and p-octopamine were found to increase lipolysis in adipocytes. The
present study approaches the question if these compounds, natural products of
the bitter orange (Citrus aurantium fruit), increase lipolysis and
fatty acid oxidation in the liver. Experiments were done in the perfused rat
hemoglobin-free perfusion was done using the Krebs/ Henseleit-bicarbonate buffer (pH 7.4) as perfusion
fluid. Both p-synephrine and p-octopamine, at the concentrations
of 100 μM, were found to stimulate the hepatic triacylglycerol lipase by 40%
and 51%, respectively. These seem to be the maximal stimulations possible in
the liver. In the perfused liver, p-synephrine,
when present at an initial concentration of 500 μM, was able to increase the
non-esterified fatty acid release after one hour of recirculating perfusion.
The effects of p-synephrine on the oxidation of exogenously supplied [1-14C]octanoate
and [1-14C]oleate were minimal. Only oxygen uptake, already
stimulated by octanoate or oleate, was additionally increased by the infusion
of p-synephrine. These results contrast with those obtained in a
previous study with p-octopamine, which increased the production of 14CO2 from both [1-14C]octanoate and [1-14C]oleate. Apparently
only the oxidation of endogenous fatty acids is stimulated by p-synephrine.
On the other hand, both p-synephrine and p-octopamine stimulate
the hepatic triacylglycerol lipase to a much lesser extent than the adipocyte
lipase. It can be concluded that p-synephrine affects much more
carbohydrate metabolism in the liver than lipid metabolism.
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