α2-adrenoceptors inhibit antidiuretic hormone-stimulated Na+ absorption across tight epithelia (Rana esculenta)

In the present study we examined the effect of alpha -adrenergic regulation of active transepithelial Na+ absorption across the isolated frog skin epithelium. alpha -Adrenergic stimulation was achieved by addition of the adrenergic agonist noradrenaline in the presence of the beta -adrenergic blocker propranolol. alpha -Adrenergic stimulation inhibited basal as well as antidiuretic hormone (ADH)-stimulated Na+ transport. The ADH-induced increase in Na+ transport was accompanied by a membrane depolarisation due to an increase in the apical Na+ permeability. The subsequent application of noradrenaline inhibited the Na+ transport and repolarised the membrane potential, suggesting that alpha -adrenergic stimulation had reduced the apical Na+ permeability. The inhibition was abolished by the alpha (2)-adrenergic antagonist yohimbine whereas it was insensitive to the alpha (1)-adrenergic antagonist prazosin. alpha -adrenergic stimulation had no effect on the cytosolic free [Ca2+] ([Ca2+](i)). Incubation of the epithelium in the presence of ADH increased the cellular adenosine 3',5'-cyclic monophosphate (cAMP) content, an increase which was abolished by a-adrenergic activation. The effect of alpha -adrenergic stimulation on cAMP production was abolished by the alpha (2)-adrenergic antagonist yohimbine. We conclude that the noradrenaline-induced inhibition of the ADH-stimulated Na+ absorption and cAMP content is mediated by activation of alpha (2)-adrenoceptors. The data further indicate that the principal cells of the epithelium do not express al-adrenoceptors. The noradrenaline-induced inhibition of the ADH-stimulated Na+ transport was concentration dependent, with 0.24 +/-0.03 muM eliciting a half-maximal response. This alpha (2)-adrenergic-mediated down-regulation of Na+ absorption is achieved at a concentration of noradrenaline which begins to activate the NaCl secretion via the skin glands. The alpha (2)-adrenoceptors therefore appear to have considerable physiological importance.