Complementary role of vasoactive intestinal polypeptide (VIP) and acetylcholine for cat submandibular gland blood flow and secretion II. Effects of cholinergic antagonists and VIP antiserum

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Abstract
The effects of atropine and hexamethonium on submandibular vasodilation, secretion and VIP release in response to parasympathetic nerve stimulation were studied in cats. It was found that salivary secretion was completely atropine sensitive at all frequencies. The vasodilatory response was characterized by an initial phase (most marked at lower frequencies) followed by a maintained phase (most pronounced at high frequencies). Atropine reduced the initial phase at all frequencies while the maintained phase was reduced only at low stimulation frequencies. At 15 Hz the maintained blood flow response was paradoxically increased after atropine, particularly with regard to the duration. The increase in blood flow after atropine was accompanied by an about eight fold increase in VIP output as compared to control stimulations at 15 Hz. This may suggest that the acetylcholine levels regulate the VIP release in a feed-back system via muscarinic auto-receptors. No increase in VIP output by atropine was, however, observed at 2 Hz while a small but significant increase was found at 6 Hz. Treatment with VIP antiserum reduced both phases of the vasodilation as well as the secretion in response to stimulation at 2 Hz. Thus, VIP and acetylcholine seem to be important for both phases of vasodilation as well as for salivary secretion. The relative contributions of VIP and acetylcholine are, however, hard to evaluate since atropine appears to increase VIP release. This fact complicates the characterization of cholinergic and noncholinergic vasodilator mechanisms by the use of atropine. Hexamethonium treatment abolished both the vasodilation, the secretion and the VIP release seen during parasympathetic nerve stimulation implying that it was preganglionic and that the preganglionic transmitter is acetylcholine which activates postganglionic neurons via nicotinic receptors. Somatostatin had no blocking effect on parasympathetic mechanisms in the cat submandibular gland.