Enhanced sensitivity to stimulation of sodium transport and cyclic AMP by antidiuretic hormone after Ca2+ depletion of isolated frog skin epithelium

Abstract

The role of Ca²⁺ in the stimulation by antidiuretic hormone (ADH) of active sodium transport across the isolated epithelium of frog skin was investigated. This has been done by bathing the blood side with Ca²⁺-free solution containing 0.1mm EGTA. This Ca²⁺ depletion halved the resistance but had no significant effect on the short-circuit current (SCC). The sensitivity of both cAMP- and SCC-stimulation to ADH was increased 40-fold by Ca²⁺ depletion. Sensitivity to stimulation by theophylline was only changed a little, while stimulation by exogenous cAMP was completely unaltered. The increase in sensitivity to ADH was dependent on the duration of preincubation in Ca²⁺-free solution, which indicates that a slowly exchanging Ca²⁺ pool is involved in the determination of sensitivity to ADH. We suggest this pool is of cellular origin and the increased sensitivity is due to the decrease of a Ca²⁺ inhibition of the ADH-stimulated adenylate cyclase. But a direct effect of Ca²⁺ on binding of ADH to the receptor cannot be excluded. Our results are not compatible with the hypothesis that entry of extracellular Ca²⁺ is an obligatory step in the natriferic action of ADH, although it may be so in the hydroosmotic action of ADH. We also found the maximal response to ADH to be higher after Ca²⁺ depletion. This is in agreement with the hypothesis of intracellular Ca²⁺ as a modulator of the sodium permeability of the outward-facing membrane.