Eosin, a potent inhibitor of the plasma membrane Ca pump, does not inhibit the cardiac Na-Ca exchanger.

The Na-Ca exchanger and the sarcolemmal/plasma membrane (SL(PM)) Ca pump are the two major pathways for Ca transport to the extracellular space in many cells. In cardiac myocytes, the Na-Ca exchanger appears to be responsible for a greater portion of this Ca flux [Bassani, R. A., et al. (1992) J. Physiol. 453, 591-608]. However, the respective contributions of these two transporters are not as well-defined in all tissues (e.g., smooth muscle). We propose that eosin (tetrabromofluorescein) may be a useful tool for quantitatively determining the proportion of Ca transported by the Na-Ca exchanger vs the SL(PM) Ca pump in various cells. Eosin is the most potent inhibitor known for the SL(PM) Ca pump (IC50 approximately 0.3 microM in red blood cell inside-out vesicles); unlike the Na/K and H/K pumps, eosin does not compete with ATP for the SL(PM) Ca pump [Gatto, C., & Milanick, M. A. (1993) Am. J. Physiol. 264, C1577-C1586]. In the present study, we have shown that eosin was a potent inhibitor of the cardiac SL(PM) Ca pump (IC50 approximately 1 microM); in contrast, eosin (< or = 20 microM) did not inhibit the cardiac Na-Ca exchanger. In experiments where Ca was being transported by both the SL(PM) Ca pump and the Na-Ca exchanger simultaneously, eosin effectively eliminated the Ca pump-mediated transport. In addition, we show that eosin can permeate the human red cell membrane; cell permeability is an attractive feature for using eosin in whole cell studies. We conclude that eosin can be used for determining the role that the SL(PM) Ca pump plays in whole cell Ca homeostasis.