The effect of monovalent cations upon 45Ca fluxes in frog sciatic nerve

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Abstract
Ca fluxes in desheathed bundles of myelinated nerve of frog were investigated. An intracellular Ca concentration of 5 .times. 10-4 mol/kg (axoplasm) and an average transmembrane flux of 6 .times. 10-8 mol/kg s at an extracellular Ca concentration of 1 mM were found. Replacement of extracellular Na by isosmotic sucrose increased Ca influx 3-fold and decreased efflux by 50%. Similar (but significantly smaller) effects were observed when Tris or choline were substituted for Na. Li replaced Na without significant changes in Ca fluxes. Ca transmembrane fluxes in this preparation apparently were sensitive to changes in the Na gradient. The observed flux changes were too small to establish a Na-Ca exchange as the sole homeostatic mechanism for intracellular Ca. Although Li appeared to serve as a good Na substitute, and even Tris and choline interacted with Ca flux, the exchange did not show the specificity described for squid axon.