Characterization of inhibitory innervation in porcine colonic circular muscle

Abstract

Effects of stimulation of intramural nerves in the circular smooth muscle layer of the porcine colon (Sus scrofa domestica) were studied using the sucrose-gap technique. Electrical field stimulation of the preparation, superfused with Krebs solution at 21 degrees C, induced a transient hyperpolarization of the smooth muscle cell membrane. This hyperpolarization was an inhibitory junction potential (IJP). The responses obtained from circular muscle originating from either the centripetal or centrifugal gyri of the ascending colon did not differ significantly. The IJP was characterized as being mediated by intramural, nonadrenergic, noncholinergic (NANC) nerves. The amplitude and latency of the IJP changed linearly with temperature (15-25 degrees C: +1 mV and -0.1 s per degree Celsius, respectively) reflecting a temperature-dependent synchronization of transmitter release. The membrane resistance decreased during the IJP. The IJP amplitude decreased or increased during conditioning hyperpolarizations or depolarizations, respectively, and reversed at membrane potentials about 30 mV more negative than the resting membrane potential. Potassium conductance blocking agents, barium (1 mM), tetraethylammonium chloride (TEA, 20 mM), 4-aminopyridine (4-AP, 5 mM), apamin (1 microM), and aminacrine (10(-4) M) added to the superfusion medium increased the membrane resistance. Only barium, TEA, and apamin depolarized the smooth muscle cell membrane. The IJP amplitude decreased in the presence of aminacrine and apamin to 75 and 35%, respectively, suggesting that apamin-sensitive Ca2+-activated K+ channels are involved in this response. ATP, adenosine, and related adenine nucleotides in concentrations up to 10(-3) M did not mimic the IJP. Superfusion with ATP for 15 min revealed a gradually increasing attenuation by up to 20% of the IJP. This might suggest that the release of neurotransmitter from intramural NANC nerves is modulated presynaptically via purinoceptors. Exogenously applied vasoactive intestinal polypeptide (VIP) in concentrations of 10(-9) to 10(-4) M did not affect the preparation. Also at elevated temperatures (up to 35 degrees C), VIP (10(-7) to 10(-4) M) did not cause measurable effects. It is concluded that the inhibitory mediator of the intramural NANC nerves present in the circular muscle layers of the porcine colon is neither a purine nor VIP.

Mediators of non‐adrenergic non‐cholinergic inhibitory neurotransmission in porcine jejunum

“…The identity of the NANC inhibitory neurotransmitter mediating the IJP in the porcine intestine is not completely known, despite the importance of the pig as a potential source for intestinal xenotransplantation to humans. In the ileum and colon of this animal, the NANC IJP was previously shown to consist of only a fast hyperpolarization and that an increase in small conductance Ca 2+ ‐activated K + outward current is involved in the IJP 28,31 . Adenosine‐5‐triphosphate is considered the NANC inhibitory neurotransmitter in ileum but not in the colon of the pig 28,31 .…”

“…The present findings showed that IJPs in porcine jejunum had only a fast component and this fast component was dependent on the opening of small conductance Ca 2+ ‐dependent K + channels sensitive to apamin. Previous studies of porcine ileum and porcine colon 28,31 showed that IJPs in these regions also consist of only a fast component. Thus, it appears that the IJP in the circular muscle layer of the pig intestine is different from the IJP in the intestine of other mammals, including humans, where it consists of both fast and slow hyperpolarizing components.…”

“…Vasoactive intestinal peptide is another candidate that could mediate IJPs 53,54 . However, like in the porcine colon and ileum , 28,31 vasoactive intestinal peptide was not the mediator of the fast IJP in pig jejunum because application of a VIP antagonist did not modify the IJP. Furthermore, activation of adenylate cyclase did not seem to be the intracellular mechanism related to the IJP and smooth muscle relaxation in pig jejunum as SQ22356, an inhibitor of adenylate cyclase, did not modify the IJP.…”

“…Inhibitory junction potentials are a prominent feature of gastrointestinal smooth muscle of many mammals including pig, dog, rabbit, opossum, hamster, guinea pig, mouse, rat, horse and human 8,16,28–36 . Typically, IJPs consist of an initial rapid hyperpolarization (‘fast component’) followed by a slow hyperpolarization (‘slow component’).…”

Galanin and Vasoactive Intestinal Polypeptide: Coexistence and Corelease from the Vascularly Perfused Pig Ileum during Distension and Chemical Stimulation of the Mucosa

“…The ascending contraction of the reflex is believed to be mediated by tachykinin motor neurons as well as cholinergic motor neurons [14]. There is strong evidence that the inhibitory response seen below the stim ulus is mediated by a VIPergic motor neuron [14,15], even though this concept has been challenged lately [27], Therefore, the in creased release of VIP in response to intralu minal agents seen in the present study could alternatively derive from muscle motor neu rons. The pattern of VIP release during chemical stimulation seems to be different from that observed in response to distension, indicating that neurons other than those ac tivated in the distension response were also involved.…”