The sodium, potassium, calcium and magnesium content in the portal vein of the rat was determined by atomic absorption spectrophotometry in different environmental conditions. It was found that minute changes in the ionic environment caused considerable variations in the Na+, K+ and Ca2+ content, and Na/K ratio. Phosphate in the solution increased the sodium and calcium content. In sodiumfree solution all muscle sodium was rapidly lost. No bound sodium was detected. The ionic content of portal vein was also dependent on dissection and blotting technique. A standard procedure and a standard solution was therefore chosen. In this solution the following values were obtained: Na+: 88, K+: 52, Ca2+: 5.7 and Mg2+: 3.7 mmolelkg freshweight respectively.
The extracellular space, determined with 14C-sorbitol was 490 ml/kg freshweight. The calculated intracellular concentrations and equilibrium potentials for Na+ and K+ were 62 and 157 mM and +21 and -86 mV respectively. The uptake of 42K showed that about 20 % of the total muscle potassium did not exchange within 5 hrs. The efflux of 42K+ showed that for the linear exchange of muscle potassium the rate constant was 0.85–0.90/hr.
K-low solution decreased the efflux of 42K+ in spite of increased electrical activity. This suggests that the permeabilty to potassium is lowered. Noradrenaline had no effect on the 42K efflux in normal solution.