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 mantled ground squirrel, Spermophilus lateralis) and a nonhibernating species (rat, Rattus norvegicus) under normothermic and hypothermic conditions. Coronary flow and left ventricular systolic pressure were measured 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 became arrhythmic and failed between 15 and 12 degrees C. Squirrel hearts remained functional at 7 degrees C. Bolus injections of acetylcholine (> 1.0 microgram) caused significant coronary vasoconstriction in rat hearts at 37 and 20 degrees C. Similar treatment caused mild coronary vasodilation in squirrel hearts at both temperatures. Squirrel hearts did not respond to acetylcholine at 7 degrees C. The responses in both species 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 contrast, do not. In this latter species there appears to be a preponderance of muscarinic (possibly endothelial-relaxing-factor-linked) dilator receptors. Given that acetylcholine acts only as a mild vasodilator at higher temperatures in squirrels, parasympathetic regulation of coronary flow in the squirrel heart is unlikely, especially during hibernation.