Atropine‐resistant submandibular responses to stimulation of the parasympathetic innervation in the anaesthetized ferret.

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Abstract
1. Submandibular salivary and vascular responses to stimulation of the peripheral end of the chorda‐lingual nerve at 20 Hz continuously for 60 min were investigated in anaesthetized ferrets, in which the sympathetic innervation to the gland was cut, in the presence and absence of atropine (2.0 mg kg‐1). 2. Both the increase in submandibular salivary flow and protein output, which occurred in response to nerve stimulation, were substantially reduced following the administration of atropine, the latency was greatly increased thereby, and both responses were more transient but neither was abolished by atropine. The fall in submandibular vascular resistance was not significantly affected by atropine, either in respect of extent or duration. 3. Chorda‐lingual stimulation produced an increase in the output of vasoactive intestinal peptide (VIP), substance P (SP) and calcitonin gene‐related peptide (CGRP) in the submandibular venous effluent blood. Each of these responses was maximal within the first 10 min after the onset of stimulation and declined thereafter. The time‐scales of both the CGRP and SP responses were similar to those of the atropine‐resistant secretory responses, both being quite short‐lived, whereas the output of VIP (like the atropine‐resistant vascular response) was significantly greater than the basal value throughout the whole of the 60 min period of stimulation. 4. The CGRP response was completely abolished by pre‐treatment with atropine, whereas the outputs of both VIP and SP were significantly enhanced thereby. Both the submandibular vascular and secretory responses to chorda‐lingual stimulation were almost completely suppressed following the administration of hexamethonium, and there was then no detectable release of peptidergic agonists from the gland. 5. The atropine‐resistant submandibular salivary secretory responses were completely abolished by pre‐treatment with a tachykinin inhibitor [( D‐Arg1, D‐Cl2 Phe5, Asn6, D‐Trp7,9, Nle11]‐SP; 0.75 mg kg‐1) without affecting the fall in submandibular vascular resistance. 6. Following pre‐treatment with hexamethonium, I.V. bolus injections of methacholine, SP and CGRP elicited increases in submandibular blood flow and secretion of saliva. VIP caused an increase in blood flow without overt secretion, although it is known to increase secretion of protein and to potentiate the secretory response to SP. Taken together, all these results are consistent with the contention that VIP contributes to the vasodilator response to stimulation of the para‐sympathetic innervation in this gland and that both SP and CGRP are likely to contribute to the secretory response.