Splanchnic blood flow and hepatic glucose production in exercising humans : role of renin-angiotensin system

The study examined the implication of the renin-angiotensin system (RAS) in regulation of splanchnic blood flow and glucose production in exercising humans. Subjects cycled for 40 min at 50% maximal O2 consumption (VO2 max) followed by 30 min at 70% VO2 max either with [angiotensin-converting enzyme (ACE) blockade] or without (control) administration of the ACE inhibitor enalapril (10 mg iv). Splanchnic blood flow was estimated by indocyanine green, and splanchnic substrate exchange was determined by the arteriohepatic venous difference. Exercise led to an ~20-fold increase (P < 0.001) in ANG II levels in the control group (5.4 ± 1.0 to 102.0 ± 25.1 pg/ml), whereas this response was blunted during ACE blockade (8.1 ± 1.2 to 13.2 ± 2.4 pg/ml) and in response to an orthostatic challenge performed postexercise. Apart from lactate and cortisol, which were higher in the ACE-blockade group vs. the control group, hormones, metabolites, VO2, and RER followed the same pattern of changes in ACE-blockade and control groups during exercise. Splanchnic blood flow (at rest: 1.67 ± 0.12, ACE blockade; 1.59 ± 0.18 l/min, control) decreased during moderate exercise (0.78 ± 0.07, ACE blockade; 0.74 ± 0.14 l/min, control), whereas splanchnic glucose production (at rest: 0.50 ± 0.06, ACE blockade; 0.68 ± 0.10 mmol/min, control) increased during moderate exercise (1.97 ± 0.29, ACE blockade; 1.91 ± 0.41 mmol/min, control). Refuting a major role of the RAS for these responses, no differences in the pattern of change of splanchnic blood flow and splanchnic glucose production were observed during ACE blockade compared with controls. This study demonstrates that the normal increase in ANG II levels observed during prolonged exercise in humans does not play a major role in the regulation of splanchnic blood flow and glucose production.