AJP - Regulatory, Integrative and Comparative Physiology, Vol 265, Issue 1 255-R260, Copyright © 1993 by American Physiological Society
Glucagon and insulin regulate lipolysis in trout liver by altering phosphorylation of triacylglycerol lipase
J. S. Harmon, L. M. Rieniets and M. A. Sheridan
Department of Zoology, North Dakota State University, Fargo 58105.
Rainbow trout were used to investigate the hormonal regulation by glucagon
and insulin of hepatic triacylglycerol (TG) lipase activation. Two purified
preparations of the trout hepatic TG lipase enzyme, the 110,000-g
preparation and the resuspended ammonium sulfate fraction (ASF), were
activated up to 58% with (in mM) 0.5 ATP, 0.01 cAMP, 5 MgCl2, and exogenous
protein kinase over control levels. ATP or cAMP alone had no effect on
activation. Activation of the trout hepatic lipase was reversible; complete
inactivation of the ASF was obtained within 3 h in the presence of
exogenous phosphorylase phosphatase. Adenosine 3',5'-cyclic monophosphate
(cAMP)/ATP-dependent 32P-phosphorylation of trout hepatic lipase was
observed within 5 min of incubation with the cAMP/ATP-Mg2+ activation
system and 25 microCi [32P]ATP. Hormonal modulation of trout hepatic lipase
phosphorylation was studied in isolated hepatocytes. Hepatocytes were
incubated with [32P]-monopotassium phosphate for 3 h, then exposed to
mammalian glucagon (GLU). Within 5 min, increased lipolysis was accompanied
by a 95% increase in phosphorylation of the enzyme. Mammalian insulin (INS)
depressed GLU-stimulated phosphorylation by 56% and inhibited
GLU-stimulated lipolysis. These results indicate that GLU and INS modulate
lipolysis in trout liver by altering phosphorylation of the TG lipase
enzyme.
