Cholinergic receptors and contraction of smooth muscle in canine portal vein.

The properties of cholinergic receptors in homogenates of canine portal vein were determined with the radioligand [3H]quinuclinidyl benzilate [( 3H]QNB), and correlated with the functional responses of that vessel to carbamylcholine (CCh) in vitro. [3H]QNB bound to a single population of sites in the homogenates, with a dissociation constant (Kd) of 125 pM (+/- 19), and a total receptor capacity of 24.4 fmol/mg of protein, which corresponded to 224 (+/- 67.3) fmol/g wet weight of the initial vascular wall sample. Competition of CCh with [3H]QNB in eight experiments revealed two binding sites of different affinity, present in about equal numbers with Kd = 0.48 microM and 31 microM (in the absence of 5'-guanylylimidodiphosphate), and Kd = 1.5 microM and 42 microM (in the presence of 100 microM 5'-guanylylimidodiphosphate). Vascular strips in vitro contracted in response to CCh, with a threshold of approximately 0.3 microM, ED50 of 1.87 microM and maximum response of 344 g/cm2 (65% of the maximum response to l-norepinephrine). Strips precontracted by l-norepinephrine or by high potassium concentrations were slightly (4%) relaxed by 0.1 microM CCh, but further contracted by higher concentrations. The contractile responses were not altered by removal of the endothelium, and were blocked by atropine but not hexamethonium. The results demonstrate the existence in this vein of muscarinic cholinergic receptors that mediate smooth muscle contraction, and the receptor properties resemble those reported in cardiac muscle and brain.