Regional Sympathetic Nervous Activity and Oxygen Consumption in Obese Normotensive Human Subjects

Abstract

Regional SNS activity is heterogeneous in the obese state. Important regional alterations, which may be clinically relevant, occur in the absence of changes in global indices of sympathetic nervous function. The enhanced renal NE spillover in obesity may have implications for the development of hypertension in this group, whereas the low cardiac sympathetic tone would be expected to be cardioprotective. Enhanced visceral oxygen consumption evident in the kidneys and hepatomesenteric circulation in proportion to body mass contributes to the greater resting oxygen consumption in obesity.

Sympathetic Overdrive and Cardiovascular Risk in the Metabolic Syndrome

“…This has been confirmed by the results of the Normative Aging Study, which in a large number of subjects has shown a close direct relationship between 24-h urinary figures of norepinephrine metabolites and body mass index values (17). This has also been confirmed by the evidence that an augmented sympathetic neural discharge to skeletal muscle as well as an increased spillover rate of norepinephrine from sympathetic nerve endings, particularly at the level of the kidney, takes place in obese normotensive subjects (18,19). Taken together, these findings provide conclusive evidence that the sympathetic overdrive plays a role in human obesity, although the magnitude of this role differs somewhat according to the specific patterns of fat distribution.…”

“…This is based on the evidence that 1) cardiovascular as well as metabolic function are modulated by sympathetic mechanisms and 2) several of the pathologic states clustering in the metabolic syndrome already display an increased adrenergic drive (2,3,10,(18)(19)(20). This is also based on the finding that when obesity and hypertension are concomitantly present in the same patient, the degree of sympathetic activation is much greater than in patients with either condition singly (24).…”

Sympathetic Vascular Tone in Human Obesity

“…Indeed, norepinephrine spillover is increased to the kidneys but not to the heart or to the hepatomesenteric circulation in obese subjects [Vaz et al, 1997]. This result is further corroborated by the observation that total body release of leptin, a marker of increased adiposity, is directly correlated with total body and renal norepinephrine spillover [Eikelis et al, 2004].…”

“…For instance, insulinemia does not correlate with renal spillover of norepinephrine in obese subjects [Vaz et al, 1997] whereas euglycemic hyperinsulinemic clamp does not increase sympathetic outflow to the kidneys in lean subjects [Gudbjornsdottir et al, 1994]. Overall, these results suggest that hyperinsulinemia per se may not cause sympathetic activation homogeneously at specific target organs.…”

“…It is possible that reductions in mSNA may reflect attenuation of sympathetic activation to other organs or systems that might be relevant to arterial pressure control. Indeed, mSNA is closely correlated with renal norepinephrine spillover [Vaz et al, 1997]. Ambulatory systolic arterial pressure decreased moderately in all obese groups whereas ambulatory heart rate decreased significantly only in obese normotensive subjects.…”