[11C]palmitate kinetics across the splanchnic bed in arterial, portal and hepatic venous plasma during fasting and euglycemic hyperinsulinemia.
OBJECTIVE
The liver is fundamental in regulating lipid metabolism, and it supplies fatty acids (FA) to the rest of the body in the form of triglycerides (TG); the time-related relevance of this process is incompletely defined. The aim of the study was to investigate the appearance of labeled TG in the hepatic vascular bed after [11C]palmitate injection during fasting and insulin stimulation.


METHODS
Plasma [11C]palmitate kinetics in arterial, portal and hepatic venous lipid fractions was studied in eight anesthetized pigs during fasting or euglycemic hyperinsulinemia. Plasma analyses were conducted at 10 and 40 min after tracer injection. Corresponding liver positron emission tomography (PET) images were acquired for the semiquantitative determination of hepatic FA uptake.


RESULTS
At 10 min, plasma levels of unchanged [11C]palmitate were lower in hyperinsulinemic than in fasting experiments in the artery and in the portal vein (P< or=.03), suggesting faster clearance. Levels of unmetabolized [11C]palmitate did not differ between portal and arterial plasma. In the fasting state, a tendency to a positive arterial and portal vs. hepatic venous gradient was observed, indicative of net hepatic [11C]palmitate extraction. Labeled TG were already detectable at 10 min (fasting vs. hyperinsulinemia, ns) and were higher in fasting than in hyperinsulinemic animals at 40 min (92+/-1% and 82+/-6% of arterial plasma radioactivity). Higher proportions of labeled TG were recovered in portal vein plasma, suggesting release by the gut. The portal and the arterial-portal vs. hepatic venous TG gradient tended to be positive. Accordingly, hepatic FA uptake was higher, but declined more rapidly during fasting than during hyperinsulinemia.


CONCLUSIONS
The study indicates that the redistribution of [11C]palmitate between different lipid pools occurs within the short time interval of most PET experiments and is strongly influenced by insulin. Labeled TG constitute an additional [11C]palmitate source in the modeling of PET data.
Guiducci
Letizia
L
SSSUP Medical Sciences Branch, Pisa 56100, Italy; Turku PET Centre, University of Turku, Turku 20520, Finland.
Järvisalo
Mikko
M
Kiss
Jan
J
Någren
Kjell
K
Viljanen
Antti
A
Naum
Alexandru G
AG
Gastaldelli
Amalia
A
Savunen
Timo
T
Knuuti
Juhani
J
Salvadori
Piero A
PA
Ferrannini
Ele
E
Nuutila
Pirjo
P
Iozzo
Patricia
P
D000818:Animals; D002250:Carbon Radioisotopes / Q000493:pharmacokinetics; D005215:Fasting / Q000378:metabolism*; D015309:Glucose Clamp Technique; D006499:Hepatic Artery / Q000378:metabolism; D006503:Hepatic Veins / Q000378:metabolism; D006946:Hyperinsulinism / Q000097:blood*; D007700:Kinetics; D008099:Liver / Q000098:blood supply* / Q000378:metabolism*; D008657:Metabolic Clearance Rate; D010168:Palmitates / Q000493:pharmacokinetics*; D011168:Portal System / Q000378:metabolism*; D013152:Splanchnic Circulation; D013552:Swine
D002250:Carbon Radioisotopes; D010168:Palmitates