Spleen volume changes during adrenergic stimulation with low doses of epinephrine

It is generally accepted that the spleen contraction is a consequence of humoral stimulation but recent data suggest a role of neural mechanisms. This study tested the hypothesis that the reduction in spleen size in response to low dose epinephrine infusion is a consequence of neurally mediated unloading of baroreceptors. Continuous ultrasonic measurements of spleen volume in response to intravenous infusion of low doses of epinephrine (0.06 μg/kg/min for 6 minutes, followed 0.12 μg/kg/min for 3 minutes) were performed with simultaneous continuous noninvasive measurements of cardiovascular parameters in thirteen subjects. In subgroup of six subjects we also continuously measured muscle sympathetic nerve activity (MSNA) as an index of peripheral sympathetic activation. Significant spleen contraction (≈30%, p=0.008) was observed early after the onset of epinephrine infusion and was preceded by a decrease in total peripheral resistance (41%, p=0.001) and mean arterial pressure (6.2%, p=0.02) and an increase in heart rate (27%, p=0.001) and total MSNA (120%, p=0.02). Our results demonstrate rapid spleen contraction induced by low-dose epinephrine infusion in conditions of decreased blood pressure and increased MSNA suggesting that the spleen may represent a constitutive part of the sympathetic nervous system under stressful situations.