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STEPI-mNSO ~ Cation Regulation in Frog Smooth Muscle I

Cation composition of frog smooth muscle ceils was investigated. Fresh stomach muscle rings resembled skeletal muscle, but marked Na gain and K loss followed immersion. Mean Na (49.8-79.7 mu/kg tissue) and K (61.880.1 mM/kg tissue) varied between batches, but were stable for long periods in vitro. Exchange of 6-30 mM Na/kg tissue with 22Na was extremely slow and distinct. Extracellular water was estimated from sucrose-14C uptake. Calculated exchangeable intraceUular 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°C, Na and K varied linearly with time over a wide range of contents, indicating constant net fluxes. Net fluxes decreased with temperature decrease. 22Na exchange in ouabain-treated tissue at 20-30°C was rapid and difficult to analyze. The best minimum estimates of unidirectional Na fluxes at 30°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. I N T R O D U C T I O N Vertebrate smooth muscle contains more Na and less K than skeletal muscle, and relatively high Na and low K concentrations in the cell water have been inferred. In consideration of both the ionic basis of electrical properties and the maintenance of ion gradients and cell volume by endergonic processes, smaller t ransmembrane cation gradients generally have been assumed for smooth muscle (7, 34). However, indirect estimates of internal cation composition depend on the extent to which nonionic components can be defined, the accuracy with which extracellular ions can be estimated, and the assumption of homogeneous distribution within the cell. A bound Na component has been suggested, but has been difficult to assess (6, 11, 12, 16, 22), and different methods of estimation of extracellular space have given varying results (2, 3, 15, 16). Studies of the kinetics of tracer Na exchange in mammal ian smooth muscle (12, 16, 25) and toad stomach muscle (6) have not characterized the