeable intracellular Na was 9 mM/kg cell water, and varied little. Thus steady-state transmembrane cation gradients appeared to be steep. K-free solution had only slight effects. Ouabain (10(-4)M) caused marked Na gain and reciprocal K loss; at 30 degrees C, Na and K varied linearly with time over a wide range of contents, indicating constant net fluxes. Net fluxes decreased with temperature decrease. (22)Na exchange in ouabain-treated tissue at 20-30 degrees C was rapid and difficult to analyze. The best minimum estimates of unidirectional Na fluxes at 30 degrees C were 10-12 times the constant net flux; constant pump efflux may explain these findings. The rapidity of Na exchange may not reflect very high permeability, but it does require a high rate of transport work.