The present study using isolated constant flow-perfused heart from adult (8 month-old) and aged (26 month-old) Fischer 344 rats for the first time demonstrated a striking enhancement of the negative chronotropic, inotropic and coronary vasoconstriction responses of the aging heart to postsynaptic cholinergic stimulation.;The approaches used and the results demonstrated were the following. (i) Acetylcholinesterase activity measured by the method of Ellman et al (1961) declined significantly in the atria and ventricles of the aged compared to adult rats. (ii) No age-related difference was seen in muscarinic receptor number in the atria and ventricles as assessed by {dollar}\lbrack\sp3\rm H\rbrack QNB{dollar} binding, but the muscarinic receptor binding affinity for carbachol, a cholinergic agonist, was increased in the atria but not ventricles of aged compared to adult rats. (iii) The relative amount of {dollar}\rm Gi\alpha{dollar} protein measured by Western immunoblotting and ADP-ribosylation techniques was significantly greater in the atria and ventricles of the aged compared to adult rats. The fidelity of the signal transduction through muscarinic receptor-linked Gi protein, as judged from the guanine nucleotide-induced decrease in receptor affinity for carbachol, was unaltered with aging in atria and ventricles. (iv) It was found that aging was accompanied by (a) enhancement in carbachol-induced hyperpolarization of the resting membrane potential in atria but not ventricles and (b) more pronounced carbachol-induced shortening of the action potential duration measured at 50% of repolarization in the ventricles but not in the atria. (v) Aging did not alter the carbachol-induced prolongation of atrioventricular conduction time measured from electrocardiograms. All of the age-related differences mentioned above may contribute to the increased negative chronotropic and inotropic responses of the aged heart to cholinergic stimuli. The age-associated increase in the coronary vascular response to a cholinergic stimulus may be caused by a relatively greater influx of extracellular {dollar}\rm Ca\sp{lcub}2+{rcub},{dollar} since the age-related difference in the coronary vascular response was attenuated in the presence of verapamil, a calcium channel blocker. Neither impaired synthesis of endothelium-derived relaxing factor (EDRF) nor reduced sensitivity of coronary vascular smooth muscle to EDRF contributes to the enhanced coronary vasoconstrictive effect to cholinergic stimulation since there was no age-related difference in the L-NMMA-induced coronary vasoconstriction in the absence and presence of carbachol. The age-related increase in the coronary response was specific for the cholinergic receptor as the {dollar}\alpha{dollar}-adrenergic response of the coronary vasculature was not altered with aging.;In summary, the cholinergic response of the heart is increased in the aged compared to the adult rats. The mechanisms for the enhanced postsynaptic response is multi-factorial, and may be attributable to the age-related changes in acetylcholinesterase activity, characteristics of the muscarinic receptor, content of the Gi protein, and electrophysiological properties of the atria and ventricles. (Abstract shortened by UMI.
