Effect of amiloride on contractility of rat cardiac muscle exposed to chronic hypercapnia and acute acidosis.

Papillary muscle preparations from rats with normal arterial oxygen and carbon dioxide tensions and from rats which had been maintained with normal oxygen tension but with hypercapnia for 28 days (FICO2 = 5%) were subjected to acute hypercapnia with or without amiloride, a competitive inhibitor of the Na+/H+ pump. Acclimatisation to hypercapnia reduced the slope of the line relating log tension against the extracellular pH from 0.96(SEM0.06) to 0.71(0.07) (p less than 0.02). Amiloride increased the slope in unacclimatised muscle to 1.39(0.09), p less than 0.001 and in muscles acclimatised to hypercapnia to 1.03(0.13), p less than 0.05. The slope in acclimatised muscles was significantly less steep than in unacclimatised muscle (p less than 0.05). The sarcolemmal Na+H+ exchanger is important in the protection of rat cardiac muscle against acute respiratory acidosis.