Atropine-resistant secretory responses of the ovine parotid gland to reflex and direct parasympathetic stimulation.

A study was made of mechanisms involved in the parasympathetic control, especially of protein concentration in, and also flow, of ovine parotid secretion. The effects of distension of the caudal thoracic oesophagus were characterized in four anaesthetized sheep each with a unilateral chronic superior cervical sympathetic ganglionectomy. Oesophageal distension produced increases in both the flow and protein concentration of saliva from the control glands similar to that from the sympathectomized glands indicating the response was mediated by the parasympathetic innervation. Atropine, in doses which completely abolished increases in flow and protein concentration produced by bethanechol infusion, reduced, but did not abolish, the responses to oesophageal distension providing evidence of non-cholinergic transmission. Electrical stimulation of the parotid (parasympathetic, postganglionic) nerve during atropinization produced ca. 70% increases in flow and ca. 100% increases in protein concentration of parotid saliva thus giving further evidence of such a mechanism. Intracarotid infusion of vasoactive intestinal polypeptide (VIP) evoked responses with similarities to those observed with oesophageal distension and nerve stimulation during atropinization. This observation adds to other evidence indicating that VIP acts as a parasympathetic neurotransmitter involved in the control of salivary secretion.