Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB371

Poster Communications

Mechanisms involved in relaxations induced by electrical field stimulation in murine airways smooth muscle.

X. Lim1, E. Bradley1, G. Sergeant1, M. Hollywood1, K. D. Thornbury1

1. Smooth Muscle Research Centre, Dundalk Institute of Technology, Dundalk, Ireland.


Purpose: The mechanism underlying electrical field stimulation (EFS)-induced relaxations in airways smooth muscle (ASM) is unknown. This study tested the hypothesis that EFS relaxes ASM by neuronal-release of substance P (SP) that activates NK1 receptors, causing release of inhibitory prostaglandin, PGE2. Methods: Isometric tension was measured in bronchial rings from mouse. Bronchi were pre-contracted with carbachol and EFS applied (0.3 ms pulse, 4 Hz frequency, amplitude 20 V, 10 sec). Results: EFS relaxed precontracted rings. Tetrodotoxin (10 μM) failed to block the relaxations (control mean: 1.61 ± 0.41 mN, TTX mean: 2.40 ± 0.40 mN; P > 0.05, n=4). Tetrodotoxin plus PF-01247324 (1 μM), which selectively blocks tetrodotoxin-resistant NaV1.8 channels, also failed to block the responses (control mean: 1.61 ± 0.41 mN, TTX plus PF-01247324 mean: 2.60 ± 0.53 mN; P > 0.05, n = 4). However, when rings were contracted with 60 mM KCl, the mean EFS-induced relaxation amplitude was reduced from 0.50 ± 0.06 mN to 0.03 ± 0.03 mN (P < 0.05, n = 4), suggesting that membrane potential was involved, hence the responses might be nerve-mediated. SP mimicked EFS-induced relaxations. Both responses were blocked by indomethacin (P< 0.05), a blocker of cyclooxygenase, which catalyses the synthesis of prostaglandins from arachidonic acid. Mean relaxation amplitude caused by EFS was significantly inhibited by indomethacin (10 μM) from 1.12 ± 0.17 mN to 0.28 ± 0.12 mN (P < 0.05, n = 8). Indomethacin (10 μM) also significantly inhibited mean relaxation amplitude caused by SP from 1.69 ± 0.28 mN to 0.10 ± 0.06 mN (P < 0.05, n = 3). SR-140333 (100 nM), a NK1 receptor blocker, abolished the SP response (from 0.74 ± 0.14 mN to 0.04 ± 0.02 mN, P < 0.05, n = 4) but had no effect on EFS-induced relaxations (control mean: 0.73 ± 0.16 mN, SR-140333 mean: 0.69 ± 0.13 mN; P > 0.05, n = 4). Mean relaxation amplitude caused by SP was significantly inhibited by SR-140333 from 0.74 ± 0.14 mN to 0.04 ± 0.02 mN (P < 0.05, n = 4). Addition of GR-159897 (100 nM), a NK2 receptor blocker, also failed to block EFS relaxations (P > 0.05, n = 4). To test if PGE2, acting on EP2 receptors was involved, PF-04418948 (100 nM), an EP2 receptor blocker, was used. This caused variable reductions in amplitude (from 2.25 ± 0.66 mN to 1.52 ± 0.32 mN, P > 0.05, n = 8) but significantly shortened the durations of relaxations. The latter was assessed by measuring the half maximal recovery times, which decreased from 213 ± 34 sec to 115 ± 28 sec (P < 0.05, n = 8). Conclusion: EFS-induced relaxations occur independently of SP release and NK1/NK2 receptors activation, but they involve cyclooxygenase activity. Also, the relaxations appear to be partially mediated by EP2 receptor activation, as evidenced by the shortening of their recovery time in PF-04418948. However, the early part of the response was less sensitive to blockade of EP2 receptors, suggesting that more than one mechanism is involved.

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