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-Adrenoceptors mediate inhibition of lipolysis in
adipocytes of tilapia (Oreochromis mossambicus)
1 Department of Integrative Zoology, Institute of Evolutionary and Ecological Sciences, University of Leiden, Van der Klaauw Laboratories, 2300 RA Leiden; and 2 Department of Molecular Pharmacology, University of Groningen, 9713 AV Groningen, The Netherlands
The regulation of triglyceride
mobilization by catecholamines was investigated in the teleost fish
Oreochromis mossambicus (tilapia) in vivo and in vitro. In
vitro experiments were carried out with adipocytes that were isolated
for the first time from fish adipose tissue. For the in vivo
experiments, cannulated tilapia were exposed to stepwise decreasing
oxygen levels (20, 10, and 5% air saturation; 3.9, 1.9, and 1.0 kPa
PO2, respectively), each level being maintained
for 2 h. Blood samples were taken at timed intervals and analyzed
for plasma lactate, glucose, free fatty acids, epinephrine,
norepinephrine, and cortisol. Hypoxia exposure did not change plasma
epinephrine levels. In contrast, the plasma norepinephrine
concentration markedly increased at all hypoxia levels. Over the same
period, plasma free fatty acid levels showed a significant continuous
decrease, suggesting that norepinephrine is responsible for the reduced
plasma free fatty acid concentration, presumably through inhibition of
lipolysis in adipose tissue. To elucidate the mechanism, adipocytes
were isolated from mesenteric adipose tissue of tilapia and incubated
with 1) norepinephrine, 2) norepinephrine + phentolamine (
1,
2-antagonist),
3) isoproterenol (nonselective
-agonist), 4)
isoproterenol + timolol
(
1,
2-antagonist), 5)
norepinephrine + timolol, and 6) BRL-35135A
(
3-agonist). The results demonstrate for the first time
that norepinephrine and isoproterenol suppress lipolysis in isolated
adipocytes of tilapia. The effect of norepinephrine is not mediated
through
2-adrenoceptors but, like isoproterenol, via
-adrenoceptors. Furthermore, this study provides strong indications
that
3-adrenoceptors are involved.
teleost fish; fat cells; adrenoceptors; free fatty acids; hypoxia
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