Mechanisms of the White Sea Cod Gadus morhua marisalbi (Gadidae) Intestinal Smooth Muscle Cholinergic Contraction: The Contribution of Various Subtypes of M-cholinergic Receptors and Rho-kinase

Studying the functioning of various body systems in different vertebrates is one of the key tasks of comparative physiology. Teleost fishes are a large group of vertebrates; however, the mechanisms of their digestive tract functioning are poorly understood and are known mainly for such a model object as Danio rerio . The aim of this work was to characterize the participation of various subtypes of M-cholinergic receptors and the Rho-kinase enzyme in the cholinergic contraction of the smooth muscle of the intestine of the White Sea cod Gadus morhua marisalbi (Gadidae). A longitudinal strip was cut out from the proximal intestine and placed in the device for recording contractile responses in the isometric mode. Contractile reactions in response to acetylcholine were completely blocked by atropine. The blockade of M 3 -cholinergic receptors with 4-DAMP resulted in a decrease in acetylcholine-induced contraction compared to the control response. The blockade of M 1 -receptors with pirenzepine also decreased contractile responses, but the effect was less pronounced compared to the effect of M 3 -cholinergic receptor blockade. The blockade of M 2 -cholinergic receptors with methoctramine did not affect the magnitude of the contractile response. The incubation of preparations with the Rho-kinase inhibitor was accompanied by a significant decrease in contractile responses compared to the control as well as a more rapid decrease in the contraction force after reaching the “peak” response. Thus, acetylcholine causes contraction of cod intestinal smooth muscle by activating M 3 - and M 1 -but not M 2 -cholinergic receptors. The activity of the Rho-kinase enzyme contributes to the development and maintenance of cod intestinal smooth muscle contraction under the action of acetylcholine. The obtained results are of interest for comparative physiology; they may be important for understanding the mechanisms of the damaging effect of environmental factors on the organism of teleost fish as well as for the use of fish in preclinical studies of drugs.