Mechanisms of sympathetic enhancement and inhibition of parasympathetically induced salivary secretion in anaesthetized 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 (4 Hz) or acetylcholine infusion (10 micrograms kg-1 min-1, i.a.), was abolished by atropine (0.05 mg kg-1, i.a.), increased by phenylephrine infusion (0.25 microgram kg-1 min-1, i.a.) and depressed by clonidine infusion (0.75 microgram 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.