Effects of variations in food intake on renal sodium pump activity and itsgene expression in Psammomys kidney

Psammomys obesus lives in an arid environment and feeds on saltbush. When a nimals are fed a laboratory diet, urine osmolarity drops. To explore the me chanism(s) of water conservation, we measured renal function, kidney solute content, Na-K-ATPase activity, and mRNA in several groups: group I (saltbu sh diet, 18 g/day, 4.2 g protein); group II (laboratory diet, 10 g/day, 1.8 g protein); and group III, the same as group I, and group IV, the same as group II, both plus a 1-day fast. Urine osmolarity was 2,223 +/- 160, 941 /- 144, 1,122 +/- 169 and 648 +/- 70.9 mosM in groups I, II, III, and IV, r espectively. Tissue osmolarities in cortex, outer medulla, and inner medull a, respectively, were 349 +/- 14, 644 +/- 63, and 1,152 +/- 34 mu osM/mg ti ssue in group I; 317 +/- 24, 493 +/- 17, and 766 +/- 60 mu osM/mg tissue in group II; 335 +/- 6,582 +/- 15, 707 +/- 35 mu osM/mg tissue in group III; and 314 +/- 18, 490 +/- 22, and 597 +/- 29 mu osM/mg tissue in group IV. Th ere were no differences in Na-K-ATPase activity and mRNA in cortex and in m edulla between groups I and II, whereas in group III Na-K-ATPase activity a nd mRNA increased in cortex and outer medulla. These results suggest a key role for urea in corticomedullary osmotic gradient of Psammomys. The absenc e of differences in Na-K-ATPase activity and mRNA between groups I and II d espite differences in tissue sodium concentrations is consistent with Na-K- ATPase-independent Na absorption. Increased Na-K-ATPase activity and mRNA i n fasting suggest transition to Na-K-ATPase-dependent Na transport.