A Novel NHE1 from Red Blood Cells of the Winter Flounder

Abstract

The ubiquitously expressed plasma membrane Na + /H + exchanger, NHE1, plays a major role in the regulatory volume increase (RVI) process after cell shrinkage. NHE1 is also activated by acidification, stimulation of receptors for a wide range of hormones and growth factors, and inhibition of Ser/Thr protein phosphatases by DNA tumor viruses or compounds such as okadaic acid and calyculin A. [1][2][3][4][5][6] Consequently, NHE1 serves important physiological functions not just as a mechanism of pH-and cell volume homeostasis but also as a signal transducer which converts growth hormone signals into pH-and/or cell volume changes away from the steady state set point. In turn, these changes can modulate cell migration, 7 cell cycle control, 8 and programmed cell death. 9 In teleost red blood cells (RBCs), a major physiological function of NHE1 is the modulation of hemoglobin (Hb) O 2 affinity. Exposure of fish to hypoxia or exercise stress elicits release of catecholamines which act on -adrenergic receptors on the RBCs, resulting in increased cellular cAMP levels, and NHE1 activation. In trout, the receptor involved was recently identified as a novel, isoproterenol-sensitive 3 isoform. 10 Both the ensuing intracellular alkalinization and cell swelling contribute to increase the O 2 affinity of Hb, the former via the Bohr/Root effects, the latter as a result of the dilution of [Hb]. 11 Reflecting this special function, NHE1s from teleost RBCs tend to be robustly activated by cAMP but display little or no activity in response to osmotic shrinkage, especially at physiological P O2 . 12,13 Given the pleiotropic roles of NHE1, it is perhaps not surprising that excessive NHE1 activity has been found to play a major role in a number of important pathological states. One example is that of hypoxia/ischemia-induced cell damage. During ischemia/ reperfusion in the heart, activation of NHE1 increases [Na + ] i , resulting in reversal of Na + /Ca 2+ exchange and Ca 2+ overload. 14 NHE1 is also activated during brain hypoxia/