Characterization of muscarinic receptors of the rabbit ear artery smooth muscle and endothelium.

Muscarinic receptors of the rabbit ear artery were characterized by observing the effect of the subtype selective antagonist pirenzepine on functional responses and radioligand binding. Pirenzepine has been shown to bind with high affinity to muscarinic receptors of certain brain regions and peripheral ganglia (M1 subtype) and with low affinity to receptors of the heart and upper gastrointestinal tract (M2 subtype). The affinity (pKB) of pirenzepine for the muscarinic sites of the endothelium was determined by the competitive antagonism of the relaxation response to methacholine. Schild analysis gave a pKB of 6.5 (320 nM) which is consistent with the low affinity, M2, subtype of muscarinic receptor. Removal of the endothelium eliminates any response to muscarinic agonists but does not decrease the density of muscarinic binding sites determined by binding of the specific ligand (-)-[3H]quinuclidinyl benzilate. This indicates a second group of muscarinic receptors most probably located on vascular smooth muscle cells for which there is no known function. The pKi for pirenzepine at these sites, as determined by the inhibition of (-)-[3H]quinuclidinyl benzilate binding, was 6.26 (550 nM) which is also consistent with a low affinity subtype. Thus, both types of vascular muscarinic binding sites, those on the endothelium which mediate relaxation and those on the vascular smooth muscle cells, are of the low affinity, M2, subtype.