Effect of changes in transmural pressure on contraction frequency of the isolated right atrium of the rabbit

Experiments were made on preparations of the rabbit right atrium maintained at 37 degrees C in oxygenated Krebs-Henseleit solution. Baseline diastolic transmural pressure was held at 2 mmHg. A step increase in diastolic pressure was accompanied by an immediate and rapid increase in atrial rate (fast response), followed by a slower increase (t(1/2) similar to 0.5 min) (slow response). The slow response to pressure steps was graded. approaching a maximum increase after a 12 mmHg step (44 +/- 4 min(-1) from a baseline of 196 +/- 5 min(-1); mean +/- SEM; n = 7; P < 0.01). In preparations where baseline atrial rate had been reduced 50% by application of carbamylcholine. the slow response to an increase in pressure was augmented (n = 7, P < 0.01); an increase of 55 +/- 9 min(-1) for a 12 mmHg step in atrial pressure. In preparations where baseline rate had been increased 63% by the application of isoprenaline, the slow response was attenuated (n = 5, P < 0.01), an increase of 22 +/- 7 min(-1) for a 12 mmHg step. During sinusoidal pressure forcing (0.002-1.0 Hz), rate responses of control and carbamylcholine-treated preparations had a high gain at frequencies less than or equal to 0.02 Hz. Carbamylcholine-treated preparations also showed a high gain at frequencies greater than or equal to 0.2 Hz. There appear to be two distinct intrinsic responses to changes in right atrial pressure: a rapid response which is augmented by cholinergic stimulation. and a slower response which is augmented by cholinergic stimulation and reduced by beta-adrenergic stimulation.