Na K ATPase mediated seasonal variation of sodium transport in Necturus kidney

The rate of net fluid reabsorption in the proximal tubule of Necturus maculosus was measured with a modification of the Gertz split droplet technique. Half time of reabsorption (T(1/2)) was twice as rapid in winter animals, (T(1/2) = 22.7 ± 1.6 min) as in summer animals (T(1/2) = 40.7 ± 3.7, summer 1971; 46.9 ± 3.8 summer 1972). This change in proximal tubular function apparently involved an important homeostatic mechanism since there was no difference in measured values of serum osmolality, sodium, or potassium in relationship to season. Further studies performed to examine the mechanism responsible for the variation in proximal tubule function demonstrated an increase in specific activity of Na K ATPase in winter (4.23 ± 0.41 μmol Pi/mg protein per h, winter 1971-72 3.92 ± 0.26, winter 1972 to 73), when proximal function was enhanced, and reduced enzyme levels in summer (2.58 ± 0.22 μmol Pi/mg protein per h), when transport was slowed. Since the administration of ouabain, in a dose sufficient to inhibit Na K ATPase, caused marked reduction of fluid reabsorption in the proximal tubule, it seems likely that the enzyme is linked to net absorption in this nephron segment. These studies suggest that Na K ATPase is involved in bulk movement of sodium in the proximal tubule of N. maculosus and extended earlier observations on the role of this enzyme in environmental adaptions of lower vertebrates. (30 references).