Acute and chronic temperature dependence of Na/H exchange activity of Pimephales promelas gills.

Na/H exchangers (NHE) mediate at least part of Na entry into gill epithelia via Na/NH exchange. For homeostasis, Na entry into and exit via Na/K ATPase from gill epithelia must balance. Na/K ATPase activity is reduced in cold- compared to warm-acclimated freshwater temperate fish. We hypothesized gill NHE activity is greater in warm- than cold-acclimated fish when measured at acclimation temperatures, and NHE activity displays a temperature dependence similar to Na/K ATPase. Since NHE mRNA expression does not differ, we measured the Na-dependence of pH-induced Na fluxes in gill vesicles from warm- and cold-acclimated fathead minnows at 20 and 7 °C, and calculated maximum transport rates (V) and Na Ks. We also measured NH-induced Na fluxes and Na-induced H fluxes. In vesicles from warm-acclimated fish, NHE Vs were 278 ± 33 and 149 ± 23 arbitrary unit/s (au/s) and Na Ks were 12 ± 4 and 6 ± 4 mmol/l when assayed at 20 and 7 °C (p < 0.004), respectively. In vesicles from cold-acclimated fish, Vs were 288 ± 35 and 141 ± 13 au/s and Na Ks 17 ± 5 and 7 ± 2 mmol/l when assayed at 20 and 7 °C (p < 0.002), respectively. Na-induced H fluxes were 98 ± 8 and 104 ± 26 au/s in warm- and cold-acclimated fish assayed at 20 °C, respectively. Na/NH exchange was 120 ± 11 and 158 ± 13 au/s in warm- and cold-acclimated fish, respectively. Conclusions: Gill NHE activity was greater in warm- than cold-acclimated fish assayed at acclimation temperatures. The temperature dependence of NHE activity was similar in both groups, but differed from that reported for Na/K ATPase suggesting complex mechanisms to maintain Na homeostasis.