Effect of extracellular Ca2+, K+ and OH- on erythrocyte membrane potential as monitored by the fluorescent probe 3,3'-dipropylthiodicarbocyanine.

Changes in fluorescence intensity of thiodicarbocyanine, DiS-C3(5), were correlated with direct microelectrode potential measurements in red blood cells from Amphiuma means and applied qualitatively to evaluate the effects of extracellular Ca2+, K+ and pH on the membrane potential of human red cells. Increasing extracellular [Ca2+] from 1.8 to 15 mM causes a K+-dependent hyperpolarization and decrease in fluorescence intensity in Amphiuma red cells. Both the hyperpolarization and fluorescence change disappear when the temperature is raised from 17 to 37 degrees C. No change in fluorescence intensity is observed in human red cells with comparable increase in extracellular Ca2+ in the temperature range 5-37 degrees C. Increasing the extracellular pH, however, causes human red cells to respond to an increase in extracellular Ca2+ with a significant but temporary loss in fluorescence intensity. This effect is blocked by EGTA, quinine or by increasing extracellular [K+], indicating that at elevated extracellular pH, human erythrocytes respond to an increase in extracellular Ca2+ with an opening of K+ channels and associated hyperpolarization of the plasma membrane.