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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.