Comparative studies of the alpha and beta adrenergic receptors in the longitudinal and circular smooth muscle layers of the simple and complex stomach

Effect of noradrenaline on smooth muscle strips from the reticular groove of adult cattle

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 x 10(-6)-55 x 10(-6) mol/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 alpha 1-receptor antagonist prazosin (10(-8) mol/l) and by higher concentrations of the alpha 2-receptor antagonist yohimbine (10(-6) mol/l) and atropine (10(-5) mol/l). Tetrodotoxin (5 x 10(-6) mol/l) and latrotoxin did not affect the contractile response of the muscle strips to noradrenaline (55 x 10(-6) mol/l). Furthermore, strips stored for 24 hours in Tyrode's solution at 4 degrees C without oxygen supply maintained their full sensitivity to noradrenaline. Propranolol (10(-4) mol/l), a beta-receptor antagonist, led to a significant increase of the contractions induced by noradrenaline (55 x 10(-6) mol/l). These results suggest that noradrenaline increases the tone of the smooth muscle of the reticular groove via alpha-adrenergic receptors and decreases its tone via beta-adrenergic receptors. Both receptor types are located on the smooth muscle cells. The alpha 1-receptor mediated effect appears to be predominant.

Effects of prostaglandin F2 alpha and indomethacin on the contractile activity of different compartments of the complex stomach

In vitro experiments have shown that PGF2 alpha or indomethacin produces different effects on muscle strips isolated from different compartments of sheep complex stomach. PGF2 alpha (10(-6) - 10(-5) M) increases the tone of the ruminal preparations and exerts a negligible effect on the reticulum and omasum strips. Indomethacin (5 X 10(-6) M) inhibits the mechanical activity of the rumen and omasum and has no effect on that of the reticulum. More pronounced are the effects on the different parts of the abomasum: PGF2 alpha (10(-9) - 10(-6) M) markedly increases the tone of the proximal abomasum and decreased the amplitude and the phasic contractions of the antral abomasum; indomethacin (5 X 10(-6) M) inhibits the tone and the phasic contractions of the middle abomasum. PGF2 alpha (10(-9) - 10(-6) M) inhibits while indomethacin (5 X 10(-6) M) stimulates the contractile activity of the antral abomasum.

Pharmacology of reticulo-ruminal motor function

experimental studies of the pharmacological control of the reticulo-rumen reveal two major central inhibitory pathways: (i) an adrenergic system which inhibits the magnitude of reticulo-ruminal movements and involves alpha 2-receptors and (ii) an inhibitory opioid system, which modulates the rate of the contractions. In addition, the coarseness of the stomach contents and the masticatory movements represent the major excitatory inputs to the gastric centres, hence the hypomotility and stasis observed in disease processes which lead to anorexia. Finally, the amplitude of reticular contractions will tend to vary inversely with the rate of contractions following the administration of stimulatory drugs acting centrally. As a practical statement, a direct action on the gastric centres is possible for drugs such as alpha blockers and/or morphine antagonists. Reflex excitatory effects may be induced by stimulation of muscarinic receptors of the reticulorumen wall as long as it remains below the threshold of a neuromuscular blockade. The highly complex mechanisms whereby reticuloruminal movements are regulated provide for (i) drug interactions, like those of histamine antagonists and opioid agonists, (ii) reflex inhibition of both amplitude and frequency of contractions arising from an enhanced abomaso-duodenal motility, like that induced by serotonin, and (iii) reflex stimulation of the rate of contractions elicited by the i.v. injection of catecholamines, an effect limited to the sheep. The multifactorial nature of reticuloruminal function suggests that treatment is unlikely to effect a cure but should materially re-establish a more normal motility. Further studies, required in diseased animals to provide for a curative use of drugs and a more thorough understanding of drug effects in normal ruminants, represent only a first step toward rational therapy. In addition, the extrapolation of drug effects from sheep to calves or cattle could be hazardous, especially for drugs whose mechanisms of action are currently unknown.

A possibility of substituting Ca2+ by Ba2+ or Mn2+ during the contractile processes of complex stomach smooth muscles

In vitro experiments were carried out on muscle strips from different compartments of sheep complex stomach. The following conclusions were drawn: 1. In Ca2+-free medium containing EDTA (0.5 mM) proximal and antral abomasum preparations are mechanically inactive. The addition of BaCl2 (0.2-1 mM) induces phasic contractions in the rumen and antrum strips. In contrast to the simple stomach Ba2+ causes a significant increase in the tone of the rumen and proximal abomasum preparations. This tonic contraction is verapamil resistant and is completely suppressed by sodium nitroprusside. 2. The Ba2+ effect is not mediated by cholinergic or adrenergic systems. Ba2+ can substitute Ca2+ in generating both phasic and tonic contractions in complex stomach smooth muscles. 3. Compared with the simple stomach Mn2+ (0.2-0.8 mM) influences neither the ACh-induced activity in normal nutrient medium nor the tone of the rumen and proximal abomasum preparations in Ca2+-free solution.

Canine gastric sling fibers: contractile properties

The contractile properties of canine sling fibers and the longitudinal and circular muscles were characterized and compared. The longitudinal and circular muscles developed spontaneous contractions, but the sling fibers developed no spontaneous phasic contractions. The cholinergic receptors of all three muscles were excitatory. The alpha- and beta-adrenergic receptors of the longitudinal and circular muscles were inhibitory, but the sling fiber alpha-adrenergic receptors were excitatory and the beta receptors inhibitory. Large concentrations of atrophine induced contractions of all three muscles. Some of the contractile properties of the sling fibers are more similar to those of the distal esophageal circular muscle than to those of the gastric longitudinal and circular muscles. The sling fibers together with the esophageal circular muscle may play an important role in the gastroesophageal sphincter mechanism.