SUBSTANCE-P EFFECTS ON BLOOD-FLOW, FLUID TRANSPORT AND VASOACTIVE INTESTINAL POLYPEPTIDE RELEASE IN THE FELINE SMALL-INTESTINE

1. Substance P (SP) infusions were given close I.A. to the feline small intestine in vivo in a dose that produced plasma concentrations of 1-5 mu M. This infusion regularly evoked a net fluid secretion measured with a gravimetric technique. Concomitantly, the release into blood of vasoactive intestinal polypeptide (VIP), a putative neurotransmitter of the enteric nervous system, increased. 2. The SP-induced fluid secretion was blocked by tetrodotoxin (7 mu g close I.A.), a blocker of fast sodium channels in excitable tissues, and hexamethonium (10 mg (kg body wt)(-1), I.V.), a nicotinic receptor antagonist, suggesting that the SP effect was mediated by the enteric nervous system. In line with this it was shown that the SP-evoked release of VIP was also significantly diminished by hexamethonium. 3. Close I.A. infusions of methionine enkephalin (Met-enkephalin; 7-23 nmol min(-1)) or electrical stimulation of the sympathetic nerve fibres (6 Hz) to the intestine markedly diminished net fluid secretion and the release of VIP caused by SP given close I.A. 4. The cyclo-oxygenase inhibitor diclofenac (5 mg (kg body wt)(-1), I.V.) or the histamine-1 receptor antagonist pyrilamine (10 mg (kg body wt)(-1), I.V.) did not influence the fluid secretion caused by SP, indicating that the effects of SP were not due to the actions of prostaglandins or histamine. 5. It is proposed that SP activates a nervous reflex arch that we have shown to be activated by various luminal stimuli, including cholera toxin. The proposed reflex is made up of at least three neurons: an afferent neuron going from the mucosa to the myenteric plexus, a cholinergic interneuron and a VIP-ergic neuron from the submucosal plexus controlling the enterocytes. Met-enkephalin and the sympathetic nerves decrease the fluid secretion by pre- or postsynaptic inhibition of reflex nervous activity.