Inhibition of food intake and reticulo-rumen motility of sheep by the alpha-2 adrenoreceptor agonist clonidine

Quantitative mRNA analysis of adrenergic receptor subtypes in the intestines of healthy dairy cows and dairy cows with cecal dilatation-dislocation

OBJECTIVE

To investigate the distribution of mRNA coding for 9 adrenoceptor subtypes in the intestines of healthy dairy cows and cows with cecal dilatationdislocation (CDD).

SAMPLE POPULATION

Full-thickness specimens of the intestinal wall were obtained from the ileum, cecum, proximal loop of the ascending colon (PLAC), and external loop of the spiral colon (ELSC) of 15 cows with CDD (group 1) and 15 healthy (control) cows (group 2, specimens collected during laparotomy; group 3, specimens collected after slaughter).

PROCEDURES

Concentrations of mRNA for 9 adrenoceptor subtypes (alpha(1A), alpha(1B), alpha(1D), alpha(2AD), alpha(2B), alpha(2C), beta(1), beta(2), and beta(3)) were measured by quantitative real-time reverse transcriptase-PCR assay. Results were expressed relative to mRNA expression of a housekeeping gene.

RESULTS

Expression of mRNA for alpha(1B)-, alpha(2AD)-, alpha(2B)-, beta(1)-, and beta(2)-adrenoceptors was significantly lower in cows with CDD than in control cows. In the ileum, these receptors all had lower mRNA expression in cows with CDD than in control cows. The same effect was detected in the ELSC for mRNA for alpha(2AD)-, alpha(2B)-, beta(1)-, and beta(2)-adrenoceptors, and in the cecum and PLAC for alpha(2B)- and beta(2)-adrenoceptors. Groups did not differ significantly for alpha(1A)-adrenoceptors. The mRNA expression for alpha(1D)-, alpha(2C)-, and beta(3)-adrenoceptors was extremely low in all groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Differences in expression of mRNA coding for adrenoceptors, most pronounced in the ileum and spiral colon, between cows with CDD and control cows support the hypothesis of an implication of adrenergic mechanisms in the pathogenesis of CDD in dairy cows.

A 2 -agonists in sheep: a review

OBJECTIVE

To review the use and adverse effects of alpha(2)-agonists in sheep.

STUDY DESIGN

Literature review.

MATERIAL AND METHODS

'Pubmed' of the United States National Library of Medicine and 'Veterinary Science' of CAB International were searched for references relating sheep to alpha(2)-agonists. The bibliographies of retrieved articles were further scrutinized for pertinent references, and relevant articles were selected manually.

RESULTS

Reports on the use of clonidine, xylazine, detomidine, romifidine, medetomidine and dexmedetomidine, MPV-2426 and ST-91 in sheep were found in the literature. Most of the studies described xylazine followed by medetomidine and clonidine. The literature on detomidine and romifidine in sheep was sparse. Reports included pharmacokinetic studies, evaluation of sedative, analgesic, and anaesthetic techniques with or without cardiovascular effects, and experimental investigations of adverse effects (mainly hypoxaemia) including the mechanisms of pulmonary oedema and impaired oxygenation after alpha(2)-agonist administration.

CONCLUSIONS

A(2)-agonists are potent and effective analgesics in sheep. In combination with ketamine, they are frequently used for the induction and maintenance of anaesthesia, in this case analgesia is satisfactory. The degree of hypoxaemia which occurs with all commercially available alpha(2)-agonists is highly variable and depends on individual or breed-related factors; the most severe reactions occur after intravenous (IV) injection and during general anaesthesia. Clinical relevance Subclinical respiratory disease is common in sheep. Rapid IV injection of alpha(2)-agonists without supplementary oxygen should be avoided whenever hypoxaemia may be critical.

Stimulation of food intake by xylazine in sheep

The effect of xylazine (0.05 and 0.15 mg/kg, intramuscularly) on food (concentrate) intake was examined in ewes. Xylazine at 0.15 mg/kg significantly increased the 2 and 24 h food intake in comparison with control values. Xylazine treatments significantly and dose-dependently decreased the heart rate, respiratory rate and ruminal motility, and increased the rectal temperature when compared with pretreatment values. The changes in the physiological variables and the slight tranquilizing effect of xylazine apparently did not interfere with the food intake. The data suggest that xylazine increases food intake in freely feeding sheep.

Vasopressin inhibits food intake in pygmy goats by activation of alpha 1-adrenergic receptors

The hypophagic effect of intraperitoneally injected VP (1.5 micrograms/kg) was blocked by concomitant injection of the alpha 1-adrenergic antagonist prazosin (27 and 40 micrograms/kg). Both the alpha 2-adrenergic antagonists idazoxan (300 micrograms/kg) and yohimbine (500 micrograms/kg) and the beta-adrenergic antagonist propranolol (500 micrograms/kg) failed to block the hypophagia induced by VP (1.5 micrograms/kg). The results suggest that the hypophagic effect of VP is mediated by alpha 1-adrenergic receptors.

Inhibitory effect of the alpha 1-adrenergic antagonist prazosin on food intake in pygmy goats

The effect of the intraperitoneally (IP) injected alpha 1-adrenergic antagonist prazosin (15, 40, 130 and 537 micrograms/kg b.wt.) on feed intake and plasma free fatty acid (FFA) levels was studied in 12 adult female African pygmy goats. Prazosin produced a dose-dependent long-lasting hypophagia and a dose-dependent increase in plasma FFA levels. The hypophagic effect of prazosin and the elevation in plasma FFA induced by prazosin were abolished by concomitant injection of the beta-adrenergic antagonist propranolol (500 micrograms/kg b.wt.). The hypophagic effect of prazosin (40 micrograms/kg) was also abolished by concomitant injection of the hypertonic agent angiotensin II (AII: 1 microgram/kg). The results suggest that blockade of alpha 1-adrenergic receptors by prazosin-producing hypotension elicits a counterregulatory sympathetic activation, which reduces voluntary food intake by activation of beta-adrenergic receptors. The dose dependence of the inhibition of food intake and of the elevation of plasma FFA after prazosin suggests that the hypophagia due to prazosin may be partly a consequence of elevated plasma FFA.