AJP - Renal Physiology, Vol 260, Issue 3 323-F330, Copyright © 1991 by American Physiological Society
Sodium transport in salamander proximal tubule at 5.5 degrees C
N. S. Morgunov and D. J. Hirsch
Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, Canada.
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.
