Stimulation of the Sodium Pump in Frog Bladder by Oxytocin

Abstract

THE stimulating effect of the neurohypophyseal hormones on the transport of sodium ions across amphibian epithelial layers in vitro has been established for frog skin¹ as well as for toad bladder². The site of action of the hormone is uncertain, however. Because the hormone increases the sodium pool in the bladder it has been suggested that the entry of sodium into the cells is facilitated by the hormone acting at the mucosal membrane³. Similarly, with frog skin, it was inferred from the kinetics of the exchange of labelled sodium that the neurohypophyseal hormone enhanced the entry of sodium across the outer membrane into the epidermal cells⁴. In neither case was it suggested that the hormone stimulated the sodium pump directly, as the sodium pump is localized at the opposite face of the epithelial layer and expels sodium from the cells. Rather, this pump was supposed to become more saturated by the increase in concentration of intracellular sodium. Other workers⁵, however, have found that neurohypophyseal hormones recover the normal direction of the short-circuit current in frog skins bathed from the epidermal side by very dilute sodium sulphate solution. They have thus shown that an active step, whether localized at the outer or the inner border of the cells, must be directly stimulated by the hormone.