Osmotic and Ionic Regulation in Amphibians

Abstract

Osmomineral regulation in four species of anurans from diverse habitats was studied under aquatic conditions. Ascaphus truei is aquatic, Rana pipiens is semiaquatic, Hyla regilla and Bufo boreas are terrestrial. The rate of osmosis is faster in terrestrial forms than aquatic forms. In μl cm⁻²h⁻¹ mean values were A. truei = 4.0, R. pipiens = 4.8, H. regilla = 5.6, and B. boreas = 16. Higher values reflect higher skin permeability to water and a higher osmolality of body fluids. Plasma Na⁺ is closely regulated at about 100 mM, which is over 100 times more concentrated than the pond water these species encounter in nature. Unidirectional fluxes of sodium were measured with 22Na or 24Na. At a given bath concentration of Na⁺ exchange rates were higher in the terrestrial species. Influx of Na⁺ displays saturation kinetics with increasing bath Na⁺ concentration. Over 90% of the influx is thermodynamically active. Compared with terrestrial species, aquatic species are characterized by high affinity-low capacity transport systems. Sodium efflux consists of renal and diffusive components. No exchange diffusion of Na⁺ was found. Renal efflux of Na⁺ was lower in aquatic species than terrestrial species (A. truei = 0.15, R. pipiens = 0.10, H. regilla = 0.35, B. boreas = 0.26 μM 10 g⁻¹h⁻¹). The integumentary loss of Na⁺ is a function of the transepithelial electrical potential difference which is a function of bath Na⁺ concentration. Body fluids become more electropositive to the bath by about 35 mV per decade increase in bath Na⁺ concentration. The constant field equation accurately predicts the increase in integumentary Na⁺ effux with increasing bath Na⁺ concentration. The most aquatic species, Ascaphus, had the lowest integumentary efflux (5.2 nM cm⁻²h⁻¹). Bufo, a terrestrial form, the highest (41 nM cm⁻²h⁻¹). All species maintained chloride balance in dilute baths in the face of an unfavorable electrochemical gradient. Chloride influx, measured with 36Cl, is predominantly carrier mediated, consisting of exchange diffusion and active transport in all species but B. boreas, in which exchange diffusion was absent. Eflux of Cl⁻ was lowest in Ascaphus and highest in Bufo. Renal loss was lower in aquatic species than terrestrial species (A. truei = 0.12, R. pipiens = 0.07, H. regilla = 0.22, and B. boreas = 0.16 μM 10g⁻²h⁻¹). The efflux was further partitioned into diffusive and exchange diffusive components. Permeability of the skin to Cl⁻, based on diffusive efflux, was lower for aquatic Ascaphus (0.4 × 10⁻⁸ cm s⁻¹) than for terrestrial Bufo (5 × 10⁻⁸cm s⁻¹). While anurans appear to possess common basic mechanisms for osmomineral regulation in freshwater, significant adaptive modifications have evolved which may affect distribution of species.