MULTINUCLEAR NMR-STUDIES OF INTRACELLULAR CATIONS IN PERFUSED HYPERTENSIVE RAT-KIDNEY

We have investigated hypertension-associated alterations in intracellular cations in the kidney by measuring intracellular pH, free Mg2+, free Ca2+, and Na+ concentrations in perfused normotensive and hypertensive rat (8-14 weeks old) kidneys using P-31, F-19, and double quantum-filtered (DQ) Na-23 NMR. The effects of both anoxia and ischemia on the Na-23 DQ signal confirmed its ability to detect changes in intracellular Na+. However, there was a sizable contribution of the extracellular Na+ to the Na-23 DQ signal of the kidney. The intracellular free Ca2+ concentration, measured using F-19 NMR and 5,5' difluoro-1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid, also increased dramatically during ischemia; the increase could be partly reversed by reperfusion. No significant differences were found between normotensive and hypertensive kidneys in the ATP level, intracellular pH, intracellular free Mg2+, and the Na-23 DQ signal or in the extent of the extracellular contribution to the Na-23 DQ signal. Oxygen consumption rates were also similar for the normotensive (5.02 +/-0.46-mu-mol of O2/min/g) and hypertensive (5.47 +/- 0.42-mu-mol O2/min/g) rat kidneys. The absence of a significant difference in intracellular pH, Na+ concentration, and oxygen consumption between normotensive and hypertensive rat kidneys suggests that an alteration in the luminal Na+/H+ antiport activity in hypertension is unlikely. However, a highly significant increase (64%, p < 0.01) in free Ca2+ concentration was found in perfused kidneys from hypertensive rats (557 +/- 48 nM, blood pressure = 199 +/- 5 mmHg, n = 6) compared with normotensive rats (339 +/- 21 nM, blood pressure = 134 +/- 6, n = 4) indicating altered renal calcium homeostasis in essential hypertension. An increase in intracellular free Ca2+ concentration without an accompanying change in the intracellular Na+ suggests, among many possibilities, that the Ca2+/Mg2+-ATPase may be inhibited in the hypertensive renal tissue.