INCREASED FUNCTION OF INHIBITORY NEURONAL M-2 MUSCARINIC RECEPTORS INDIABETIC RAT LUNGS

1 The function of inhibitory neuronal M-2 muscarinic receptors in diab etic rat lungs was investigated. 2 Neuronal M-2 muscarinic receptors i nhibit acetylcholine release from parasympathetic nerves. Thus, stimul ation of neuronal M-2 muscarinic receptors with muscarinic agonists, s uch as pilocarpine, inhibits acetylcholine release and vagally induced bronchoconstriction. In contrast, blockade of neuronal M-2 muscarinic receptors with selective M-2 muscarinic antagonists, such as AF-DX 11 6, potentiates acetylcholine release and vagally induced bronchoconstr iction. 3 Rats were made diabetic by streptozotocin (65 mg kg(-1), i.v .). After 7-14 days the rats were anaesthetized with urethane (1.5 g k g(-1), i.p.), tracheostomized, vagotomized, ventilated and paralysed w ith suxamethonium (30 mg kg(-1), i.v.). Some 7 day diabetic rats were treated with low doses of long acting (NPH) insulin (2 units day(-1), s.c.) for 7 days before experimentation. This dose of insulin was not sufficient to restore normoglycaemia in diabetic rats. Thus, insulin-t reated diabetic rats remained hyperglycaemic. 4 Distal electrical stim ulation (5-70 Hz, 6 s, 40 V, 0.4 ms) of the vagi caused bronchoconstri ction, measured as an increase in inflation pressure and bradycardia. In diabetic rats, vagally induced bronchoconstriction was significantl y depressed vs controls. In contrast, bronchoconstriction caused by i. v. acetylcholine was similar in diabetic and control animals. 5 The fu nction of neuronal M-2 muscarinic receptors was tested with the muscar inic agonist pilocarpine (0.001-100.0 mu g kg(-1), i.v.) and the antag onist AF-DX 116 (0.01-3.0 mg kg(-1), i.v.). Pilocarpine inhibited vaga lly-induced bronchoconstriction (30 Hz, 20-40 V, 0.4 ms at 6 s) and AF -DX 116 potentiated vagally-induced bronchoconstriction (20 Hz, 20-40 V, 0.4 ms at 6 s) to a significantly greater degree in diabetic rats c ompared to controls. 6 Both frequency-dependent vagally-induced bronch oconstriction and M-2 muscarinic receptor function could be restored t o nearly control values in diabetic rats treated with low doses of ins ulin. 7 Displacement of [H-3]QNB (1 nM) with the agonist carbachol (10 .0 nM-10.0 mM) from diabetic cardiac M-2 muscarinic receptors revealed a half log increase in agonist binding affinity at both the high and low affinity binding sites vs controls. In contrast, M-2 receptors fro m insulin-treated diabetic rat hearts showed no significant difference in binding affinity vs controls. 8 These data show that neuronal M-2 muscarinic receptors in the lungs have increased function in diabetic rats, suggesting that insulin modulates M-2 muscarinic receptor functi on.