Original Article

Volume-sensitive K⁺/Cl^- Cotransport in Rabbit Erythrocytes: Analysis of the Rate-limiting Activation and Inactivation Events

Correspondence to: Michael L. Jennings, Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, 4301 W. Markham St., Mail Slot 505, Little Rock, AR 72205. Fax:501-686-8167 E-mail:jenningsmichaell{at}exchange.uams.edu.

Released online: 15 November 1999

The kinetics of activation and inactivation of K⁺/Cl^- cotransport (KCC) have been measured in rabbit red blood cells for the purpose of determining the individual rate constants for the rate-limiting activation and inactivation events. Four different interventions (cell swelling, N-ethylmaleimide [NEM], low intracellular pH, and low intracellular Mg²⁺) all activate KCC with a single exponential time course; the kinetics are consistent with the idea that there is a single rate-limiting event in the activation of transport by all four interventions. In contrast to LK sheep red cells, the KCC flux in Mg²⁺-depleted rabbit red cells is not affected by cell volume. KCC activation kinetics were examined in cells pretreated with NEM at 0°C, washed, and then incubated at higher temperatures. The forward rate constant for activation has a very high temperature dependence (E_a ~ 32 kCal/mol), but is not affected measurably by cell volume. Inactivation kinetics were examined by swelling cells at 37°C to activate KCC, and then resuspending at various osmolalities and temperatures to inactivate most of the transporters. The rate of transport inactivation increases steeply as cell volume decreases, even in a range of volumes where nearly all the transporters are inactive in the steady state. This finding indicates that the rate-limiting inactivation event is strongly affected by cell volume over the entire range of cell volumes studied, including normal cell volume. The rate-limiting inactivation event may be mediated by a protein kinase that is inhibited, either directly or indirectly, by cell swelling, low Mg²⁺, acid pH, and NEM.

Key Words: osmoregulation, red blood cell, phosphatase, kinase, kinetics

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