DIFFERENTIAL-EFFECTS OF ACETYLCHOLINE ON CORONARY FLOW IN ISOLATED HYPOTHERMIC HEARTS FROM RATS AND GROUND-SQUIRRELS

This study was designed to determine whether cholinergic receptors are operative in the coronary vessels of a hibernating species (golden ma ntled ground squirrel, Spermophilus lateralis) and a nonhibernating sp ecies (rat, Rattus norvegicus) under normothermic and hypothermic cond itions. Coronary flow and left ventricular systolic pressure were meas ured in isolated perfused hearts from squirrels at 37, 20 or 7-degrees -C and from rats at 37 and 20-degrees-C. During cooling, rat hearts be came arrhythmic and failed between 15 and 12-degrees-C. Squirrel heart s remained functional at 7-degrees-C. Bolus injections of acetylcholin e (> 1.0 mug) caused significant coronary vasoconstriction in rat hear ts at 37 and 20-degrees-C. Similar treatment caused mild coronary vaso dilation in squirrel hearts at both temperatures. Squirrel hearts did not respond to acetylcholine at 7-degrees-C. The responses in both spe cies were blocked by atropine. Rat coronary vessels appear to contain muscarinic constrictor receptors similar to those described in humans, sheep, cattle and pigs. The coronary vessels of squirrels, by contras t, do not. In this latter species there appears to be a preponderance of muscarinic (possibly endothelial-relaxing-factor-linked) dilator re ceptors. Given that acetylcholine acts only as a mild vasodilator at h igher temperatures in squirrels, parasympathetic regulation of coronar y flow in the squirrel heart is unlikely, especially during hibernatio n.