Regulatory Volume Decrease in Carp Red Blood Cells: Mechanisms and Oxygenation-Dependency of Volume-Activated Potassium and Amino Acid Transport

Hypo-osmotic swelling of carp red blood cells (RBCs) induced a regulatory volume decrease (RVD), which restored the original cell volume within 140 min in oxygenated RBCs, whereas volume recovery was incomplete in deoxygenated RBCs. The complete RVD in oxygenated RBCs resulted from a sustained volume-activated release of K⁺, Cl⁻ and amino acids (AAs). In the absence of ouabain, the cells also lost Na⁺ as released K⁺ was partially regained via the Na⁺/K⁺ pump. Inorganic osmolytes contributed approximately 70%, and organic osmolytes approximately 30 %, to the RVD of oxygenated RBCs. Oxygenation in isotonic medium per se activated a K⁺ efflux from the RBCs. Hypo-osmotic cell swelling stimulated an additional K⁺ release. The oxygenation-activated and the volume-activated K⁺ efflux were both inhibited by DIDS and by the replacement of Cl⁻ with NO₃⁻, showing that both types of K⁺ efflux were Cl⁻-dependent and probably occurred via the same K⁺/Cl⁻ cotransport mechanism. Once activated by oxygenation, the K⁺/Cl⁻ cotransport was further stimulated by cell swelling. Deoxygenation inactivated the oxygenation-induced Cl⁻-dependent K⁺ release and cell swelling was not a sufficient stimulus to reactivate it significantly. In deoxygenated RBCs, the volume-induced K⁺ release was transient and primarily Cl⁻-independent and, in the absence of ouabain, the cell K⁺ content recovered towards control values via the Na⁺/K⁺ pump. The Cl⁻-independent K⁺ efflux seemed to involve K⁺/H⁺ exchange, but other transport routes also participated. Swelling-activated AA release differed in kinetics between oxygenated and deoxygenated RBCs but was important for RVD at both oxygenation degrees. Approximately 70 % of the AA release was inhibited by DIDS and substitution of NO₃⁻ for Cl⁻ produced a 50 % inhibition, suggesting that the AA permeation was partly Cl⁻-dependent. In oxygenated RBCs, a reduction in pH lowered the volume-activated Cl⁻-dependent K⁺ efflux but not the AA efflux. In deoxygenated RBCs, the acute volume-stimulated K⁺ and AA release were both increased by acidification. The data are discussed in relation to possible transducer mechanisms and physiological implications.