@entity934 release by a stimulus train lowers XXXX threshold.

@entity6839 @entity235 @entity4486 @entity3897 @entity955 @entity934
This study was designed to evaluate the importance of local release of autonomic neuromediators when electrical stimuli are applied to the right atrium to measure the @entity955 threshold (AFT). Experiments were performed in 16 open-chest @entity235 anesthetized with @entity4486 . The @entity235 were denervated by bilateral transection of the stellates and cervical vagi. The AFT was determined in 11 @entity235 by delivering either a train of stimuli (14 pulses, 4 ms, 100 Hz) or a single stimulus (10 ms) to the right atrium during its vulnerable period. In eight @entity235 , beta-adrenergic blockade with @entity6839 (0.1 mg/kg) had no effect on the AFT determined with either method. @entity3897 (0.2 mg/kg), given after @entity6839 , significantly increased the train-of-pulses AFT from 4.7 +/- 0.4 to 32.3 +/- 4.6 mA (P less than 0.001). The single-pulse AFT increased from 16.5 +/- 1.5 to 17.8 +/- 1.5 mA (P less than 0.05). @entity3897 had a similar effect on the AFT when it was given in the absence of @entity6839 (n = 3). In five additional @entity235 , a monophasic action potential was recorded while a 10-mA train was delivered to the atrium during its absolute refractory period. There was marked shortening of the monophasic action potential duration (55 +/- 6 ms) in the first beat after the train. The shortening was totally abolished by @entity3897 (0.2 mg/kg). The results suggest that a train of stimuli liberates local stores of @entity934 , which cause a shortening of atrial repolarization time and a profound decrease in the current necessary to evoke @entity955 .
