Splenic neurohormonal modulation of mesenteric vascular tone

In portal hypertension, development of a hyperdynamic circulation is preceded by transient mesenteric vasoconstriction. Portal hypertension increases splenic venous outflow pressure. We hypothesized that this causes direct reflex activation of mesenteric vasoconstrictor nerves and splenorenal reflex-mediated activation of the renin-angiotensin system. In anaesthetized male rats, we measured mesenteric efferent nerve activity and mesenteric vascular conductance (MVC) after selectively elevating splenic venous pressure. Partial splenic vein occlusion raised splenic venous pressure (from 4.8 ± 0.4 to 24.1 ± 0.3 mmHg; n = 18) and induced a significant increase in mesenteric efferent nerve activity (from 23.2 ± 3.3 to 31.6 ± 3.5 spikes s(-1); n = 11); this response was abolished by prior splenic denervation (from 32.4 ± 2.4 to 31.2 ± 1.6 spikes s(-1); n = 7). Mesenteric vascular conductance, the ratio of superior mesenteric artery blood flow to mean arterial pressure, fell upon splenic vein occlusion (ΔMVC = -0.0120 ± 0.0014 ml min(-1)mmHg(-1); P < 0.05, n = 10). This was attenuated by splenic denervation (ΔMVC = -0.0044 ± 0.0018 ml min(-1)mmHg(-1); P < 0.05, n = 8), but unaffected by mesenteric denervation (ΔMVC = -0.0145 ± 0.0020 ml min(-1)mmHg(-1); n = 6) or bilateral renal denervation (ΔMVC = -0.0106 ± 0.0021 ml min(-1)mmHg(-1); n = 5). Localized blockade of mesenteric vascular angiotensin II type 1 (AT(1)) receptors significantly attenuated the response (ΔMVC = -0.0058 ± 0.0017 ml min(-1)mmHg(-1); P < 0.05, n = 5), whereas blockade of both AT(1) and α(1)-adrenergic receptors caused a significant increase in mesenteric conductance (ΔMVC = +0.0033 ± 0.0010 ml min(-1)mmHg(-1); P < 0.05, n = 6). Our evidence suggests that increased splenic venous outflow pressure reflexly activates adrenergic/angiotensinergic mesenteric nerves, vasodilator mesenteric nerves and the renin-angiotensin system. We propose that obstruction to splenic venous outflow, such as would normally accompany portal hypertension, induces reflex mesenteric vasoconstriction independently of the increase in portal venous pressure.