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

Abstract

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.

Airway epithelial cells regulate membrane potential, neurotransmission and muscle tone of the dog airway smooth muscle.

“…Although no differences were found in membrane potential among upper, middle, and lower segments of the trachea, it was reported that the resting membrane potential of the smooth muscle cells of the bronchial airway (second generation) is -63 mV, that is, slightly larger than that of the trachealis (Souhrada et al 1983). We found that the resting membrane potential of the bronchiolar smooth muscle cells (about 1 mm in diameter) is about -70 mV (Inoue & Ito, 1986;Ito & Inoue, 1989; present study), a value significantly larger than those observed in the muscle cells in trachealis or bronchial wall. Thus, the smaller the airway, the larger the resting membrane potential of the airway smooth muscle cell, and it was assumed that the lower membrane potential in the trachealis, compared to that of bronchioles, is probably due to a large sodium conductance rather than to a smaller potassium conductance in the membrane (Ito & Inoue, 1989).…”

“…3A). We repeated the experiments in the presence of indomethacin (10-5 M) and guanethidine (106 M), since it was reported that 10-5 M-indomethacin suppressed the decremental responses in the helically cut bronchiolar tissues (Inoue & Ito, 1986). However, the twitch contraction of the intact bronchiolar ring evoked by EFS gradually decreased as shown in Fig.…”

“…that this compound at this concentration induces sustained contraction in bronchioles (Inoue & Ito, 1986). EFS applied during contraction evoked by 5-HT (10-5 M) produced an initial phasic contraction with a subsequent phasic relaxation.…”

“…Thus, the decremental responses observed in the mucosa-free trachealis and epitheliumdenuded bronchioles are due to endogenous prostaglandins which inhibit ACh release from nerve terminals of the vagus following stimulation of nervous elements in muscle tissues (Inoue et at. 1984;Inoue & Ito, 1986;Ito, 1991). Using cultured tracheal epithelial cells and intact tissues, it was found that a cyclo-oxygenasedependent inhibitory factor is produced by the epithelium, and that the factor is PGE2 (Barnett, Jacoby, Nadel & Lazarus, 1988).…”

“…Small airways (about 1 mm in diameter) were carefully excised from the lung tissue (under the microscopic observation), and lung parenchyma and pulmonary vessels running along a bronchiolar branch were removed. Histological investigations revealed that the tissue used for the present experiments had a diameter of 0 8-1P1 mm and was composed of smooth muscle layers and mucous membrane but lacked cartilage, thereby indicating that the tissue comprised bronchioles (Cumming, 1972;Inoue & Ito, 1986;Ito & Inoue, 1989). A piece of bronchiole 1 mm in diameter, 4-5 mm in length),and ring preparations (0 8-1 mm in diameter, 1-2 mm in length) were used for the microelectrode and tension-recording experiments, respectively.…”