SEPARATION OF THE EFFECTS OF ALPHA AND BETA ADRENERGIC RECEPTOR STIMULATION ON TAENIA COLI

¹ Department of Pharmacology, University of Michigan, Ann Arbor, Michigan; Parke, Davis & Company, Research Laboratories, Ann Arbor, Michigan

The effects of catecholamines on isolated taenia coli from guinea pigs were studied with strain gauge and sucrose gap techniques. Elevated concentrations of potassium (35 and 90 mM) were used to stimulate the smooth muscle. In the presence of 35 mM K⁺, norepinephrine and epinephrine inhibited spike activity and relaxed the muscle by acting mainly on alpha receptors. A beta receptor component in the relaxation induced by norepinephrine or epinephrine could be demonstrated only when higher concentrations of the agonists were used after blockade of the alpha receptors. Isoproterenol in low concentrations elicited relaxation by beta receptor stimulation, although at higher concentrations relaxation was mediated by alpha receptor stimulation. Relaxation elicited by the lower concentrations of isoproterenol had no effect on the electrical activity. In the presence of 90 mM K⁺, all three catecholamines caused relaxation through stimulation of beta receptors only. There were no changes in the electrical activity accompanying the relaxation. No alpha receptor- mediated effects on electrical or mechanical activity could be demonstrated in the depolarized taenia. In contrast to the catecholamines, aminophylline elicited relaxation of the taenia in both 35 and 90 mM K⁺ and was not blocked by catecholamine antagonists. We conclude that when taenia coli is stimulated by increased concentrations of extracellular potassium, alpha receptor activation leads to relaxation by a suppression of the spike activity of the cell membrane; beta receptor activation leads to relaxation by an effectnot involving the electrical activity of the membrane.