EFFECTS OF BETA-ADRENERGIC AGONISTS ON SPLANCHNIC VASCULAR VOLUME AND CARDIAC-OUTPUT

The effect of beta-adrenergic agonists on splanchnic intravascular volume (SIV), measured with radionuclide imaging, and the subsequent influence of such volume changes on cardiac output (CO) were examined in 40 anesthetized dogs. Isoproterenol (6-mu-g/min) caused a decrease in total SIV of 12 +/- 1% (P < 0.001). The decrease was due entirely to a decrease in splenic volume of 24 +/- 3% (P < 0.001), since volume increased in the remainder of the splanchnic vasculature [hepatic and mesenteric volume increased 12 +/- 2% (P < 0.001) and 11 +/- 3% (P < 0.02), respectively]. CO increased from 1,724 +/- 187 to 3,138 +/- 321 ml/min (P < 0.001); after subsequent splenectomy, isoproterenol caused a similar increment. Isoproterenol-associated SIV changes were not altered by carotid denervation and vagotomy or by beta-1-adrenergic inhibition with metoprolol but were abolished by nonselective beta-adrenergic inhibition with propranolol. With a larger dose of metoprolol and smaller dose of isoproterenol to minimize beta-1-adrenergic effects, the isoproterenol-associated CO increment was attenuated (P < 0.01) by splenectomy. With the beta-2-agonist terbutaline (41-mu-g/min) after metoprolol, total SIV decreased 15 +/- 4% (P < 0.001). After subsequent alpha-adrenergic inhibition with phenoxybenzamine, terbutaline caused no change in SIV and an attenuated (P < 0.05) increase in CO. Thus beta-adrenergic agonist administration causes a decrease in total SIV due entirely to a decrease in splenic volume. The SIV decrement is dependent on beta-2- and alpha-adrenoceptor stimulation and appears to enhance CO only if beta-1-adrenergic effects are minimized.