Effects of dietary sodium and potassium on mineral balance in captive meadow voles (Microtus pennsylvanicus)

Na+ and K+ balance of captive meadow voles (Microtus pennsylvanicus) were studied at dietary levels of 100 and 1000 ppm Na+ and 6000, 18 000, and 36 000 ppm K+. Na+ loss and Na+ balance were not significantly affected by high dietary K+ content; this result does not support the hypothesis that K+ loading is a major cause of the apparent Na+ imbalances shown by some herbivores during spring and early summer. Voles maintained or approached Na+ balance on the low Na+ diets. The relative size of the zona glomerulosa of the adrenal gland was not increased on the low Na+ diet. In contrast, high dietary K+ had a significant effect on zona glomerulosa size; this pattern and other data suggest that excreting K+ under conditions of high dietary K+ may represent a much greater physiological challenge to meadow voles than conserving Na+ on low Na+ diets. There was no indication that bone Na+ played an important role in Na+ balance in this species. Levels of Na+ and K+ in hair varied significantly with respective dietary levels.