The significance of changes in thermodynamic affinity induced by aldosterone in sodium-transporting epithelia.

The energetics of sodium transport were examined in toad (and occasionally frog) skin, with particular emphasis on the effect of aldosterone. Thermodynamic affinity was computed according to Essig and Caplan. Following treatment with antidiuretic hormone or drugs believed to affect only the apical membrane barrier, no change in thermodynamic affinity was observed either acutely (after one to two hours) or chronically (after 18-odd hours hours). By contrast, following treatment with aldosterone overnight, thermodynamic affinity was considerably increased, whether or not incubation was conducted in the presence of sodium in the outer solution; addition of glucose at the end of incubation, whereby sodium transport was stimulated further, failed to influence affinity as measured. The stoichiometry between sodium transport and oxygen consumption was, however, unchanged by aldosterone treatment in short-circuit conditions, neither was that fraction of aerobic metabolism unrelated to sodium transport influenced. It is concluded that the change observed with aldosterone can be directly ascribed to the hormone, as it is independent of glucose availability and of sodium transport. Aldosterone action, at least following prolonged incubation, therefore does not involve only an increase in apical conductance for sodium.