Is the red cell calcium pump electrogenic?

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Abstract
In inside-out vesicles of high K permeability, prepared from human red cell membranes, volume changes accompanying the action of the Ca2+ pump were measured by recording the intensity of light scattered by a suspension of these vesicles. Replacing Cl- by the impermeant gluconate anion changed swelling into shrinking. Assuming that the Cl- media 2 Cl- accompany 1 Ca2+ moved by the pump and in gluconate media 2 K+ are exchanged for 1 Ca2+ resulted in a good agreement between relative Ca2+ transport rate obtained from the volume change and from direct measurement of 45Ca uptake in the 2 media. The fact that it was possible to change cotransport of Ca+ with Cl- into countertransport of Ca2+ for K+ ruled out that within the pump there was an obligatory coupling of Ca2+ movement with movement of another ion species (including the proton). The Ca2+ pump must be electrogenic. The combination of measurement of volume change with direct measurement of 45Ca movement yielded 5-6 .mu.l/mg protein for the volume of the vesicles.