Abstract
The intravenous administration of glucagon to anesthetized rats resulted within 5 min in a 20% drop in the hepatic phosphorylase phosphatase activity, as measured in a post-mitochondrial supernatant at low dilution, but it did not affect the activity of glycogensynthase phosphatase. On the other hand, the injection of insulin plus glucose caused increases by about 35% in both phosphatase activities. Upon subcellular fractionation these effects were recovered in the cytosol, but not in the glycogen/microsomal fraction. However, activity changes in the latter fraction were observed after recombination with the liver cytosol from a hormone-treated animal. Preincubation of the liver cytosol with modulator protein (a specific inhibitor of type-1 protein phosphatases) cancelled the activity changes induced by insulin plus glucose. No hormonal effects on hepatic protein phosphatase activities were observed when the fractions were either diluted an additional 10-fold or pretreated with trypsin. An acute hormonal regulation of protein phosphatases could also be demonstrated in the perfused liver. When added to the perfusion medium, glucose as well as insulin increased the cytosolic protein phosphatase activities by about 25%. Their effect was additive, irrespective of the order of addition. On the other hand, the addition of glucagon and/or vasopressin resulted in a 20% drop in the phosphorylase phosphatase activity. The presence of glucagon did not interfere with the effectiveness of insulin, and vice versa. The changes in the phosphorylase phosphatase activities induced by glucagon, insulin, and glucose represented changes in the Vmax only. We propose that the acute control of the hepatic glycogen synthase phosphatase and phosphorylase phosphatase activities is mediated by transferable, cytosolic effector(s).
Highlights
The intravenous administration of glucagon to an- that areassociated with the microsomal fraction andwith the esthetized rats resulted within 5 min in a 20% drop in glycogen particles in the liver have been classified as type-1 the hepatic phosphorylase phosphatase activity, as ( 4 5 )
Several studies have indicated that protein phosphatases When added to the perfusion medium, glucose as well do not merely serve a “passive” function to reverse the action as insulin increased thecytosolic protein phosphatase of protein kinases, but are the target of hormonal activities by about 25%
We propose that the acutecontrol of the rylation is antagonistically controlled by insulin and adrenahepatic glycogen synthase phosphatase and phospho- line (10, 11)
Summary
Centration of modulator that depressed the cytosolic synthase phosphatase activity to one-third This concentration of modulator lowered the synthase phosphataseactivity to thesame final value when a treatment with insulin plus glucose had first increased the activity by 56%.The same was true for the phosphorylase phosphatase activity, the adopted concentration of modulator protein decreased the latteractivity by only 50%. The modulator decreased the latter activity by about 40%, both before and after administration of glucagon The subsequent phosphorylase phosphatase assays were performed at 5 "C instead of the usual 25 "C
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
