Transmural electrical stimulation-induced relaxation of the guinea-pig gall bladder

The responses of isolated gal bladder to transmural electrical stimulation with rectangular pulses (50 volt, 0.5 ms) of 30 Hz for 10 s at intervals of 30 min were observed for 5 to 6 hours continuously and compared for different values of resting tonus in Krebs solution at 27 degrees C and 37 degrees C Krebs solution, the resting tonus increased gradually with time and became stable at 0.56 +/- 0.07 g (mean +/- S.D.) after 5 h. In 37 degrees C Krebs solution, the resting tonus increased gradually with time and became stable at 1.48 +/- 0.49 g (mean +/- S.D.) to 1.5 +/- 0.19 g (mean +/- S.D.) after 5 h. Monophasic contraction was observed when the resting tonus was lower than 1.49 +/- 0.15 g (mean +/- S.D.), but above this value, biphasic response consisting of contraction and relaxation was observed. Both monophasic and biphasic responses were blocked by atropine and tetrodotoxin. Physostigmine enhanced monophasic contraction, increased the duration of contraction, and abolished the relaxation of biphasic contractile response. The results suggest that contractile response of isolated guinea-pig gall bladder to transmural electrical stimulation was directly induced by acetylcholine released from postganglionic cholinergic fibers. Transmural electrical stimulation with rectangular pulses (50 volt, 0.5 ms) at 10, 20, 30 or 40 Hz for 10 s induced biphasic response in which the contraction varied with frequency, peaking at about 30 Hz, while the relaxation was almost constant. Biphasic relaxation was reduced or inhibited by 2,4-dinitrophenol, ouabain, or K+ -free solution. This suggests that the relaxation phase of the biphasic response induced by transmural electrical stimulation was an active process.