Köhn, Eva (2000)
Motility of the small and large intestine in horses
The aim of this study was to investigate the in vitro motility pattern of circular and longitudinal muscle strips of equine jejunum and colon. In addition the study addressed the differentiation between myogenic and nerve mediated contractile patterns and the contribution of excitatory and inhibitory neurotransmitters. In a separate set of experiments it was assessed whether there are differences between normal tissue and preparations from horses with colic.
Jejunal and colonic preparations showed different motility patterns under basal, non-stimulated, conditions. While both muscle layers in the colon showed periods of quiescence follwed by clustered contractions which were associated with an increase in muscle tone, the jejunum exhibited a more regular contractile activity. Electrical field stimulation (EFS) using bipolar platin electrodes was used to activate nerves in the preparation. The transmitter cocktail released during EFS mediates motor responses which contain inhibitory and excitatory components. Dependent on which of the
transmitters will be released the excitatory or the inhibitory component will dominate. Both muscle layers in the colon responded to EFS with a pronounced inhibition of motility. The responses in the jejunum varied between different preparations. Most longitudinal muscle preparations (9 out of 12) responded to EFS with enhanced motility; the three remaining preparations showed an inhibition.
While 7 circular muscle preparations showed excitatory responses during EFS 6 preparations
responded with an inhibition. It is noteworthy that the occurrence of excitatory and inhibitory response in the jejunum circular muscle was dependent on the basal tone of the tissue. Preparations with a high tone showed excitation and those with a relatively low tone responded with an inhibition. These results showed that nerve mediated motor responses in the equine colon were all inhibitory. In the
longitudinal muscle of the jejunum excitatory responses predominate whereas the responses of the jejunal circular muscle is very much dependent on the tone of the tissue.
A neuropharmacological approach enabled the identification of putative transmitters involved in those nerve mediated responses evoked by EFS. This has been done firstly by direct application of various transmitters with the aim to mimick the EFS evoked responses and secondly by evaluating the capability of various antagonists to block EFS responses. Direct bath application of the transmitter nitric oxide (NO), by addition of the NO donor Sodium
Nitroprusside (SNP), Adenosine 5'-Triphosphate (ATP), Vasoactive Intestinal Polypeptide (VIP) and Pituitary Adenylat Cyclase Activating Polypeptide (PACAP) all inhibited ongoing motility in both muscle layers of the colon and in the circular muscle of the jejunum.
Application of ATP evoked a transient inhibition of motility whereas NO, VIP and PACAP led to a sustained inhibition which lasted for the entire application of the substances. In the jejunum longitudinal muscle only NO and ATP were effective inhibitors. In all preparations the most potent effect was observed for NO. PACAP and VIP effects were comparable between the longitudinal muscle of the colon and the circular muscle of the jejunum. The circular muscle of the jejunum was more sensitive to PACAP than to VIP, whereas VIP had the strongest effect in the circular muscle of the jejunum.
The NO synthase blocker L-NAME reduced significantly the duration of the EFS induced inhibition in the colon, but was uneffective in the jejunum. Apamin, a blocker of small conductance calcium-activated potassium channels, totally abolished the inhibitory response during EFS in all preparations of colon and jejunum. These results indicate that the inhibitory responses in the jejeunum are exclusively mediated by apamin sensitive mechanisms whereas inhibitory responsees in the colon contain nitrergic and apamin sensitive components.
After blockade of the inhibitory response by L-NAME and Apamin, EFS evoked increased motility.
This increased motility was significantly reduced by atropine which suggested that acetylcholine acting on muscarinic receptors was the mediator of this response.
The results so far showed that nerve mediated response during EFS contain inhibitory and excitatory components. At least in the equine colon, the inhibitory response dominates. Blockade of inhibition unmasked the cholinergic excitatory response. In contrast to the colon, cholinergic excitatory response occur in the equine jejunum without pharmacological intervention. The EFS responses in the
longitudinal muscle have a very prominent cholinergic component. The EFS responses in the circular muscle of the jejunum are excitatory if the muscle tone is high and inhibitory if the muscle tone is low.
Since apamin quite unselectively blocks mechanisms through which various inhibitory transmitters relax the muscle it was investigated whether apamin blocks the effects of the inhibitory transmitters identified in this study. It was revealed that the sensitivity of the transmitters to apamin was regions specific. In all regions apamin sensitive mechanisms appeared to be involved in the effects of ATP.
Only in the longitudinal muscle of the colon and the circular muscle of the jejunum apamin antagonised also the responses to PACAP. It can therefore be concluded in these regions that the effects of PACAP were due to activation of the PACAP specific, apamin-sensitive receptor subtype.
The ATP induced inhibition could not be blocked by suramine, indicating that neither P2x nor P2y
receptors were involved.
The alpha adrenergic blocker phentolamine had no effect on the basal motility patterns but reduced the EFS evoked contractile response in the longitudinal muscle of the jejunum. These results further support the significance of the EFS evoked non-adrenergic non-cholinergic responses mediated by enteric neurones and the relative minor importance of sympathetic tone in the equine gut, at least under the experimental conditions of this study.
Immunohistochemical techniques were used to demonstrate the presence of NOS-, ChAT- and VIP-positive nerve cell bodies and nerve fibers in the myenteric plexus of the equine colon and jejunum.
Compared to control preparations there were no significant differences in basal motility or nerve mediated responses in preparations from horses with colic. However, it was found that in colic horses longitudinal muscle preparations from the jejunum showed a significantly increased sensitivity to the muscarinic agonist carbachol than preparations from control animals.
This study identified the regulation mechanisms of the myogenic and nerve mediated contractile patterns of equine jejunal and colonic smooth muscle. These findings may suggest new therapeutic targets for motility disorders, such as agonists or antagonists that influence the action of nitrergic and purinergic inhibitory mechanisms.