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Biphasic Relaxant Response of Ovine Trachealis Muscle to Electrical Field Stimulation: Influence of Cooling

Electrical field stimulation of ovine trachealis muscle produced neurogenic atropine-sensitive contractions under resting conditions. However, when the tissues were precontracted with 5-hydroxytryptamine in the presence of atropine, electrical field stimulation induced a frequency-dependent tetrodotoxin-sensitive relaxation. The relaxation was biphasic, consisting of fast and slow phases. The fast component was attenuated by propranolol, indicating an action on β-adrenoceptors. The slow phase was attenuated by capsaicin and, therefore, involved release of a peptide. These results showed that excitatory responses in ovine trachealis muscles are cholinergically mediated, while both adrenergic and peptidergic components mediate electrically induced relaxation in the trachea. We also examined the influence of lowering bath temperature to 20°C on electrically evoked responses. These were significantly reduced by cooling. At 20°C, under resting conditions, the time-to-peak tension was lengthened, and the amplitude of the contractile responses was significantly (p < 0.05) reduced. In the same preparation, carbachol-induced contractions were not reduced by cooling, indicating that the reduction in electrically induced contractions was probably due to a reduction in transmitter release. Cooling also abolished the fast inhibitory phase (adrenergic in nature) without significantly inhibiting the slow (non-adrenergic, non-cholinergic) component. Propranolol (1 µmol/l) and capsaicin (100 µmol/l) did not affect significantly the slow relaxation observed during cooling. It was concluded that cooling inhibited cholinergically mediated, electrically induced contractions and selectively abolished the adrenergic component of electrically induced relaxant responses.