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 th e tissues were precontracted with 5-hydroxytryptamine in the presence of at ropine, electrical field stimulation induced a frequency-dependent tetrodot oxin-sensitive relaxation. The relaxation was biphasic, consisting of fast and slow phases. The fast component was attenuated by propranolol, indicati ng an action on beta-adrenoceptors. The slow phase was attenuated by capsai cin and, therefore, involved release of a peptide. These results showed tha t excitatory responses in ovine trachealis muscles are cholinergically medi ated, while both adrenergic and peptidergic components mediate electrically induced relaxation in the trachea. We also examined the influence of lower ing bath temperature to 20 degrees C on electrically evoked responses. Thes e were significantly reduced by cooling. At 20 degrees C, under resting con ditions, the time-to-peak tension was lengthened, and the amplitude of the contractile responses was significantly (p < 0.05) reduced. In the same pre paration, carbachol-induced contractions were not reduced by cooling, indic ating 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 th e slow (non-adrenergic, non-cholinergic) component. Propranolol (1 mu mol/l ) and capsaicin (100 mu mol/l) did not affect significantly the slow relaxa tion observed during cooling. It was concluded that cooling inhibited choli nergically mediated, electrically induced contractions and selectively abol ished the adrenergic component of electrically induced relaxant responses.