Reflex effects of left atrial pressure elevation on total intravascular volume

The reflex autonomic influence of left atrial baroreceptor stimulation on the total capacitance vasculature has not been examined. To this end, left atrial pressure was increased in 25 anesthetized dogs, in which blood from the vena cavae was drained into an extracorporeal reservoir and returned to the right atrium at a constant rate, so that changes in intravascular volume could be recorded as reciprocal changes in reservoir volume. Left atrial pressure was elevated from 5 ± 1 (mean ± SE) to 11 ± 1 mmHg by inflating a balloon at the mitral orifice for 12-20 min. With left atrial pressure elevation, total intravascular volume decreased 25 ± 10 ml (P < 0.025). In six of the dogs, intravascular volume decreased 37 ± 12 ml with left atrial pressure elevation before bilateral cervical vagectomies and increased 66 ± 8 ml with atrial pressure elevation after vagectomies (P < 0.001). In eight of the dogs, volume decreased 42 ± 19 ml with atrial pressure elevation before propranolol administration, and increased 44 ± 29 ml after propranolol (P < 0.03). Phenoxybenzamine in five of the animals and atropine in three did not attenuate the change in intravascular volume with left atrial pressure elevation. Thus left atrial baroreceptor stimulation is associated with an autonomic reflux, which acts to decrease intravascular volume. The afferent limb is mediated by the vagi, and the efferent limb, by β-adrenergic receptor stimulation.