The response of type B atrial receptors to distension and acute cardiac failure

Type B atrial receptors have been implicated in various reflexes which regulate body fluid balance and heart rate. In the present study type B atrial fibers were dissected from slips of the cervical vagus in open chest dogs and their response to changes in left atrial pressure (LAP) examined. Left atrial contractility was monitored using a strain gauge arch. LAP was varied by volume expansion and hemorrhage or by inflating a balloon in the left atrium. Acute cardiac failure was induced by propranolol (40 to 80 mg i.v.). Increases in LAP caused a nearly linear increase in discharge rate. Hemorrhage from expanded states resulted in a decrease in discharge concurrent with a fall in LAP; however, this decrease did not follow the previous expansion curve but resulted in a distinct hysteresis. A similar hysteresis was observed between receptor discharge and atrial segment length. Propranolol decreased atrial contractility and increased discharge rate per cardiac cycle in proportion to the associated increase in LAP. The relationship between receptor discharge and LAP was altered by propranolol only at high atrial pressures when discharge rate decreased as LAP was further increased. These experiments demonstrate that a depression of atrial contractility per se does not alter the relationship between B fiber discharge and atrial pressure.