Sodium transport in salamander proximal tubule at 5.5 degrees C

Na+ transport and electrophysiology of isolated perfused proximal tubules of the salamander Ambystoma tigrinum were compared at 22 and 5.5 degrees C, a range over which these animals normally live. Both intracellular Na+ activity and basolateral membrane potential were unaffected by temperature, whereas transepithelial potential depolarized from -6.5 +/- 0.8 mV at 22 degrees C to -3.5 +/- 0.6 mV at 5.5 degrees C (P less than 0.05). Compared with 22 degrees C, reduction of temperature to 5.5 degrees C included major increases in apical membrane resistance (2,052 +/- 473 omega.cm2 to 18,464 +/- 2,667 omega.cm2) and basolateral membrane resistance (491 +/- 113 omega.cm2 to 1,780 +/- 256 omega.cm2) (P less than 0.01). Sequential increases of luminal glucose concentration allowed characterization of the Na(+) -glucose cotransporter at both temperatures. The Km was stable (2 mM), but the maximal activity (Vmax) at 5.5 degrees C of 167 peq/5 cm2 increased to 1,000 peq/5 cm2 at 22 degrees C (P less than 0.05). In parallel with this temperature sensitivity of apical Na+ entry, basolateral Na+ pump activity was reduced at low temperature. Rubidium uptake at 22 degrees C was reduced by 40% at 5.5 degrees C. The rate of decrease of intracellular Na+ activity when tubules were perfused with substrate-free solution was -2.6 +/- 0.7 mM/min at 5.5 degrees C, compared with -4.9 +/- 1.2 mM/min at 22 degrees C. We conclude that low temperature reduces both Na+ uptake and efflux, allowing stability of intracellular milieu despite reduction in net transepithelial transport.