Amiloride-sensitive sodium-hydrogen exchange in osmotically shrunken rabbit red blood cells

As part of a detailed study of cell volume regulation in high-potassium mammalian erythrocytes, we have characterized ouabain-insensitive sodium transport in normal and osmotically shrunken rabbit red cells. In cells of normal volume and physiological pH, there is no amiloride-inhibited component of the sodium efflux (into either sodium-containing or sodium-free media). Osmotic shrinkage activates an amiloride-sensitive (50% inhibitory concentration = 10(-5) M) sodium transport system that can catalyze net sodium movement in either direction. This system appears to be distinct from the sodium-sodium (sodium-lithium) counter-transporter that operates in cells of normal volume. Replacement of chloride with acetate does not inhibit the sodium flux, but replacement with either nitrate or thiocyanate is inhibitory. An inward sodium gradient in shrunken cells induces a net uphill efflux of acid equivalents, indicating that the sodium transport is a sodium-hydrogen exchange. However, a sevenfold inward gradient of hydrogen ions (pHo = 6.4; pHi = 7.2) does not stimulate net sodium efflux in shrunken cells. This suggests that the extracellular affinity of the transport site for hydrogen ions is high, and that there is an extracellular noncompetitive inhibitory site for proton binding. Bilateral pH reduction stimulates an amiloride-inhibitable sodium flux in cells of normal volume; this indicates that, as has been found in kidney, brain, and lymphocytes, there is an intracellular protonation site that can activate the transport. Shrinkage of the cells shifts the pH dependence of the transport, suggesting that part of the signal for the osmotic activation of the transport is a shift in the pKa of this modifier site.