MECHANISMS OF SYMPATHETIC ENHANCEMENT AND INHIBITION OF PARASYMPATHETICALLY INDUCED SALIVARY SECRETION IN ANESTHETIZED DOGS

1 The effects of superimposed and continuous sympathetic nerve stimulation on submandibular parasympathetic salivation were investigated in anaesthetized dogs. 2 Superimposed sympathetic nerve stimulation (1-2 min) initially enhanced and later inhibited salivary secretion induced by parasympathetic nerve stimulation (2-8 Hz) in glands with uncontrolled blood supply or constant-flow vascular perfusion. Propranolol (0.05 mg kg(-1), i.a.) did not affect the diphasic sympathetic action whereas phentolamine (0.1 mg kg(-1), i.a.) abolished it. Prazosin (0.025 mg kg(-1), i.a.) greatly lessened the initial enhancement while yohimbine (0.025 mg kg(-1), i.a.) alleviated the late inhibition. 3 Salivary secretion, induced by parasympathetic nerve stimulation (4Hz) or acetylcholine infusion (10 mu g kg(-1) min(-1), i.a.), was abolished by atropine (0.05 mg kg(-1), i.a.), increased by phenylephrine infusion (0.25 mu g kg(-1) min(-1), i.a.) and depressed by clonidine infusion (0.75 mu g kg(-1) min(-1), i.a.). Hexamethionium (12.5 mg kg(-1), i.a.) abolished the nerve-induced secretion but had no effect on the acetylcholine-induced secretion. 4 Continuous background sympathetic nerve stimulation decreased parasympathetic nerve-induced salivary secretion in glands with uncontrolled blood supply or constant-flow vascular perfusion. 5 These results show that parasympathetic salivation can be modified by the sympathetic system at the postsynaptic level; enhancement is via alpha(1)-adrenoceptors whereas inhibition is via alpha(2)-adrenoceptors.