Transmural inhomogeneity of extracellular [K+] and pH and myocardial energy metabolism in the isolated rat heart during acute global ischemia; dependence on gaseous environment

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Abstract
We investigated in the isolated rat heart the influence of the gas surrounding the globally ischemic heart on transmural inhomogeneity of energy metabolism, extracellular K+ accumulation, and change of extracellular pH. Hearts were made ischemic in 100% N2 (N2-ischemia), 100% O2 (O2-ischemia) or 100% CO2 (CO2-ischemia). We measured: 1) Midmural, subepicardial, and epicardial changes of extracellular [K+] and pH during successive 6-min periods of global ischemia, and 2) content of creatinephosphate (CrP) in consecutive tissue sections of 100 μm, from the subepicardium after 10 min of ischemia. During O2-ischemia both extracellular [K+] and change of pH in the subepicardium are significantly less than in the midmyocardium. During N2-ischemia only minor differences exist in [K+] and pH between the subepicardium and the midmyocardium. During CO2-ischemia midmural and subepicardial [K+] are similar to those during N2-ischemia. The midmural change of pH resembles that during N2-ischemia; subepicardial change of pH, however, was slightly larger. Midmural changes in [K+] and pH were not influenced by the nature of the surrounding gas. After 10 min of O2-ischemia a gradient of tissue content of CrP extends from the epicardium (CrP about 30 μmoles/g dry weight) to a distance of about 1000 μm (CrP 1 μmoles/g dry weight). In N2-and CO2-ischemia a CrP gradient is absent; CrP is appreciably less than 1 μmoles/g dry weight at any distances from the epicardium. We conclude that diffusion of O2 into the myocardium and of CO2 from the myocardium affects transmural gradients of [K+], pH, and energy metabolism during ischemia. Local availability of O2 increases the capacity of the ischemic tissue to generate high energy phosphates and mitigates ischemia-induced changes of transsarcolemmal ion gradients.