Question

Glucagon binds and activates the GPCR glucagon receptor and in the liver produces a number of responses. Select all of the results of glucagon signaling in liver cells. Choose one or more: A. inhibition of glycogen synthesis B. activation of PKA, which turns on glycogen degradation C. activation of adenylate cyclase and an increase in CAMP levels D. increased glycogen production leading to an insulin-like response of lowering blood glucose levels E. net glucose export F. increase in PKA activity due to DAG activation and IP3-mediated calcium release

Answer 1

A. Inhibition of glycogen synthesis.

B. Activation of PKA, which turns on glycogen degradation.

C. Activation of adenylate cyclase and an increase in cAMP levels.

E. Net glucose export.

Step-by-step explanation:

The process begins in the islets of Langerhans, where glucagon is released when the concentration of glucose in the bloodstream falls too low. It reaches the hepatocytes, connects with a membranous receptor, which activates the Gs protein. Since it activates the adenylate cyclase, cAMP is generated and kinase A is activated, whose catalytic subunits phosphorylate two cytosolic enzymes: phosphorylase kinase and glycogen synthase. The phosphorylase kinase activates and phosphorylates another enzyme, the glycogen phosphorylase, which degrades glycogen, releasing glucose-1-P. In contrast, glycogen synthase is inhibited and stops synthesizing glycogen from glucose molecules.

After an isomerization to glucose-6-P, an enzyme located in the SER membrane, glucose 6-phosphatase, transforms glucose 6-P into glucose, which leaves the hepatocyte.