SWELLING ACTIVATION OF K-CL COTRANSPORT IN LK SHEEP ERYTHROCYTES - A 3-STATE PROCESS

K-Cl cotransport in LK sheep erythrocytes is activated by osmotic swel ling and inhibited by shrinkage. The mechanism by which changes in cel l volume are transduced into changes in transport was investigated by measuring time courses of changes in transport after osmotic challenge s in cells with normal and reduced Mg concentrations. When cells of no rmal volume and normal Mg are swollen, there is a delay of 10 min or m ore before the final steady-state flux is achieved, as there is for sw elling activation of K-Cl cotransport in erythrocytes of other species . The delay was shown to be independent of the extent of swelling. The re was also a delay after shrinkage inactivation of cotransport. Reduc ing cellular Mg concentration activates cotransport. Swelling of low-M g cells activates cotransport further, but with no measurable delay. I n contrast, there is a delay in shrinkage inactivation of cotransport in low-Mg cells. The results are interpreted in terms of a three-state model: A half arrow right over half arrow left k21 k12 B half arrow r ight over half arrow left k32 k23 C in which A state, B state, and C s tate transporters have relatively slow, intermediate, and fast transpo rt rates, respectively. Most transporters in shrunken cells with norma l Mg are in the A state. Swelling converts transporters to the B state in the rate-limiting process, followed by rapid conversion to the C s tate. Reducing cell Mg also promotes the A --> B conversion. Swelling of low-Mg cells activates transport rapidly because of the initial pre dominance of B state transporters. The results support the following c onclusions about the rate constants of the three-state model: k21 is t he rate constant for a Mg-promoted process that is inhibited by swelli ng; k12 is not volume sensitive. Both k23 and k32 are increased by swe lling and reduced by shrinkage; they are rate constants for a single p rocess, whereas k12 and k21 are rate constants for separate processes. Finally, the A --> B conversion entails an increase in J(max) of the transporters, and the B --> C conversion entails an increase in the af finity of the transporters for K.