INDUCTION OF EXPRESSION OF THE SODIUM-HYDROGEN EXCHANGER IN RAT MYOCARDIUM

Objective: The aim was to examine the regulation of the cardiac Na+/H+ exchanger NHE-1 isoform mRNA in response to ischaemia and acidosis in the mammalian myocardium. Methods: Male Sprague Dawley rat hearts were perfused in a non-circulated retrograde fashion according to the Langendorff method. Hearts were perfused for 3 h at flow rates of either 10 ml.min(-1) (control), or 3, 1, or 0 ml.min(-1) (ischaemia) followed by 5 min of reperfusion. Hearts were immediately frozen in liquid Nz, and stored at -80 degrees C until ready for RNA isolation. Northern blot analysis was used to examine expression of the NHE-1 isoform of the Na+H+ exchanger message in these isolated perfosued hearts. Activity of the Na+/H+ exchanger was assessed in primary cultures of neonatal rat myocytes under either control conditions or after treatment with chronic, low external pH. Results: A decrease in developed tension and an increase in resting tension was observed which was dependent upon the severity of the ischaemic episode. Low flow ischaemia of 3 ml.min(-1) caused increased Na+/H+ exchanger message levels, while perfusion at more reduced flow rates eliminated the increase. Treatment of primary cultures of isolated myocytes with low external pH resulted in increased ability to recover from an acute acid load. Conclusions: Low flow ischaemia can increase the Na+/H+ exchanger message in the intact mammalian myocardium. More severe ischaemia prevents the increase, suggesting that severely damaged tissue may not be capable of the ischaemic response. Primary cultures of isolated myocytes can respond to chronic low external pH by increasing Na+/H+ exchanger activity.