A Depolarizing Electrogenic Pump in Frog Muscle

Surface fibers of Na-enriched muscles, bathed in K-free Na- containing saline, depolarize when the external sodium is replaced isosmotically with Tris. Subsequent addition of strophanthidin to the Na- and K- free bathing solution results in a hyperpolarization. If the main effect of strophanthidin in these experiments is to inhibit the electrogenic pump, then it may be concluded that before addition of the drug the net pump current was inward-going. This contrasts with the usual observation that cell membranes generally have hyperpolarizing electrogenic pumps which are caused by Na-K pump ratios greater than unity. These direct electrical measurements suggest that the enzyme machinery responsible for the active transport of sodium and potassium is not always obligated to extrude sodium out of the cell faster than potassium is actively accumulated. Since the muscles retain virtually all of their internal potassium when bathed in Na- and K-free saline, the passively lost potassium would appear to be largely regained by an efficient back-pump process, which may only modestly stimulate active Na efflux.