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

Abstract.

In the present study we examined the effect of α-adrenergic regulation of active transepithelial Na⁺ absorption across the isolated frog skin epithelium. α-Adrenergic stimulation was achieved by addition of the adrenergic agonist noradrenaline in the presence of the β-adrenergic blocker propranolol. α-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 α-adrenergic stimulation had reduced the apical Na⁺ permeability. The inhibition was abolished by the α₂-adrenergic antagonist yohimbine whereas it was insensitive to the α₁-adrenergic antagonist prazosin. α-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 α-adrenergic activation. The effect of α-adrenergic stimulation on cAMP production was abolished by the α₂-adrenergic antagonist yohimbine. We conclude that the noradrenaline-induced inhibition of the ADH-stimulated Na⁺ absorption and cAMP content is mediated by activation of α₂-adrenoceptors. The data further indicate that the principal cells of the epithelium do not express α₁-adrenoceptors. The noradrenaline-induced inhibition of the ADH-stimulated Na⁺ transport was concentration dependent, with 0.24±0.03 µM eliciting a half-maximal response . This α₂-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 α₂-adrenoceptors therefore appear to have considerable physiological importance.