Intracellular Free Magnesium and High Energy Phosphates in the Perfused Normotensive and Spontaneously Hypertensive Rat Heart A 31P NMR Study

We have employed 31P nuclear magnetic resonance (NMR) spectroscopy to examine the relationship between cytosolic free Mg2+ ([Mg2+]in), intracellular pH, high energy phosphates, and genetic hypertension using the Wistar-Kyoto rat (WKY) as a control and the spontaneously hypertensive rat (SHR) as a model for essential hypertension. The mean systolic blood pressures (measured using the tail cuff method) of control and hypertensive rats (aged 7 to 12 weeks) were 113 +/- 4 mm Hg (mean +/- 2 SE, n = 14) and 162 +/- 5 mm Hg (mean +/- 2 SE, n = 17), respectively. Intracellular free Mg2+ levels were significantly depleted in the isolated Langendorff perfused hypertensive rat hearts (452 +/- 39 mumol/L, mean +/- 2 SE, n = 17) compared to control hearts (756 +/- 52 mumol/L, n = 14); however, intracellular pH did not differ in the SHR hearts (7.02 +/- 0.03, mean +/- 2 SE, n = 7) compared with controls (7.03 +/- 0.03, n = 7). Although we could not demonstrate a statistically significant difference in the levels of P-creatine or ATP, intracellular Pi was two-fold higher (5.71 +/- 2.28 mmol/L v 2.92 +/- 0.66 mmol/L, n = 4) and the phosphorylation potential, [MgATP]/[MgADP][Pi], was correspondingly lower (3.0 X 10(4) +/- 1.6 x 10(4) v 8.3 X 10(4) +/- 1.4 X 10(4) (mol/L)-1, n = 4) in SHR compared to WKY hearts. These data demonstrate free magnesium depletion in heart muscle and indicate an alteration in cardiac bioenergetics in essential hypertension.