Aspartate transporter activity in hypertrophic rat heart and ischaemia-reperfusion injury

Abstract

This study's rationale was that the expression and activity of aspartate transporters in hypertrophied hearts might be different from normal hearts, which could affect the use of aspartate in myocardial protection of hypertrophied hearts. mRNA expression of system Xag⁻ transporters in hearts from normal (Wistar Kyoto) and hypertrophied (spontaneously hypertensive rat) rats was investigated by RT-PCR. EAAT3 protein expression in isolated cells and vesicles from normal and hypertrophied hearts was investigated by Western blotting. The same vesicles were also used to measure aspartate uptake. The effects of 0.5 mmol l⁻¹ aspartate supplementation on cardiac performance during ischaemia–reperfusion were investigated in isolated and perfused hearts. Both normal and hypertrophied hearts expressed EAAT1 and EAAT3 mRNA. EAAT3 protein expression was significantly greater in cells and vesicles from hypertrophied hearts compared to normal hearts. The velocity (Vmax) of aspartate uptake was faster at 24.4 ± 2.2 pmol mg⁻¹ s⁻¹ in vesicles from hypertrophied hearts compared to 8.2 ± 0.8 pmol mg ⁻¹ s ⁻¹ (P<0.001, t test, n= 6, means ± s.e.m.) in normal heart vesicles. The affinity (Km) was similar for both preparations. When recoveries were matched, 0.5 mmol l⁻¹ aspartate addition reduced reperfusion injury and increased functional recovery of hypertrophied hearts but not normal hearts. This was associated with a greater preservation of ATP, glutamate and glutamine and less lactate production during ischaemia in aspartate-treated hypertrophied hearts compared to all other experimental groups. These results suggest that increased aspartate transporter expression and activity in hypertrophy helps facilitate aspartate entry into hypertrophied cardiomyocytes, which in turn leads to improved myocardial protection.