Sodium transport in salamander proximal tubule at 5.5°C

Na⁺ transport and electrophysiology of isolated perfused proximal tubules of the salamander Ambystoma tigrinum were compared at 22 and 5.5°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°C to -3.5 ± 0.6 mV at 5.5°C (P < 0.05). Compared with 22°C, reduction of temperature to 5.5°C included major increases in apical membrane resistance (2,052 ± 473 Ω·cm² to 18,464 ± 2,667 Ω·cm²) and basolateral membrane resistance (491 ± 113 Ω·cm² to 1,780 ± 256 Ω·cm²) (P < 0.01). Sequential increases of luminal glucose concentration allowed characterization of the Na⁺-glucose cotransporter at both temperatures. The K(m) was stable (2 mM), but the maximal activity (V(max)) at 5.5°C of 167 peq/5 cm² increased to 1,000 peq/5 cm² at 22°C (P < 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°C was reduced by 40% at 5.5°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°C, compared with -4.9 ± 1.2 mM/min at 22°C. We conclude that low temperature reduces both Na⁺ uptake and efflux, allowing stability of intracellular milieu despite reduction in net transepithelial transport.