In the isolated, blood-perfused @entity235 right atrium, which was pretreated with @entity1169 , negative chronotropic and inotropic responses were evoked by stimulation of the intramural parasympathetic nerve fibers or by intra-arterial infusion of @entity934 ( @entity3451 ). Successive cholinergic interventions were applied; first, a conditioning intervention for 2 min was given, then this was followed by a test intervention for 4 min. The two interventions were separated by a rest period that varied from 15 to 240 s. The cardiac responses to the conditioning parasympathetic nerve stimulation quickly reached maximum levels, and then they "faded" or progressively diminished back toward the control level. The inotropic responses to the conditioning infusion of @entity3451 (1 microgram/min) faded slightly but the chronotropic response did not. After the rest period, the test nerve stimulation evoked responses that also gradually faded with time. The maximal amplitude of the responses to the test stimuli were less than those to the conditioning stimuli. This reduction in the maximal amplitude of the cardiac responses to the test stimuli was more pronounced with high frequency stimulation (30 Hz) than with low frequency stimulation (5 Hz). The decrement was also more pronounced the shorter the rest period, and it was greater at earlier times after beginning the stimulation. Conversely, the maximal cardiac responses to test infusions of @entity3451 were not appreciably less than the responses to the conditioning infusions. We conclude, therefore, that the diminution of the cardiac responses to the second test stimulation of the parasympathetic nerve fibers was mainly ascribable to a prejunctional rather than to a postjunctional mechanism.

Attenuation of the responses to repeated cholinergic interventions in the isolated XXXX atrium.

@entity235 :: ('9615', 'Species') :: ['canine'] @entity3451 :: ('D000109', 'Chemical') :: ['ACh'] @entity934 :: ('MESH:D000109', 'Chemical') :: ['acetylcholine'] @entity1169 :: ('MESH:D011433', 'Chemical') :: ['propranolol']
@entity235 :: (9615,Species) :: ['dog']
