Relationship between the shape and the membrane potential of human red blood cells

Microscopic observations of isotonic suspensions of human red blood cells demonstrate that cell shape is unaltered when the transmembrane electrical potential, or Em, is set in the range -85 to + 10 mV with valinomycin at varied external K+, or Ko X Em was measured with the fluorescent potentiometric indicator, diS-C3(5), as calibrated by a delta pH method. Repeating Glaser's experiments in which echinocytosis was attributed to hyperpolarization, we found that at low ionic strength the pH-dependent effects of amphotericin B appear to be unrelated to Em. The effects of increased intracellular Ca2+, or Cac, on echinocytosis and on Em are separable. With Ca ionophore A23187 half-maximal echinocytosis occurs at greater Cao than that which induces the half-maximal hyperpolarization associated with Ca-induced K+ conductance (Gardos effect). Thus, cells hyperpolarized by increased Cac remain discoidal when Ca is below the threshold for echinocytosis. With A23187 and higher Cao, extensive echinocytosis occurs in cells which are either hyperpolarized or at their resting potential. The Ca-activation curve for echinocytosis is left-shifted by low Ko, a new observation consistent with increased DIDS-sensitive uptake of 45Ca by hyperpolarized cells. These results support the following conclusions: (1) the shape and membrane potential of human red blood cells are independent under the conditions studied; (2) in cells treated with A23187, the Gardos effect facilitates echinocytosis by increasing Cac.