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Comparison of the movements of rubidium and potassium between blood and tissues, and between blood and external medium, was undertaken in seawater adapted rainbow trout (Salmo gairdneri) and sculpin (Leptocottus armatus). Qualitative observations suggested that the two ions behaved similarly in exchanges within both fish. More than 90% of an injected dose of both ions disappeared from the blood within 1 h. Complete equilibration of both required about 5 h, and the decrease in blood concentration during the last 4 h followed similar kinetics. After injection and equilibration of 86Rb into trout the specific activity of most tissue pools (c.p.m. 86Rb/microeq tissue K) was close to that of blood (c.p.m. 86Rb/microeq plasma K). Skeletal muscle was the sole exception; its specific activity was less than 15% that of plasma even 4–5 h after injection. Injection of ouabain into trout caused cells to lose both 86Rb and K at similar rates. The same thing occurred when [K] in the external medium was raised abruptly. Exchanges of the two ions between blood and the external medium were also similar. Both ions were extruded and taken up rapidly across the gills. Double-labelling (86Rb and 42K fluxes measured simultaneously) showed that JK/JRb = 1.27 for both influx and efflux in trout. The same value was obtained for effluxes in sculpin. Exposing sculpin to seawater lacking Na and K caused both 42K and 86Rb efflux to fall by 75–80%; 86Rb efflux decreased similarly in trout. Alkali metal-free sea water also caused the voltage across the trout gill to change from +10 mV to −20 mV, and in the sculpin from +25 mV to −25 mV. Addition of either Rb or K to the ion-deficient medium partially repolarized the gill in both fish. Repolarization was slightly greater after K repletion than after Rb repletion. These observations suggest that 86Rb is an adequate tracer for potassium movement in seawater-adapted fish. The gills of trout and sculpin are a little more permeable to K than to Rb, and 86Rb fluxes must be multiplied by 1.3 to provide accurate estimates of transbranchial K movement. The quantitative relationship between JK and JRb within the animal was not established, but some data suggest that cell membranes in fish, like the gills, are somewhat more permeable to K than to Rb.

Potassium Metabolism in Seawater Teleosts : I. The Use of 86Rb as a Tracer for Potassium