Characterization of muscarinic cholinergic receptors in human and dog cerebral arteries.

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Abstract
Various concentrations of acetylcholine (ACh) produced dose dependent relaxations of isolated, helical preparations of human cerebral arteries and these responses were blocked by atropine. The median effective concentration (EC50) of ACh was 6.1 +/- 0.2 X 10(-6)M. ACh produced dual responses in isolated dog cerebral arteries. ACh in low concentrations produced relaxation, and contraction occurred when the concentration was raised to 1 X 10(-5)M. The EC50 of ACh which produced relaxation, in dog cerebral arteries was 7.2 +/- 0.2 X 10(-7)M. Muscarinic cholinergic receptors in these arteries were analyzed directly using 3H-QNB as the ligand. The specific 3H-QNB binding to the arteries was saturable and of KD = 1.5 nM and Bmax = 93 fmol/mg protein in human cerebral arteries and KD = 0.59 nM, Bmax = 340 fmol/mg protein in dog cerebral arteries. Specific binding of 3H-QNB was displaced by muscarinic cholinergic agents. Ki values and Hill coefficients were as follows: QNB, 1.0 X 10(-9)M, 0.89; atropine, 1.1 X 10(-8)M, 0.95; ACh, 0.8 X 10(-8)M and 2.1 X 10(-6)M, 0.54; carbachol, 1.2 X 10(-7)M and 4.3 X 10(-5)M, 0.33 in human cerebral arteries and QNB, 0.55 X 10(-9)M, 0.85; atropine 0.9 X 10(-9)M, 1.00; ACh, 0.9 X 10(-9)M and 1.1 X 10(-6)M, 0.43; carbachol 6.3 X 10(-8)M and 1.1 X 10(-5)M, 0.36 in dog cerebral arteries. Endogenous choline acetyltransferase (ChAT) activity was 1.6 +/- 0.4 nmol/mg protein/hr in human cerebral arteries and 3.7 +/- 0.1 nmol/mg protein/hr in dog cerebral arteries.(ABSTRACT TRUNCATED AT 250 WORDS)