THE SIGNIFICANCE OF CHANGES IN THERMODYNAMIC AFFINITY INDUCED BY ALDOSTERONE IN SODIUM-TRANSPORTING EPITHELIA

The energetics of Na 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 1-2 h) or chronically (after .apprx. 18 h). By contrast, following treatment with aldosterone overnight, thermodynamic affinity was considerably increased, whether or not incubation was conducted in the presence of Na in the outer solution; addition of glucose at the end of incubation, whereby Na transport was stimulated further, failed to influence affinity as measured. The stoichiometry between Na transport and O2 consumption was, however, unchanged by aldosterone treatment in short-circuit conditions; nor was the fraction of aerobic metabolism unrelated to Na transport influenced. The change observed with aldosterone can be directly ascribed to the hormone, as it is independent of glucose availability and of Na transport. Aldosterone action, at least following prolonged incubation, therefore does not involve only an increase in apical conductance for Na.