Transport pathways for Na+ and Br- (Cl-) in noradrenaline-stimulated frog skin (Rana temporaria)

(1) Transport pathways for Na+ and Br- across noradrenaline-stimulated frog skin, in which the sodium channels in the apical membrane had been blocked with amiloride, were examined by pre-steady-state flux ratio experiments. (2) To analyse the experiments, equations are derived from which the fluxes through two parallel pathways can be determined if these differ with respect to the flux ratio as well as the mean passage time for the ion in question. (3) The fluxes of Na+ and Cl- were followed over a period of 3 h and it was found that about 1 h after addition of noradrenaline time-invariant fluxes could be achieved. In that period inward and outward tracer fluxes of Na+ and Br- (used as a substitute for Cl-) were followed during the build-up of isotope in the tissue until steady-state fluxes were attained. (4) The experiments showed that when the Na channels of the capital cells are blocked the glands provide the major pathway for sodium transport across the skin. The efflux of Br- can be separated into two components. A fast one through the mitochondria-rich cells, which is the pathway for passive anion fluxes, and a slower one through the glands. No influx through the slow transport pathway could be detected. In some skins exchange of Br- through the mitochondria-rich cells is revealed. An anion exchange mechanism also appears to be present in the basolateral membrane of the gland cells.