Delayed activation of calcium pump during transient increases in cellular Ca 2+ concentration and K + conductance in hyperpolarizing human red cells

Abstract

The net Ca 2+ influx was increased in human red cells in suspension by adding moderate concentrations of the Ca 2+ ionophore A23187, and due to the increased cellular Ca 2+ concentration ([Ca] i) the K + channels opened (the ‘Gardos effect’). At low K + concentration and with the protonophore CCCP in the buffer-free medium the cells hyperpolarized and the extracellular pH (pH 0) increased, enhancing the A23187-mediated net Ca 2+ influx. This elicited a prolonged response, viz. a primary transient increase of pH 0 and [Ca] i followed by one or more spontaneous pH 0 and [Ca] i transients. We explored the pump-mediated Ca 2+ efflux by blocking the A23187-mediated Ca 2+ flux with CoCl 2 at appropriate times during the prolonged response. The Ca 2+ pumping was higher during the descendent than during the ascendent phase of the primary transient at equal values of [Ca] i. The data were anlyzed using a mathematical model that accounts for the prolonged oscillatory response, including pH 0 and [Ca] i. In conclusion, the activation of the Ca 2+ pump is delayed due to slow binding of cellular calmodulin, which is a hysteretic response to a rapid increase of the cellular Ca 2+ concentration. This mechanism may be important for generation and execution of transient signals in other types of cell.