INTERACTIVE EFFECTS OF K- IMPLICATIONS FOR EXERCISE(, ACID, NOREPINEPHRINE, AND ISCHEMIA ON THE HEART )

We tested the hypothesis that cardiac ischemia uncouples the beneficia l interaction among hyperkalemia, acidosis, and raised plasma catechol amines when these chemicals are changed to mimic their exercise levels . Potassium chloride, lactic acid, and norepinephrine (NE) were infuse d intravenously for 2 min into anesthetized, artificially ventilated, thoracotomized rabbits during either occlusion of the left circumflex artery (3 min; n = 10) or after a period of prolonged ischemia (20 min ; n = 7) that led to a small infarction. NE (1 mu g . kg(-1). min(-1) iv) offset the negative cardiac effects of hyperkalemia (up to 8.7 +/- 0.7 mM) and acidosis (arterial pH 7.09 +/- 0.03) in normal hearts. Ca rdiac performance was not significantly depressed by either acute or c hronic ischemia before any infusions. However, the protective effect o f NE during acute ischemia or after prolonged ischemia with hyperkalem ia and acidosis was substantially reduced. These results show that car diac ischemia attenuates the protective action of NE and increases the depressive effects of hyperkalemia and acidosis. Whether myocardial i schemia amplifies the cardiotoxic effects of hyperkalemia and acidosis during vigorous exercise by attenuating the beneficial effect of cate cholamines remains to be determined.