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AB STRACT Sodium transport and oxygen consumption have been simultaneously studied in the short-circuited toad skin. A constant stoichiometric ratio was observed in each skin under control condition (NaCI-Ringer's solution bathing both sides of the skin) and after block of sodium transport by ouabain. During alterations of sodium transport by removal and addition of K to the internal solution the stoichiometric ratio is constant although having a value higher than that observed in other untreated skins. The coupling between active sodium transport and oxygen consumption was studied after a theoretical nonequilibrium thermodynamic model. Studies were made of the influence of Na chemical potential difference across the skin on the rates of Na transport and oxygen consumption. A linear relationship was observed between the rates of Na transport and oxygen consumption and the Na chemical potential difference. Assuming the Onsager relationship to be valid, the three phenomenological coefficients which describe the system were evaluated. Transient increases in the rate of sodium transport and oxygen consumption were observed after a transitory block of sodium transport by removal of Na from the external solution. Cyanide blocks completely the rate of oxygen consumption in less than 2 min and the short-circuit current measured after that time decays exponentially with time, suggesting a depletion of ATP from a single compartment.

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The possible role of changes in the sodium (Na) affinity of the carrier for inorganic phosphate (Pi) in the adaptation of Pi transport to low Pi diet was examined in both renal and intestinal brush border membranes vesicles (BBMV) obtained from the same animal. This role was assessed by measuring the Na concentration resulting in half maximal activation of Pi transport (K0.5 Na) in renal and intestinal BBMV prepared from animals adapted to either low (LPD) or high (HPD) phosphorus diet for 7 days. The K0.5 Na was not modified by dietary Pi, in both renal and intestinal BBMV. LPD increased maximal Pi transport from 1794.8 +/- 198.0 to 2964.0 +/- 362.0 in renal and from 28.2 +/- 3.4 to 80.5 +/- 7.2 pmol/mg 10 s in intestinal BBMV. For both LPD and HPD lowering pH from 7.4 to 6 dramatically increased K0.5 Na in renal and intestinal BBMV. As compared to pH 7.4, it was enhanced by approximately 200% in both renal and intestinal membranes. This change of Na affinity with acidic pH prevented the expression of Pi transport adaptation at 100 mM Na concentration. However, at saturating Na concentrations (500 mM for renal, 300 mM for intestinal membranes), Pi transport adaptation was equally expressed at pH 6 and 7.4 in both types of membranes. Hill coefficient analysis indicates a 2:1 stoichiometry of Na to Pi in renal and intestinal membranes isolated from high or low Pi diet animals. This ratio was not modified by changes of the medium pH.