DIFFERENCES IN THE EFFECTS OF VASOPRESSIN AND OXYTOCIN ON FELINE GASTRIC CORPUS MOTILITY - SELECTIVE ACTION OF VASOPRESSIN ON LONGITUDINAL MUSCLE

The effect of vasopressin and oxytocin on the contractile activity of preparations isolated from the feline gastric corpus wall was investig ated. Vasopressin (1.5 x 10(-9)-2.1 x 10(-7) M), but not oxytocin, evo ked concentration-dependent tonic contractions only of longitudinal mu scle strips. At the same time, vasopressin (1.5 x 10(-9)-2.1 x 10(-7) M) potentiated the magnitude of amplitudes, but not the frequency, of spontaneous contractions. Both the vasopressin V1 receptor antagonist d(CH2)(5)-(Me)(2)-Tyr-AVP and the predominantly vasopressin V2 recepto r antagonist d(CH2)(5), D-Ile(2), Ile(4)-AVP, the non-selective muscar inic receptor antagonist, atropine, the predominantly selective muscar inic M1 receptor antagonist, pirenzepine, the predominantly selective muscarinic M2 antagonist, methoctramine, the predominantly selective m uscarinic M3 receptor antagonist, para-fluoro-hexahydrosiladifenidol, and the calcium channel blocker, nifedipine, but not the ganglion bloc king agent, mecamylamine, depressed or blocked the tonic contractions induced by vasopressin. Among the antagonists, only atropine and nifed ipine inhibited the spontaneous contractions. On the other hand, the a nticholinesterase, physostigmine, potentiated both the vasopressin-ind uced tonic and spontaneous contractions. With regard to the receptors, the vasopressin-induced tonic contractions are mediated at least in p art through vasopressin V-1 and V-2 receptors, non-selective muscarini c and selective muscarinic M-1, M-2 and M-3 receptors. The increase in amplitudes of spontaneous contractions is mediated only via-nonselect ive muscarinic receptors. Vasopressin receptors appear to be located m ostly pre-synaptically, although the direct effect of vasopressin on p ost-synaptic receptors cannot be excluded. The pA(2) values suggests r ather V-1a than V-1b vasopressin receptor subtype involvement in tonic contractions vasopressin had produced. The tonic as well as spontaneo us contractions are calcium-dependent. In addition, these results poin t to the existence of non-selective muscarinic receptors, which partic ipate in the regulation of both tonic and spontaneous contractions, wh ile muscarinic M-1, M-2 and M-3 receptors subserve only the tonic cont ractions. (C) 1998 The Italian Pharmacological Society.