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1 Baker Heart Research Institute, Melbourne, Victoria, Australia; Department of Medicine, Central and Eastern Clinical School, Monash University, Melbourne, Victoria, Australia
2 Baker Heart Research Institute, Melbourne, Victoria, Australia
3 Department of Pharmacology, Melbourne University, Melbourne, Victoria, Australia
* To whom correspondence should be addressed. E-mail: murray.esler{at}baker.edu.au.
The link between the human sympatho-adrenalmedullary system and the adipocyte hormone, leptin, is controversial. We measured total and regional norepinephrine spillover, epinephrine secretion rate and extra-adipocyte leptin release in 22 lean (BMI<26) and 20 obese (BMI>28) normotensive men who underwent arterial and central venous catheterisation. Since plasma clearance of leptin is primarily by renal removal, at steady-state we could estimate whole-body leptin release to plasma from renal plasma leptin extraction. Whole-body leptin release was 1950±643 (mean±SEM) ng/min in obese men and 382±124 ng/min in lean men (P<0.05). Total and renal norepinephrine spillover rates correlated directly with whole-body leptin secretion rate. Leptin is released from multiple non-adipocyte sites, which we tested for using simultaneous arterio-venous blood sampling. We found a surprisingly large contribution of brain leptin release to the plasma leptin pool, 529±175 ng/min (>40% whole-body leptin release), with greater leptin release in obese than lean men, 935±321 vs. 160±59 ng/min (p=0.045). In parallel with leptin measurements, we also quantified brain serotonin turnover and jugular overflow of neuropeptide Y (NPY). Brain serotonin turnover was higher in obese than lean men, 227±112 vs. 21±14 ng/min, (p=0.019), as was overflow of NPY from the brain, 12.9±1.4 vs. 5.3±2.2 ng/min, (p=0.042). These results suggest that leptin is released within the brain, and at an increased rate in obese humans, in whom activation of brain serotonergic and NPY mechanisms also exists.
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