SmoothB-Adrenoceptor-Mediated Responses in the Muscle of Hypert nsive Rats Gastric

Effects of isoproterenol on contraction and membrane potential of gastric smooth muscle were studied in stroke-prone spontaneous]y hypertensive rats (SHRSP) and normotensive Wistar Kyote rats (WKY). Circular muscle preparation from the gastric fundus developed tonic contraction by re-administration of Ca2' to a nominally Ca2'-free solution, The contraction was inhibited by nifedipine or nicardipine, Isoproterenol induced relaxation when it was applied to the Ca2t・ induced contraction. The amplitude of isoproterenolinduced relaxation was concentration-dependent. Propranolol 10'6 M abolished the relaxa・ tion induced by isoproterenol 10'7M. In the preparation from SHRSP, the amplitude of isoproterenol-induced relaxation was smaller than that from WKY between 3× 10'e and 10" M. Forskolin, an adenylate cyclase activator, incluced concentration-dependent relaxation. There was no difference in the relaxation induced by forskolin between preparations from WKY and SHRSP, Dibutilyl cyclic AMP, a membrane permeable analogue of cyclic AMP, also induced similar relaxation in preparations from WKY andSHRSP. Resting membrane potential of smooth muscle cell was not different between preparatiuns from WKY ancl SHRSP. Isoproterenol hyperpolarized the membrane concentration-dependently. Isoproterenol-induced hyperpolarizatien in the preparation from SHRSP was smaller than that from WKY between 10'S and 10 eM. When the membrane was depolarized by Tyrode's solution containing 40 rnMK', isoproterenol-induced hyperpolarization was almost abolished. In this conclition, the isoproterenol-induced relaxation was inhibited partly, ho"Tever, there was no difference in the amplitude of relaxatien between preparations frorn WKY and SHRSP. Therefore, isoproterenol-induced hyperpolarization contributed at least partly to the relaxation. Forskolin hyperpolarizecl the membrane by the sarne amplitude in the preparatiens from WKY and SHRSP. These resurts indicate that a decrease in hyperpolarizatien may contribute to the decreased relaxation by isoproterenol in the preparation from SHRSP.