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The effects of duodenal distension on forestomach and abomasal motility were investigated in conscious sheep chronically fitted with intraparietal electrodes, a duodenal cannula, and an intracerebroventricular cannula. Duodenal distensions with a balloon inflated with 40 ml (DD40) of water reduced the frequency of forestomach and abomasal contractions by 45 and 32%, respectively, while distension with 80 ml (DD80) induced a total inhibition. Methysergide, a mixed 5HT1-5HT2 antagonist administered intravenously (200 micrograms/kg) or intracerebroventricularly (20 micrograms/kg) suppressed the DD40-induced inhibition and reduced that induced by DD80. Spiroxatrine, a selective 5HT1A antagonist, intravenously (100 micrograms/kg) or intracerebroventricularly (10 micrograms/kg), suppressed the DD40 and DD80-induced inhibition, which was also attenuated by the 5HT2 antagonist ritanserin given intravenously (200 micrograms/kg) or intracerebroventricularly (20 micrograms/kg). Granisetron, a 5HT3 antagonist, injected intravenously (150 micrograms/kg), abolished the effects of DD40 and DD80 while it had no antagonistic action on DD40 and DD80 when given intracerebroventricularly (15 micrograms/kg). It is concluded that in sheep, duodenal distension inhibits forestomach and abomasal motility through 5HT1A and 5HT2 receptors at the level of the central nervous system and 5HT3 receptors located peripherally.

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The effects of peripheral (intravenous, i.v.) and central (intracerebroventricular, ICV) administration of agonists of 5-HT1A, 5-HT2, 5-HT3 and 5-HT4 receptors were investigated in conscious sheep chronically fitted with intraparietal electrodes on the reticulum and the dorsal, ventral and caudo-ventral rumen. The 5-HT1A agonist 8-hydroxydipropylaminotetralin increased reticular and decreased ruminal spike burst frequency when given i.v. (80 micrograms/kg) and ICV (8 micrograms/kg). The 5-HT2 and 5-HT3 agonists, alpha-methylserotonin and 2-methylserotonin, induced a moderate inhibition of rumino-reticular contractions when given i.v. at 100 and 150 micrograms/kg, respectively, while marked inhibition was observed after ICV administration at doses of 10 and 5 micrograms/kg, respectively. The 5-HT4 agonist 5-methoxytryptamine strongly stimulated rumino-reticular motility by the ICV (10 micrograms/kg) route, whereas it induced a moderate inhibition when administered i.v. (200 micrograms/kg). The selective antagonist of 5-HT1A, 5-HT2, 5-HT3 and 5-HT4 receptors, spiroxatrine, ritanserin, granisetron and DAU 6285, respectively, blocked the responses of the respective agonists given by the same route. Moreover, the antagonists given ICV blocked the effects of the agonists given i.v. except for DAU 6285 ICV, which did not antagonize the inhibition induced by 5-methoxytryptamine i.v. It is concluded that the four types of serotonergic receptors investigated control rumino-reticular motility at the central level. However, according to the receptor type and the forestomach area (reticulum or rumen) this control may be stimulatory or inhibitory, demonstrating a pleiotropic role of serotonin in the control of rumino-reticular motility in sheep.

Intravenous injection of the a-adrenergic receptors agonist xylazine (0.3 mg/kg b. w.) produced hyperglycaemia with its peak (1 10 mg/100 ml) 30 min later in sheep. This hyperglycaemia was statistically significant for 3 hours. When xylazine was administered after pretreatment of the animals with either the a,-adrenergic receptors blocker prazosin (0.5 mg/kg i. v.) or the a2-blocker yohimbine (0.5 mg/kg i. v.) a hyperglycaemia was also produced but in the case of prazosin its peak did not exceed 95 mg/100 ml, and in the case of yohimbine it was only 70 mg/100 ml (control value 55 mg/ lOOml). When animals were pretreated with either yohimbine plus prazosin or the a,-and a,-blocker tolazoline (3 mg/kg i. v.) the hyperglycaemic effect of xylazine was inhibited. These results suggest that, in the regulation of blood glucose levels by the a-adrenergic receptors, the a, play the main role, while the a, are also involved.

Summary The involvement of adrenoreceptors in the control of reticulo‐ruminal (R‐R) motility was electromyographically studied in five conscious adult ewes. Intravenous (i. v.) administration of the β2‐agonist ritodrine (10 or 20μg/kg ṁ min for 15 min) or α1‐blocker prazosin (20μg/kg ṁ min for 30 min) or the β1‐and β‐blocker acebutolol and propranolol, respectively, (30μg/kg ṁ min for 30 min) had no significant effect on R‐R phasic myoelectrical grouped discharges (PMGD). Administration of the α1‐agonist phenylephrine (4 μg/kg ṁ min i. v. for 15 min) significantly increased for 25 min both the frequency of R‐R PMGD and the percentage of occurrence of secondary ruminal PMGD and provoked rumination. Pretreatment of animals with prazosin prevented these R‐R responses to phenylephrine. Administration of the α2‐agonist naphazoline (2.5 μg/kg ṁ min i. v. for 15 min) stopped the reticular PMGD for 1 hour, after an initial increase in their frequency, and greatly decreased the frequency of ruminal PMGD. When animals were pretreated with the α2‐blocker yohimbine (20μg/kg ṁ min i. v. for 30 min), naphazoline failed to produce any effect on R‐R PMGD. Yohimbine, given either alone or followed by naphazoline, abolished secondary ruminal PMGD for about 2 hours. The β1‐agonist (+)‐dobutamine (30μg/kg ṁ min i.v. for 15 min) provoked a pause in R‐R PMGD, which reappeared soon after the end of drug administration. Pretreatment of ewes with acebutolol or propranolol prevented dobutamine from producing its effects on R‐R PMGD. It is concluded that, among the adrenoreceptors, the α1‐, α2‐ and β1‐ones are involved in regulating R‐R motility in sheep, α1‐receptors being stimulatory receptors, while α2‐ and β1‐receptors are inhibitory ones. Further, a hypothesis is put forward that there may exist receptors affected by yohimbine, having an inhibitory effect on secondary ruminal contractions. Zusammenfassung Zur Rolle der adrenergenen Rezeptoren bei der Kontrolle der reticulo‐ruminalen myoelektrischen Aktivität beim Schaf Bei 5 adulten weiblichen Schafen wurde die Rolle adrenergener Rezeptoren bei der Kontrolle der Hauben‐Pansen‐Motilität elektromyographisch untersucht. Die intravenöse (i.v.) Applikation des β‐Agonisten Ritodrin (10 oder 20 μg/kg ṁ min für 15 min), des α1‐Blockers Prazosin (20 μg/kg ṁ min für 30 min) oder der β1‐ bzw. β‐Blocker Acebutolol bzw. Propanolol (30μg/kg ṁ min für 30 min) hatte keine signifikanten Veränderungen der phasischen myoelektrischen Gruppenentladungen (PMGD) im Hauben‐Pansen‐Bereich zur Folge. Die Applikation des α1‐Agonisten Phenylephrin (4μg/kg ṁ min i.v. für 15 min) führte zu einer 25 min dauernden Erhöhung der Frequenz der PMGD im Hauben‐Pansen‐Bereich, sowie dem prozentualen Auftreten von sekundären ruminalen PMGDS. Des weiteren wurde Wiederkauen ausgelöst. Die Vorbehandlung mit Prazosin verhinderte diese Effekte von Ephedrin. Nach Verabreichung des α2‐Agonisten Naphazolin (2,5 μg/kg ṁ min für 15 min) si...