Noradrenaline induced secretion of nonelectrolytes through frog skin

Addition of noradrenaline (4 x 10(-5) M) to the inner bathing fluid in the skin of the frog Rana esculenta results in increased unidirectional fluxes of urea, thiourea, N-methyl-thiourea, N-N'-dimethylthiourea and mannitol. Fluxes towards the external medium (phi o) undergo a much greater increase than those moving the opposite direction (phi i). The effect of noradrenaline on phi o is higher for urea and thiourea than mannitol, while its effect on phi o thiourea derivatives is related to lipid solubility. This phenomenon does not occur for phi i of the same molecules. FCCP (10(-6) M) pretreatment strongly inhibits the noradrenaline effect on phi o. In skin pretreated with colchicine (2 x 10(-5) M) both urea fluxes are increased to the same extent by noradrenaline. Noradrenaline is concluded to exert two separate effects: (1) a change in permeability in both directions; (2) a secretion of nonelectrolytes towards the external fluid. Such secretion is most probably associated with the hormone-induced secretion of fluid and electrolytes, perhaps mediated by an exocytotic mechanism.