THE EFFECTS OF ACETYLCHOLINE ON THE MEMBRANE AND CONTRACTILE PROPERTIES OF SMOOTH MUSCLE CELLS OF THE RABBIT SUPERIOR MESENTERIC ARTERY

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Abstract
1 Effects of acetylcholine (ACh) on the membrane potential and mechanical properties of rabbit superior mesenteric artery were investigated by the use of microelectrode and isometric tension recording methods. The membrane potential was −62.5 ± 3.0 mV (s.d.). The maximum slope of the membrane depolarization produced by tenfold increase in [K]0 plotted on a log scale was 48 mV. Excess [K]0 and low [K]0 depolarized the membrane and produced contraction (contracture). The minimum depolarization to produce contraction was 10 mV. 2 Low concentrations (10 and 100 ng/ml) of ACh hyperpolarized the membrane. Increased concentrations of ACh (1 and 10 μg/ml) hyperpolarized the membrane further in adult rabbit, while increased concentrations of ACh produced a smaller hyperpolarization in young rabbit. These potential changes produced by ACh in immature and adult rabbits were suppressed by treatment with atropine (0.1 μg/ml). 3 ACh (10 ng to 1 μg/ml) consistently generated contraction in Krebs solution. However, ACh relaxed the contraction induced by either K+ or noradrenaline in the adult rabbit, and it enhanced contraction produced by this treatment in the immature rabbit. In Ca-free EGTA solution, the action of ACh on the mechanical response was markedly suppressed, although high concentrations of ACh still evoked contraction. However, treatment with atropine (1 μg/ml) completely prevented these actions of ACh. 4 ACh-induced relaxation during either K+-induced or noradrenaline-induced contraction was not caused by the hyperpolarization of the membrane. 5 It is concluded that ACh possesses dual actions on smooth muscle cells of the rabbit superior mesenteric artery in Krebs solution, i.e. ACh hyperpolarizes the membrane, while it consistently generates contraction. These ACh actions on the muscle cells were modified by aging.