This report describes the influence of fluid flow and osmotically induced volume changes on Na+-Ca2 exchange (NCX) activity in transfected CHO cells. Exchange activity was measured as Na+-dependent Ca2 or Ba2 fluxes using the fluorescent probe fura-2. When exchange activity was initiated by superfusing Ba2+-containing solutions over the cells for a 20 s interval, a high rate of Ba2+ uptake was observed while the solution was being applied but the rate of Ba2+ uptake declined > 10-fold when the solution flow ceased. Ba2+ efflux in exchange for extracellular Na+ or Ca2+ (Ba2+-Ca2+ exchange) was similarly biphasic. During NCX-mediated Ca2+ uptake, a rapid increase in cytosolic [Ca2+ ]to a peak value occurred, followed by a decline in [Ca2+] i to a lower steady-state value after solution flow ceased. When NCX activity was initiated by an alternate procedure that minimized the duration of solution flow, the rapid phase of Ba2+ influx was greatly reduced in magnitude and Ca2+ uptake became nearly monophasic. Solution superfusion did not produce any obvious changes in cell shape or volume. NCX-mediated Ba2+ and Ca2+ influx were also sensitive to osmotically induced changes in cell volume. NCX activity was stimulated in hypotonic media and inhibited in hypertonic media; the osmotically induced changes in activity occurred within seconds and were rapidly reversible. We conclude that NCX activity is modulated by both solution flow and osmotically induced volume changes.
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