AIRWAY EPITHELIAL-CELLS REGULATE MEMBRANE-POTENTIAL, NEUROTRANSMISSION AND MUSCLE TONE OF THE DOG AIRWAY SMOOTH-MUSCLE

1. The effects of epithelial cells were investigated on resting membrane potential and neuro-effector transmission in smooth muscle cells of the dog tracheal and bronchiolar tissues. 2. The mean value of the resting membrane potential of the epithelium-intact bronchiolar smooth muscle cells of the dog was -70.0 +/- 1.1 mV (+/- S.D., n = 40) and mechanical denudation of the epithelial layer depolarized the membrane to -57.0 +/- 2.5 mV (+/- S.D., n = 40). Application of isolated and dispersed epithelial cells (> 2 x 10(5) cells/ml) to the perfusing solution repolarized the membrane of epithelium-denuded bronchiolar smooth muscle cells to -67.0 +/- 2.7 mV (+/- S.D., n = 20). The mean resting membrane potential of the mucosa-free tracheal smooth muscle cells was -59.1 +/- 1.4 mV (+/- S.D., n = 50), and application of isolated and dispersed cells (> 2 x 10(5) cells/ml) hyperpolarized the membrane to -67.2 +/- 1.8 mV (+/- S.D., n = 50). These repolarizing actions were not modified by indomethacin (10(-5) M). 3. In the epithelium-denuded bronchioles, ACh (> 10(-9) M) dose-dependently depolarized the smooth muscle cells, while in the epithelium-intact bronchioles, ACh (10(-11)-10(-8) M) did not affect the resting membrane potential. At a concentration of 10(-7) M, ACh significantly depolarized the membrane. 4. Electrical field stimulation (EFS; 50-mu-s in duration and about 10-20 V in strength) applied to ring preparations of the bronchioles evoked twitch-like contractions (hereafter referred as twitch contraction), and size of the twitch contractions gradually and continuously decreased in the presence or absence of indomethacin (10(-5) M) and guanethidine (10(-6) M). When similar experiments were performed using epithelium-denuded bronchiolar ring preparations, in no case was there a prominent reduction in the amplitude of the twitch contractions in the presence of indomethacin and guanethidine. 5. The decremental response of the twitch contraction observed in the epithelium-intact bronchioles was overcome by application of the leukotriene synthesis inhibitor AA861 (10(-6) M) and the leukotriene antagonist ONO1078 (10(-5) M). 6. Leukotrienes C4 and D4 (LTC4 and LTD4, > 10(-8) M) evoked muscle contraction with a steady increase in muscle tone, up to a certain level. However, at 10(-9) M, LTC4 increased and LTD4 decreased the amplitude of the twitch contractions evoked by EFS in the epithelium-intact bronchioles. On the other hand LTE4 (10(-8) M) consistently suppressed the relationship between the number of stimuli and the relative amplitude of the twitch contractions. 7. In the epithelium-denuded bronchioles, indomethacin (10(-5) M) markedly enhanced the amplitude of the excitatory junction potentials (EJPs, to 1.56 +/- 0.07 of the control value, +/- S.D., n = 4). However, this compound only slightly increased the EJP amplitude in the epithelium-intact bronchioles. The leukotriene synthesis inhibitor AA861 (10(-6) M) enhanced the EJP amplitude to 1.66 +/- 0.25 times the control value (+/- S.D., n = 4) in the epithelium-intact bronchioles, but had little effect on the epithelium-denuded bronchioles. In the intact bronchioles LTC4 (10(-8) M) markedly and LTB4, D4 and E4 (10(-8) M) slightly enhanced the EJP amplitude. 8. Application of isolated and dispersed epithelial cells (3.5 x 10(5) cells/ml) to the bath significantly suppressed the EJP amplitude of mucosa-free trachealis to 0.48 +/- 0.09 (+/- S.D., n = 5) times the control value, and these effects were only partially suppressed by indomethacin (10(-5) M). In the presence of indomethacin (10(-5) M), the dispersed epithelial cells suppressed the EJP amplitude to 0.64 +/- 0.03 times the control, and the additional application of AA861 (10(-6) M) further enhanced the EJP amplitude to 0.70 +/- 0.06 (+/- S.D., n = 4). 9. Hydroxytryptamine (10(-5) M) produced a tonic contraction of the intact bronchiole, and electrical field stimulation (EFS) applied during tonic contraction produced an initial phasic contraction and a subsequent relaxation in the presence of indomethacin (10(-5) M) and guanethidine (10(-6) M). Atropine (10(-6) M) and mechanical denudation or hypo-osmolarity shock selectively abolished the phasic contraction and relaxation, respectively. 10. These observations suggest that a factor(s) released from epithelial cells in the airway plays multiple roles in controlling the resting membrane potential, neuro-effector transmission and muscle tone of the smooth muscle cells.