The intrinsic rate response of the isolated right atrium of the rat, Rattus norvegicus

Experiments were performed on rat right atria maintained at 37 degrees C in oxygenated Krebs-Henseleit solution, at a baseline diastolic transmural pressure of 2 mmHg. A step increment in right atrial pressure caused an increase in atrial rate which reached a steady value after 2-3 min (rate response). An 8-mmHg increase in atrial pressure caused an 8% increase in atrial rate (n = 9, P < 0.01). When the atrial rate was reduced by carbamylcholine, the rate response was augmented. After a 34% reduction in atrial rate, an 8-mmHg increase in atrial pressure increased atrial rate by 51% (n = 7, P < 0.01). When atrial rate was elevated 71% by isoprenaline, the rate response was reversed (atrial rate decreased 3% following an 8-mmHg increase in atrial pressure; n = 7, P < 0.01). In another series of experiments, atrial rate was adjusted to a wide range of values by exposure to carbamylcholine and isoprenaline, first applied singly, and then in combination. At any given atrial rate, the rate response was always larger when both agonists were present; this difference was greatest when atrial rate was near the control untreated value.