Rapid Reciprocal Changes in Rat Hepatic Glycolytic Enzyme and Fructose Diphosphatase Activities following Insulin and Glucagon Injection

Abstract

Abstract Insulin and glucagon injected into the portal vein produced rapid reciprocal changes in the activities of certain hepatic glycolytic enzymes and fructose diphosphatase in the rat. Insulin produced a rapid increase in hepatic phosphofructokinase and pyruvate kinase activities and a decrease in fructose diphosphatase activity. Glucagon produced a rapid but reciprocal response in each of these enzyme activities. Fructose diphosphate aldolase activity was unaltered by either hormone. The insulin effect on these enzymes was detected within 5 min following injection, was maximal by 10 min, and then gradually declined over the remaining 30 min of testing. The magnitude of the insulin effect was dependent on the amount of insulin injected. No effect was detected when 0.005 unit per kg (0.001 unit) was injected; phosphofructokinase and pyruvate kinase activities were slightly increased with 0.015 unit per kg (0.003 unit); phosphofructokinase, pyruvate kinase, and fructose diphosphatase activities were altered with 0.15 unit per kg (0.03 unit) and 1.5 units per kg (0.3 unit), with the higher dose giving a slightly greater effect. The insulin effect was not associated with a significant change in cyclic adenosine 3':5'-monophosphate concentrations. The glucagon effect on the enzyme activities was preceded by a significant increase in cyclic adenosine 3':5'-monophosphate concentrations, which occurred within 30 s after hormone injection. A change in enzyme activities was found at 2 to 5 min and was maximal 5 to 10 min following glucagon injection. The hormone effect on enzyme activities and cyclic adenosine 3':5'-monophosphate levels was dependent on the amount of glucagon injected, being maximal at doses of 150 to 300 µg. Pretreatment of the rat with actinomycin D or puromycin did not alter the response of the enzymes to insulin and glucagon, indicating that de novo protein synthesis was not responsible for the change in enzyme activities. Glucagon injected 5 min following insulin reversed the insulin effect; insulin given 5 min after glucagon also partially reversed the glucagon effect on cyclic adenosine 3':5'-monophosphate and the enzyme activities. Intravenous insulin produced rapid changes in the activities of renal cortical, skeletal muscle, and epididymal fat glycolytic enzymes and fructose diphosphatase in the rat. Insulin (0.15 unit per kg) produced rapid increases in pyruvate kinase and phosphofructokinase and rapid decrease in fructose diphosphatase activities in all tissues. Fructose diphosphate aldolase activity was unchanged following insulin infusion. Intravenous glucagon (0.15 mg) produced rapid changes, reciprocal to those seen with insulin, in fructose diphosphatase activity in all tissues. Glucagon significantly decreased epididymal fat phosphofructokinase activities but did not alter the activity of this enzyme in the renal cortex and skeletal muscle. Glucagon significantly decreased renal cortical pyruvate kinase but had no effect in the epididymal fat and skeletal muscle. Fructose diphosphate aldolase activity was unchanged in all tissues following glucagon infusion. Cyclic adenosine 3':5'-monophosphate infusion (0.05 mmole) produced significant changes in pyruvate kinase, phosphofructokinase, and fructose diphosphatase activities in all tissues which resembled those changes seen following glucagon infusion. These data suggest that the glucagon responses seen in the four tissues studied are mediated by cyclic adenosine 3':5'-monophosphate.