INDIRECT ALPHA ADRENERGIC RESPONSE TO ISOPROTERENOL IN THE DOG SPLEEN

¹ Division of Pediatric Cardiology, Department of Pediatrics, University of Washington School of Medicine, Seattle, Washington

Although the spleen is reported to respond only to alpha adrenergic stimulation, intravenous isoproterenol causes vigorous contraction. To resolve this apparent contradiction, we have studied the splenic response in 22 dogs. Ultrasonic dimension transducers were placed on the splenic capsule and an artery was cannulated. Contraction of the spleen to intravenous phenylephrine occurred within 15 seconds after injection. Injection of small quantities of isoproterenol (ISO) directly into the splenic artery produced a slight dilatation of the spleen. Intravenous injection of ISO (1 µg/kg) caused a slight dilatation of the spleen at 14 seconds, followed by contraction at 27 seconds, coincident with a substantial fall in mean arterial blood pressure, but little change in systolic pressure. Surgical denervation of the spleen resulted in prolongation of dilatation with intravenous ISO and a delayed splenic contraction which could be blocked by bilateral adrenalectomy. Splenic contraction after intravenous ISO thus represents secondary alpha adrenergic activity induced by a fall in blood pressure, consisting of an early neural phase and a later humoral phase. This induced contraction occurs during the phase of arteriolar dilatation and indicates a therapeutic advantage of isoproterenol: increased flow without venous pooling.