Endothelium-mediated negative dromotropic effects of intravascular acetylcholine

Acetylcholine acting through specific muscarinic membrane receptors causes a negative dromotropic effect and, in blood vessels, causes a vasodilation which results from its action on the endothelial cells via release of nitric oxide (NO). We decided to study this effect in isolated Krebs-Henseleit retrogradely perfused guinea pig hearts. A pair of stimulating electrodes was placed in the right atrium and to record the auricular-ventricular interval (A-V delay) one recording electrode was placed on the left atrium and the other on the tip of the ventricle. Hearts were paced at a rate of 3.8+/-0.1 Hz and perfused at a coronary flow rate of 9+/-0.25 ml/min. To obtain dose-response curves, single doses (as boluses) of acetylcholine were infused and the maximal A-V delay induced by each dose was determined. Perfusion of agents that inhibit NO accumulation (L-Arginine methyl ester (L-NAME) (0.5 mM)) or oxyhemoglobin (6 microM) caused displacement of the acetylcholine dose-response curve downward and to the right. Perfusion of NO-sparing agents like superoxide dismutase and dithiothreitol caused an upward and leftward displacement. Infusion of NO solutions or a NO donor (diethylamine-nitric oxide [DEA-NO]) caused a dose-dependent negative dromotropic effect. In contrast, inhibition of the prostaglandin metabolic pathway by Indomethacin (0.01 mM) caused potentiation of acetylcholine effects which were reversed when it was co-perfused with L-NAME. When endothelial intravascular muscarinic receptors were selectively blocked by perfusion of a non-permeable macromolecule: dextran ( > 2000 kDa) covalently complexed to the receptor blocker (3-(2'-aminobenzhydryloxy) tropane)), the negative dromotropic effect of intravascular acetylcholine was diminished in a concentration-dependent manner up to complete blockade. Our data indicate that the dromotropic effect caused by intracoronary administration of acetylcholine is the result solely of activation of intravascular endothelial muscarinic receptors, that nitric oxide and prostaglandins are non-synergistic endothelial mediators of this effect and that there may be an interaction between NO and prostaglandin metabolic pathways.