Alpha adrenergic drugs inhibit [3H]-QNB binding to muscarinic receptors of rat heart, brain and parotid gland membranes

Alpha adrenergic agonists and antagonists as clonidine, guanfacine, yohimbine, phenylephrine and prazosin inhibited the [3H]-QNB binding to rat brain cortex muscarinic acetylcholine receptor (mAChR, M-1 subtype), heart (M-2 subtype) and parotid gland homogenate (M-3 subtype) in a dose-dependent competitive fashion. Ki values were between 10(-6) and 10(-3) M. Hill coefficients were about 1. No correlation was found between mAChR inhibiting capacity of these drugs and their activity on alpha adrenergic receptors. In contrast, other transmitters, as dopamine, GABA, glutamic acid, histamine, serotonin, isoproterenol and platelet activating factor (PAF) did not affect the QNB binding.