Characteristics of neuro-effector transmission in the smooth muscle layer of dog bronchiole and modifications by autacoids.

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Electrical and mechanical properties of smooth muscle cells and of neuro-effector transmission in the smooth muscle layer of the dog bronchiole (about 1 mm in diameter) were studied with double-sucrose-gap, micro-electrode or tension recording methods. Electrical field stimulation of short duration (50 microseconds) applied to the whole tissue excited the intrinsic nerves, and evoked excitatory junction potentials (e.j.p.s) followed by a twitch-like tension; these were abolished by tetrodotoxin (10(-7) M) or by atropine (10(-6) M). 5-Hydroxytryptamine (10(-5) M) produced a tonic contracture of the bronchiole, and electrical field stimulation applied during the tonic contracture produced an initial phasic contraction and a subsequent relaxation. Atropine (10(-6) M) and propranolol (10(-6) M) selectively abolished the phasic contraction and relaxation, respectively, indicating that dog bronchiolar muscles are innervated by excitatory cholinergic and inhibitory adrenergic nerve fibres. E.j.p.s and twitch contractions evoked by electrical field stimulation at 3 min intervals showed a gradual reduction in amplitude during superfusion with normal Krebs solution, and this reduction was overcome by pre-treatment with indomethacin (10(-5) M). The mean value of the resting membrane potential of the bronchiole smooth muscle cells was -70.2 +/- 2.2 mV (+/- S.D., n = 150), and an action potential was superimposed on e.j.p.s in 50% of the muscle cells examined when the amplitude of e.j.p.s exceeded 35 mV. During repetitive field stimulation at 0.1-0.2 Hz, the amplitude of the e.j.p.s gradually increased (facilitation); this phenomenon was markedly enhanced by indomethacin (10(-5) M) and was depressed by exogenously applied prostaglandins in low concentrations (10(-11)-10(-8) M). Histamine (5 X 10(-8)-5 X 10(-7) M) enhanced the amplitude of e.j.p.s and twitch contraction evoked by field stimulation, and this effect was antagonized by mepyramine (10(-7) M). Histamine (10(-7) M) enhanced the amplitude of the ACh-induced contraction when a relatively low concentration (10(-7) M) of ACh was applied, but not when concentrations of 10(-6) M- or 10(-5) M-ACh were applied. Histamine had no effects on the facilitation of e.j.p. Bronchiolar smooth muscle cells therefore showed larger resting potentials and a greater tendency to fire action potentials than trachealis muscle, and prostaglandins and histamine are involved in inhibitory and accelerative mechanisms related to excitatory neuro-effector transmission, respectively.

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