The effect on the efferent vagal nerves to the heart of stimulating atrial receptors in the dog.

In chloralose-anaesthetized dogs, distension of small balloons at the pulmonary vein-atrial junctions to stimulate atrial receptors with myelinated vagal afferent nerves causes an increase in heart rate but does not influence the activity in efferent vagal cardiac nerves. However, distension of these small balloons also stimulates atrial receptors with non-myelinated vagal and sympathetic afferent nerves, which are thought to affect the heart rate and activity in efferent vagal cardiac nerves. In the present investigation, seven dogs anaesthetized with chloralose were studied by distension of small balloons at the pulmonary vein-atrial junctions and in the left atrial appendage, and by graded cooling of the vagal nerves in the neck; cooling to 9 degrees C was used to prevent the increase in activity in myelinated vagal afferent nerves to distension of the small balloons and cooling to 0 degree C was used to prevent responses to the distension in all vagal afferent nerves. Eleven vagal efferent nerve fibers were studied which responded to stimulation of carotid baroreceptors and chemoreceptors. Distension of the small balloons did not affect the activity in these eleven efferent vagal nerve fibres, with the vagi at 37 degrees C or during vagal cooling to 9 degrees C or to 0 degree C. The results indicate that upon distension of the small balloons, none of the three types of atrial receptor influence the activity in efferent vagal cardiac nerves. The results support the conclusion that stimulation of atrial receptors with myelinated vagal afferent nerves, responsible for the reflex increase in heart rate, does not influence the activity in efferent vagal cardiac nerves.