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Ethylisopropylamiloride diminishes changes in intracellular Na, Ca and pH in ischemic newborn myocardium
H Liu1, P M Cala, S E Anderson
1Department of Human Physiology, University of California, Davis, CA, 95616-8644, USA.
Insights
Inhibition of the sodium-hydrogen exchanger (Na/H) using ethylisopropylamiloride (EIPA) protects newborn rabbit hearts from ischemic injury. EIPA treatment reduces intracellular sodium and calcium overload, preserving heart function and limiting damage.
Area of Science:
- Cardiovascular Physiology
- Neonatal Cardiology
- Biochemistry
Background:
- Myocardial ischemic injury in adults is partly due to proton-stimulated Na/H exchange, increasing intracellular sodium (Nai) and calcium ([Ca]i).
- Inhibiting Na/H exchange is hypothesized to reduce ion accumulation and improve cardiac function post-ischemia.
Purpose of the Study:
- To investigate the role of Na/H exchange in myocardial ischemic injury in newborn rabbit hearts.
- To test whether inhibiting Na/H exchange with ethylisopropylamiloride (EIPA) protects neonatal hearts from ischemia-reperfusion injury.
Main Methods:
- Isolated 4-7-day-old rabbit hearts were Langendorff-perfused.
- Nuclear Magnetic Resonance (NMR) spectroscopy measured intracellular pH (pHi), Nai, [Ca]i, and high-energy phosphates.
- Hearts underwent global ischemia followed by reperfusion, with or without EIPA treatment prior to ischemia.
Main Results:
- EIPA treatment maintained higher pHi during ischemia compared to controls.
- EIPA limited the rise in Nai and [Ca]i during ischemia and improved their recovery during reperfusion.
- EIPA preserved cellular ATP, reduced inorganic phosphate accumulation, improved left ventricular developed pressure recovery, and decreased creatine kinase release.
Conclusions:
- The findings in newborn hearts support the hypothesis that Na/H exchange contributes to ischemic injury.
- Inhibition of Na/H exchange by EIPA offers cardioprotection in the neonatal heart, similar to adult hearts.
Abstract:
Numerous studies suggest that in adult hearts myocardial ischemic injury is in part the result of proton stimulation of Na/H exchange which increases intracellular Na (Nai) and thus leads to increases in intracellular Ca concentration ( [Ca]i) due to changes in Na/Ca exchange flux. Corollary to the hypothesis, inhibition of Na/H exchange diminishes Na and Ca accumulation and improves heart function after ischemia. To test this hypothesis and its corollary in newborn hearts, NMR spectroscopy was used to measure intracellular pH (pHi), Nai, [Ca]i, and high energy phosphates in isolated, 4-7-day-old rabbit hearts, Langendorff-perfused with Krebs-Henseleit solution at pH 7.4+/-0.5 equilibrated with 95% O2/5% CO2 at 36+/-1 degrees C. Control hearts were perfused for 30 min before initiating 40 min of global ischemia followed by 40 min of reperfusion. In a second group of hearts ethylisopropylamiloride (EIPA-10 microM) was added to the perfusate 20 min before global ischemia to inhibit Na/H exchange. After 15 min ischemia, pHi in EIPA-treated hearts (6.41+/-0.04) was higher than that of the control hearts (6.20+/-0.08; P<0.05). EIPA also limited the increase in Nai and [Ca]i during ischemia and improved Nai and [Ca]i recovery during reperfusion (P<0.05). Nai (mEq/kg dry weight) rose from 18. 1+/-3.2 to 110.6+/-14.0 and recovered to 53.3+/-12.3 in the control group. The corresponding Nai values for EIPA-treated hearts were 16. 2+/-2.4, 39.6+/-9.6 and 12.6+/-3.5, respectively. In control hearts [Ca]i (nM/l) rose from 332+/-42 to 1157+/-89 and recovered to 842+/-55, whereas in EIPA-treated hearts the values were 255+/-32, 616+/-69 and 298+/-34, respectively. EIPA also preserved cellular ATP during ischemia and reperfusion and diminished inorganic phosphate during reperfusion (P<0.05). Finally, EIPA treatment improved recovery of left ventricular developed pressure (68.2+/-8.9 v 16.2+/-3.6% of control) and limited myocardial injury as indicated by decreased total creatine kinase release during reperfusion (348+/-132 v 2432+/-639 IU/g dry weight). Thus, as in adults, the results from newborn hearts are consistent with the hypothesis.