Effects of epinephrine, glucagon and insulin on the activity and degree of phosphorylation of fructose-1,6-bisphosphatase in cultured hepatocytes.

The effects of epinephrine, glucagon and insulin on the activity and degree of phosphorylation of fructose-1,6-bisphosphatase in isolated hepatocytes maintained in cell culture for 24 h were investigated. Epinephrine caused a rapid decrease in the apparent Km monitored as the activity ratio between the activity at 12.5 and 83 microM fructose-1,6-bisphosphate, reaching a maximum after 5 min. Glucagon caused a slower and less pronounced activation, and insulin caused an equally slow increase in Km. The effect of epinephrine and glucagon was completely reciprocated by insulin and the action of insulin was totally erased by the other two. Glucagon stimulated the incorporation of [32P]phosphate into fructose-1,6-bisphosphatase from about 2.5 to 4.2 mol/mol enzyme and epinephrine to 3.5 mol/mol. The effect of the two hormones acting together was cumulative. Insulin brought about a decrease in the degree of phosphorylation to 2.0 mol/mol. The effect of epinephrine was shown to be caused by the beta-receptors, since it was completely blocked by propanolol (a beta-antagonist) and remained unaffected by the presence of phentolamine (an alpha-antagonist).