Characterization of alpha-adrenoceptor subtypes in smooth muscle of equine ileum

Dexmedetomidine Has Differential Effects on the Contractility of Equine Jejunal Smooth Muscle Layers In Vitro

α2 agonists are frequently used in horses with colic, even though they have been shown to inhibit gastrointestinal motility. The aim of this study was to determine the effect of dexmedetomidine on small intestinal in vitro contractility during different phases of ischaemia. Experimental segmental jejunal ischaemia was induced in 12 horses under general anaesthesia, and intestinal samples were taken pre-ischaemia and following ischaemia and reperfusion. Spontaneous and electrically evoked contractile activity of the circular and longitudinal smooth muscles were determined in each sample with and without the addition of dexmedetomidine. During a second experiment, tetrodotoxin was added to determine if the effect was neurogenic. We found that the circular smooth muscle (CSM) contractility was not affected by ischaemia, whereas the longitudinal smooth muscle (LSM) showed an increase in both spontaneous and induced contractile activity. The addition of dexmedetomidine caused a decrease in the spontaneous contractile activity of CSM, but an increase in that of LSM, which was not mediated by the enteric nervous system. During ischaemia, dexmedetomidine also mildly increased the electrically induced contractile activity in LSM. These results may indicate a stimulatory effect of dexmedetomidine on small intestinal contractility. However, the influence of dexmedetomidine administration on intestinal motility in vivo needs to be further investigated.

Effects of α2-adrenergic drugs on small intestinal motility in the horse: an in vitro study

The effects of selective α(2)-agonists (xylazine, detomidine and medetomidine) and antagonists (yohimbine and atipamezole) on in vitro small intestine motility in the horse were evaluated. Samples of equine jejunum were placed in isolated organ baths and drug-induced modifications of motility were measured by means of an isotonic transducer. All tested α(2)-agonists dose-dependently reduced both spontaneous and electrically-evoked phasic contractions. Conversely, α(2)-antagonists were ineffective when tested alone, and showed a heterogeneous and dose-independent ability to inhibit agonist activity. In particular, the antagonism exerted by higher concentrations of both yohimbine and atipamezole against α(2)-agonists was weaker than when lower concentrations were used. The data are indicative of the presence of both pre- and post-synaptic α(2)-adrenoceptors with inhibitory activity on equine jejunum motility, and support a possible therapeutic utility of these drugs in horse intestinal disorders associated with hypermotility.

Are so many adrenergic receptor subtypes really present in domestic animal tissues? A pharmacological perspective

Adrenergic receptors (ARs) are the cellular membrane binding sites through which natural catecholamines and sympathomimetic drugs exert their physiological and pharmacological effects. In recent decades, studies to clarify the distribution and function of ARs have been performed mostly on cultured cells, laboratory animals and human target tissues, but little is known about these aspects in domestic animals. This review focuses on AR structure, classification and signalling pathways and on AR subtype distribution in target tissues of some domestic animals, namely dogs, horses and bovines. In these species, different alpha- and beta-AR subtypes have been characterized and the functions controlled by the adrenergic systems have been studied. In the dog, the role played by the adrenergic system in the pathogenesis of cardiovascular disorders and in the modulation of canine aggression has roused particular interest. In dogs affected by dilated cardiomyopathy a significant down-regulation of beta-ARs has been observed both in the heart and circulating lymphocytes. This finding confirms the involvement of the adrenergic system in the pathogenesis and progression of the disorder and suggests new therapeutic strategies. In the horse, AR distribution has been studied in the cardiac, respiratory and gastrointestinal systems as well as in digital veins and arteries. The cardiac beta-ARs in healthy horses seem to be predominantly represented by the beta(1) subtype. In this species, heart failure may increase the expression of the beta(2) subtype, rather than causing AR down-regulation. Different beta- and alpha-AR subtypes have been characterized in the smooth muscle of equine ileum. The sympathetic relaxation of equine ileum smooth muscle seems to depend mainly on beta(3)-AR subtype activation, with minor involvement of the beta(2) subtype. In the respiratory tract, regional differences have been evidenced in the functionality of beta-AR subtype. The beta(2) subtype predominates in all segments but the beta(2) subtype-mediated adenyl cyclase response is tissue-dependent, with higher activity in tracheal membranes than bronchial or pulmonary ones. Both alpha- and beta-AR subtypes are present in the genital tract of cows. Bovine ovarian and myometrial cell membranes express higher concentrations of beta(2)-ARs than the beta(1) subtype, whereas as far as alpha-ARs are concerned, a single class of alpha(1)-ARs and two distinct classes of alpha(2)-AR binding sites have been discriminated. Interestingly, it has been observed that the activation of the sympathetic system could play an important role in the pathogenesis of bovine ovarian cysts as suggested by the modifications in beta-AR levels in the hypophysis and ovary of cows affected by ovarian cysts. In this species, the phenomenon of down-regulation has been well studied in different organs of veal calves treated with clenbuterol as a "partitioning agent". Since differences exist in AR distribution among species, data obtained in laboratory animals or in human beings cannot be extrapolated to domestic animals and further investigation on AR subtypes in domestic animal tissues is necessary.

Retrospective study of the risk factors and prevalence of colic in horses after orthopaedic surgery

The records of 496 orthopaedic operations on 428 horses were reviewed to estimate the prevalence of, and identify the risk factors for, the development of colic in horses after surgery. Colic was defined as any recognised sign of abdominal pain that could not be attributed to a concurrent disease. Fourteen of the horses developed colic; eight of them were undiagnosed, three were classified as impactions, one as tympanic colic of the colon, one as incarceration of the small intestine in the epiploic foramen, and one as left dorsal displacement of the colon in the nephrosplenic space. Morphine was associated with a four-fold increased risk of colic compared with the use of no opioid or butorphanol, and out-of-hours surgery was also associated with an increased risk.

Distribution of mRNA that codes for subtypes of adrenergic receptors in the gastrointestinal tract of dairy cows

OBJECTIVE

To describe the distribution of mRNA that codes for 9 subtypes of adrenergic receptors in the digestive tract of dairy cows.

SAMPLE POPULATION

Fresh full-thickness wall specimens from the abomasum (fundus, corpus, and antrum), ileum, cecum, proximal loop of ascending colon, and 4 locations of the spiral colon collected from 10 healthy cows at slaughter.

PROCEDURE

Concentrations of mRNA that code for 9 subtypes of adrenergic receptors in the bovine gastrointestinal tract (alpha1A, alpha1B, alpha1D, alpha2AD, alpha2B, alpha2C beta1, beta2, and beta3) were measured by use of a quantitative real-time reverse transcription-polymerase chain reaction assay. Results were reported in relation to mRNA expression of the housekeeping gene glyceraldehyde phosphate dehydrogenase (GAPDH).

RESULTS

Mean mRNA contents of adrenergic receptors in the bovine digestive tract were low (range, 0.00006% to 5.04% of GAPDH). Distribution of receptor subtypes was similar in all tissues, with lowest expression of alpha1D receptors, followed by alpha2B, alpha2C, beta3, alpha1B, alpha1A, beta1, and beta2 in the abomasum, whereas alpha2AD and beta2 in the intestines were highest. In comparison with the intestines, relative concentrations of mRNA for receptors beta2 and beta3 were significantly lower in the abomasum.

CONCLUSIONS AND CLINICAL RELEVANCE

Relative concentrations of mRNA that code for adrenergic receptors differed among receptor subtypes and among locations in the bovine gastrointestinal tract. Comparison of these values established in healthy cattle with results for cows with motility disorders, such as abomasal displacement and cecal dilatation, may lead to improved therapeutic or prophylactic approaches for these diseases.

Identification of functional alpha-adrenoceptor subtypes in the bovine female genital tract during different phases of the oestrous cycle

The concentration and functionality of the alpha-adrenoceptor (alpha-AR) subtypes in the genital tract of cyclic heifers were investigated. In each tissue sample, a single class of alpha1-ARs was observed, whereas two distinct classes of alpha2-ARs were discriminated: low-affinity (LA) and high-affinity (HA) alpha2-ARs. Statistical analysis showed the presence of significantly (p < 0.05) higher concentrations of all alpha-AR subtypes in the follicle than in the corpus luteum. No significant differences were found in the ovary or myometrium between the luteal and follicular phases. In the ovary, the density of alpha1-ARs was significantly (p < 0.05) higher than that of alpha2-ARs. By contrast, there were significantly (p < 0.05) more alpha2-ARs than alpha1-ARs in the myometrium. As far as alpha2-ARs are concerned, LA alpha2-ARs were significantly (p<0.05) higher than HA alpha2-ARs in all tested tissues. Competition studies suggested that the rank order of potency of antagonists for alpha1-ARs was prazosin > phentolamine > yohimbine, whereas for alpha2-ARs the order of potency was yohimbine > or = phentolamine>prazosin. Functional assays performed on myometrium showed that noradrenaline, phenylephrine and clonidine elicited concentration-dependent contractions only in dioestrus and pro-oestrus preparations and that clonidine was more effective than phenylephrine as a contractile agent. It appeared that there were no significant modifications in alpha-AR affinity or concentration during the different stages of bovine oestrous cycle, whereas the uterine spontaneous activity and the responsiveness to alpha-adrenergic stimulation was strongly influenced by hormonal levels. The modifications of uterine contractility observed during the oestrous cycle may be related to modifications induced in the transductional mechanisms of alpha-ARs.