Mechanism of action of somatostatin on the canine ileal circular muscle.

The aim of this study was to investigate the mechanism of action of somatostatin on the circular muscle of the isolated canine ileum using the microelectrode technique. The membrane potential from circular muscle cells was recorded in two preparations: 1) whole thickness circular and longitudinal muscle (with intact myenteric and deep muscular plexuses, n = 13) and 2) isolated circular muscle (with only deep muscular plexus, n = 9). In this preparation, inhibitory junction potentials (IJPs), elicited after field stimulation, are mediated by a nitric oxide-related (NO-R) compound. Somatostatin (10(-6) M) transiently (2-5 min) depolarized the circular muscle cells in both whole thickness (3.6 +/- 1.0 mV, P < 0.01) and isolated circular muscle preparations (8.0 +/- 0.8 mV, P < 0.01). Somatostatin did not reduce either the amplitude or duration of the IJP in the isolated circular muscle but reduced slow-wave amplitude. In contrast, a reduction (20-50%) in the amplitude of the IJP was observed in the whole thickness preparation, and there was little effect on slow-wave amplitude. Somatostatin did not affect the induced slow wave observed in the whole thickness preparation after field stimulation. Apamin significantly reduced the amplitude of the IJP in both preparations. Somatostatin (10(-6) M) did not modify the apamin-resistant IJP. A reduction in the slow-wave amplitude was observed in the isolated circular muscle preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of mechanisms and sites of actions of mu and delta opioid receptor activation in the canine intestine.

Perfusion with ([N-Me-Phe3,D-Pro4]morphiceptin (PL017)), [D-Pen2,5]enkephalin (DPDPE) and MEt5 and Leu5 enkephalin induced circular muscle contractions and decreased immunoreactive vasoactive intestinal polypeptide (VIP) venous output in canine ileal segments. Motility and VIP responses to PL017 were abolished by the mu antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) and unchanged by the delta antagonist ICI 174,864 ([N,N-dially-Tyr1,Aib2,3]Leu-enkephalin) which abolished DPDPE motility and VIP responses. The VIP response to DPDPE was unchanged by CTAP, which reduced motility responses, suggesting a DPDPE interaction with endogenous mu opioids, at a mu/delta(complexed) receptor. ICI 174,864 abolished Met5 and Leu5 enkephalin motility responses and Leu5 enkephalin VIP responses while CTAP was ineffective on Leu5 enkephalin motility responses or on both enkephalin VIP responses. CTAP increased Met5 enkephalin motility responses suggesting mu actions to inhibit excitatory nerves. ICI 174,864 reduced Met5 enkephalin VIP output decrements requiring CTAP addition for abolition, suggesting actions at mu/delta(complexed) receptors. Inhibition of nitric oxide synthase with N-omega-L-arginine methyl ester (L-NAME) abolished delta opioid and reduced by 30% mu opioid motility responses, leaving the VIP response intact. Hexamethonium and atropine abolished tonic VIP output, leaving intact motility responses to PL017 and DPDPE. Subsequently L-NAME eliminated delta opioid and reduced by 1/3 mu opioid motility responses. All opioids reduced the NO-mediated IJPs in myenteric plexus-free ileal circular muscle. Thus mu or delta opioids inhibit both NO and VIP release but removal of NO, not VIP, disinhibits circular muscle motility.

Peptide YY stimulates circular muscle contractions of the isolated perfused canine ileum by inhibiting nitric oxide release and enhancing acetylcholine release.

Peptide YY (PYY) and neuropeptide Y (NPY), infused into the quiescent isolated perfused canine ileum, dose-dependently increased phasic activity of the circular muscle and decreased tonic output of immunoreactive vasoactive intestinal peptide (VIP) in the venous effluent. The contractions subsided before the prolonged inhibition of VIP output. Motor excitation by PYY, an abundant neuropeptide in this tissue, was reduced by blockade of muscarinic or nicotinic receptors, or inhibition of nitric oxide (NO) synthase, despite continued inhibition of VIP output. A combination of atropine and hexamethonium eliminated the PYY-induced decrement in VIP output and left motor excitation unchanged. Blockade of NO synthase eliminated motility increases under these conditions. Intracellular microelectrode recordings of myenteric plexus-free circular muscle strips found no effect of NPY on the resting membrane potential, or on the field stimulation-induced inhibitory junction potential. Inhibition of VIP release plays no essential role in changing motility. These results suggest that PYY/NPY induce motility by stimulating release of acetylcholine and inhibiting NO release at a locus proximal to but not on nerve terminals.

Inhibitory and excitatory mechanisms of neurotensin action in canine intestinal circular muscle in vitro.

The effect of neurotensin on canine ileal circular muscle devoid of myenteric plexus was investigated using single and double sucrose gap techniques. Similar results were obtained with microelectrode techniques. Neurotensin caused a temperature-sensitive and dose-dependent biphasic response, an initial hyperpolarization associated with inhibition of contractile activity, followed by an excitatory phase, usually consisting of spike discharge and tonic and phasic contractions, for which depolarization was not required. Neither response was affected by tetrodotoxin, phentolamine, propranolol, or atropine. The hyperpolarization was associated with decreased membrane resistance, blocked by 10(-7) M apamin, and converted to tonic depolarization by apamin (10(-6) M). Tachyphylaxis to neurotensin occurred when the stimulation interval was less than 20 min. After Ca2+ depletion, depolarization was observed instead of the hyperpolarization; this depolarization was not affected by nitrendipine and was gradually abolished with repetitive stimulation at 20-min intervals. When Ca2+ was present, nifedipine did not alter the hyperpolarizing phase of the response but inhibited spiking and blocked all contractions. The excitatory phase of the response was enhanced by Bay K-8644. Neuromedin N elicited a response identical with that of neurotensin. The responses of the two peptides were completely cross tachyphylactic. Inhibitory junction potentials were not affected by neurotensin tachyphylaxis. It is concluded that neurotensin and neuromedin N activate apamin-sensitive, calcium-dependent potassium channels in circular muscle, causing membrane hyperpolarization and inhibition of muscle contraction. Release of intracellular calcium is involved in the activation of these potassium channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide may be the final mediator of nonadrenergic, noncholinergic inhibitory junction potentials in the gut.

This study tested the hypothesis that the final mediator of nonadrenergic, noncholinergic (NANC) inhibitory junction potentials (ijps) and associated relaxation responses was nitric oxide (NO) or a related substance and not vasoactive intestinal polypeptide (VIP). We used opossum esophagus body circular muscle and canine intestine circular muscle. In both these tissues, ijps had reversal potentials near the potassium equilibrium potential, (EK); in esophagus the ijps were apamin insensitive, but in the intestine they were partially apamin sensitive. N omega-Nitro-L-arginine methyl ester (NAME) (10(-5) to 5 x 10(-4) M) abolished ijps in both tissues, an effect overcome by 10(-3) M L-arginine but not D-arginine. NAME increased input resistance of esophagus tissues in the double sucrose gap but caused no significant depolarization in the sucrose gap or in studies with microelectrodes. Contractions and basal tension were increased in both tissues by NAME. The apamin sensitive and insensitive ijp components in canine muscle were both abolished by NAME, but the time course of this abolition was different for the two components. Methylene blue (10-50 microM) with variable rapidity and extent inhibited ijps in both tissues, but L-arginine could not overcome this effect. Methylene blue, like NAME, did not depolarize detectably but enhanced the contractile activity. VIP (10(-6) M) had very small effects in both tissues, little or no hyperpolarization and increased input resistance in esophagus, these effects were not changed by NAME, and VIP did not affect ijps. We conclude that NO may be the final mediator of NANC-initiated inhibitory junction potentials in gastrointestinal circular smooth muscle.