Determination of the driving force of the Na(+) pump in toad bladder by means of vasopressin.

Vasopressin stimulates Na(+) transport across toad bladder largely or entirely by decreasing the resistance to Na(+) entry into the transporting epithelial cells. Therefore, the hormone should induce proportional changes in short circuit current (I S ) and tissue conductance; the ratio of these changes should equal the driving force (E Na) of the Na(+) pump.Administration of vasopressin provided a rapid, reversible and reproducible technique for the measurement ofE Na. Values calculated forE Na ranged from 74 to 186 mV, in agreement with previously published estimates. The results were not dependent on the vasopressin concentration over a wide range of concentrations.Ouabain, an agent thought to inhibit specifically the Na(+) pump, decreased bothI S andE Na. On the other hand, amiloride, a diuretic thought to block specifically Na(+) entry, markedly reducedI S , without reducingE Na.It is concluded that vasopressin constitutes a probe for the rapid reproducible determination ofE Na under a wide variety of physiological conditions.