Summary The effects of adrenaline, dopamine, serotonin, and different cholinergic agents on isolated muscle strips from the ansa proximalis coli in cattle were studied. Adrenaline (5 · 10−5 mol/l) evoked a relaxing effect (p < 0.05) on longitudinal and circular muscle strips, mediated via beta‐ and alpha1‐receptors respectively. High concentration of dopamine (5 · 10−4 mol/l) caused a non‐significant inhibitory effect on longitudinal smooth muscles, which was probably mediated by beta‐ and not by dopamine‐receptors. Serotonin (5 · 10−5mol/l) and cisapride (75 · 10−6mol/l) had no effect. Carbachol (2 or 10 · 10−6 mol/l) and bethanechol (5 or 10 · 10−5 mol/l) caused a dose‐dependent (p < 0.05) contraction of both smooth muscle types. This excitatory effect was inhibited (p < 0.05) by metoclopramide (1 · 10−4mol/l), as well as the muscarinic agents RS‐86 (5 · 10−5mol/l) and CGP‐37,218 (37.5 or 75 · 10−6 mol/l).

The effect of VIP was studied in muscle strips isolated from sheep rumen. This neuropeptide caused a concentration-dependent (10-9-5.10-8 mol/l) reduction of acetylcholine-induced (5.5. 10-6mol/l) isotonic contraction of the muscle strips. The inhibitory effect of VIP was not affected by incubation of the preparations with phentolamine, propranolol or tetrodotoxin; the relaxation exerted by VIP is therefore myogenic. Because nerve fibres containing VIP have recently been identified in ruminal smooth muscle, VIP probably acts on ruminal smooth muscles as an inhibitory transmitter.

Summary The effects of various neurotransmitters and electrical field stimulation on muscle strips from the distal equine esophagus were studied. Acetylcholine (ACH) caused concentration dependent (1.1–55 · 10−6 mol/l) contractions of the longitudinal and circular muscle strips from the distal esophagus as well as from the lower esophageal sphincter (LES). Atropine (10−5 mol/l) blocked these contractions. Noradrenaline (NA) induced concentration related (1.1–55 · 10−6 mol/l) contractions of the muscle strips from the LES. This excitatory effect of noradrenaline was antagonized by the α1‐receptor antagonist prazosin. Tetrodotoxin (5 · 10−6 mol/l) did not affect the contractile response of the muscle strips to noradrenaline (55 · 10−6 mol/l). Noradrenaline (1.1–55 · 10−6 mol/l) had no excitatory effect on the circular and the longitudinal muscle strips from the esophagus. Furthermore, noradrenaline induced a concentration dependent (1.1–55 · 10−6 mol/l) relaxation of the longitudinal muscle strips from the esophagus. The relaxing effect of NA was antagonized by the β‐receptor antagonist propranolol (10−5 mol/l). Histamine (10−7‐10−6 mol/l) elicited a contraction in 4 out of 18 muscle preparations from the LES. The histamine induced contractions were partly antagonized by the H1‐receptor antagonist clemastine (10−4 mol/l) and fully abolished by the H2‐receptor antagonist clemastine (10−4 mol/l). Electrical field stimulation (EFS, 5 Hz, 2 ms; 500 mA; 10 Hz, 2 ms; 500 mA) produced tetrodotoxin sensitive contractions in all three types of muscle strips. Atropine (10−5 mol/l) fully suppressed these contractions in most preparations. Prazosin (10−4 mol/l) had no effect on the contractions produced by EFS. Substance P (1–5.5 · 10−6 mol/l) serotonin (10−6‐10−4 mol/l) cholecystokinin (10−5‐106 mol/l), and VIP (10−8‐5 · 10−7 mol/l) did not produce any effect on all three types of muscle strips. These results suggest that aceylcholine is the most important excitatory transmitter in the distal equine esophagus.

The effect of Vasoactive Intestinal Peptide (VIP) on muscle strips from the reticular groove, the reticulo-omasal orifice (ROO) and the omasal canal was studied. VIP caused a concentrationdependent (10-8-5 .10-7 mol/l) reduction of the acetylcholine-induced (55 . mol/l) contraction of the reticular groove muscle preparations from calves and adult cattle. VIP was more effective in the muscle strips from calves than in those from adult cattle. Moreover, the circular muscle strips from the reticular groove were more sensitive to VIP than the longitudinal muscle strips.VIP also induced a concentration-related (10+-5 . io-'mol/l) relaxation of muscle strips from the calf ROO. Furthermore, both circular and longitudinal muscle strips from the omasal canal of the calf were relaxed by VIP (10-'-5~10-7m0l/l). The relaxing effect of VIP on the circular muscle strips from the reticular groove of the calf was not significantly affected during incubation with 5 . 10dmoVl tetrodotoxin. The relaxing effect of VIP therefore seems to be mediated by VIP receptors of smooth muscle cells.These properties of VIP in conjunction with its presence in nerve fibres located in the wall of the reticular groove and ROO are consistent with a role of VIP as inhibitory transmitter in this region of the bovine stomach.

Summary The effect of noradrenaline (NA) on smooth muscle strips from the reticular groove of adult cattle was studied. The mechanical activity of the muscle strips was recorded isometrically. Noradrenaline caused concentration dependent (1.1 · 10−6 ‐ 55 · 10−6mol/l) contractions of both the transversal muscle strips from the floor and the longitudinal muscle strips from the lips of the reticular groove. This excitatory effect of noradrenaline was antagonized by the α‐receptor antagonist prazosine (10−8 mol/1) and by higher concentrations of the α2‐receptor antagonist yohimbine (10−6 mol/1) and atropine (10−5 mol/1). Tetrodotoxin (5 ·10−6 mol/1) and latrotoxin did not affect the contractile response of the muscle strips to noradrenaline (55 · 10−6 mol/l). Furthermore, strips stored for 24 hours in Tyrode's solution at 4 °C without oxygen supply maintained their full sensitivity to noradrenaline. Propranolol (10−4 mol/1), a β‐receptor antagonist, led to a significant increase of the contractions induced by noradrenaline (55 · 10−6 mol/l). These results suggest that noradrenaline increases the tone of the smooth muscle of the reticular groove via α‐adrenergic receptors and decreases its tone via β‐adrenergic receptors. Both receptor types are located on the smooth muscle cells. The α1‐receptor mediated effect appears to be predominant.