Effects of ischemia on intracellular sodium and phosphates in the in vivo rat liver

Metabolic factors that influence the transition from reversible to irreversible ischemic injury were studied in the rat liver in vivo with ³¹P-nuclear magnetic resonance (NMR) spectroscopy. Hepatic ischemia for 15, 35, or 65 min was produced by occlusion of the hepatic artery and portal vein in rats. Ischemia caused a rapid decrease in the ATP concentration ([ATP])- to-P(i) concentration ratio and pH within 5 min, but there was little change in these variables detectable by ³¹P-NMR with longer periods of ischemia. After reperfusion, the [ATP] and P(i) concentration returned toward normal values in livers exposed to 15 or 35 min of ischemia, but 65 min of ischemia were associated with only modest recovery in [ATP], and the [ATP] later decreased. Because the ³¹P-NMR spectrum was similar after brief compared with prolonged ischemia, it appears that neither ATP depletion, P(i) accumulation, nor acidosis predicts metabolic recovery. Hepatic intracellular Na⁺ was also measured in separate groups of animals by ²³Na-NMR in the presence of a shift agent, thulium (III) 1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetrakis(methylene-phosphonate) (TmDOTP^5-), and by atomic absorption spectroscopy. Under baseline conditions, the concentration of intracellular Na⁺ was 15.2 mM by atomic absorption spectroscopy and 16.5 mM by ²³Na-NMR. Although the ³¹P-NMR spectrum responded very rapidly to the onset of ischemia, intracellular Na⁺ concentration measured by ²³Na-NMR increased gradually but steadily at ~1.0 mM/min during early (up to 15 min) ischemia. These observations demonstrate that a rise in intracellular Na⁺ does occur during early ischemia, that TmDOTP^5- can be applied in vivo for analysis of intracellular Na⁺ in the ischemic liver, and that ³¹P-NMR spectroscopy is very sensitive to early ischemic injury.