NMR-STUDIES OF THE EFFECT OF HYPERGLYCEMIA ON INTRACELLULAR CATIONS IN RAT-KIDNEY

Alterations in intracellular cation concentrations in the kidney during hyperglycemia may play a role in a number of complications associated with diabetes mellitus such as renal hypertrophy and hypertension. In this study, we have investigated the effect of 25 mM glucose on intracellular pH and free Mg2+, free Ca2+, and Na+ concentrations in the perfused kidneys of Sprague-Dawley rats using P-31, F-19, and Na-23 triple-quantum filtered NMR. No significant alteration occurred in the intracellular free Mg2+ concentration, pH, or ATP concentration during hyperglycemia. However, a sizable (approximately 50%) increase in the intracellular Na+ concentration was inferred from Na-23 triple-quatum filtered NMR after 30-45 min of perfusion with 25 mM glucose. Intracellular free Ca2+, measured to be 390 +/- 15 nM at 5 mM glucose, increased significantly (95%; p < 0.001) to a value of 765 +/- 28 nm after 30 min of perfusion with 25 mM glucose. This effect of glucose was reversible. Only small increases (less-than-or-equal-to 20%) in free Ca2+ were found with addition of comparable concentrations of a nontransportable sugar (20 mM mannitol), indicating that glucose entry into the cell (through the Na+/glucose cotransporter) plays a role in causing the free Ca2+ increase. No effect on free Ca2+ was, however, seen with 1 mM ouabain, which caused a sizable increase in intracellular Na+, indicating that Na+/Ca2+ exchange does not play a significant role in the maintenance of low intracellular free Ca2+ in the kidney and that the observed increase in free Ca2+ is probably due to a decrease in the Ca2+/Mg2+-ATPase activity during hyperglycemia. Increased concentrations of Na+ and free Ca2+ during hyperglycemia may be involved in the mechanism of renal hypertrophy and hypertension, frequently associated with diabetes mellitus.