Human Liver

Preincubation of human liver slices (n= 6) with 10-5 mol/L glucagon resulted in a 20% inhibition of phosphofructokinase and 34-39% inhibition of pyruvate kinase. When insulin (10-8 mol/L) was added, the inhibition by glucagon was reduced by 80% for phosphofructo-kinase and by 70% for pyruvate kinase. Insulin alone has only a small and not significant stimulatory effect. This is in keeping with the view that insulin acts primarily by opposing the effects of counter-regulatory hormones. Since both the enzymes studied are regulatory in glycolysis, their inhibition by glucagon favors the prevalence of the opposing gluconeogenic enzymes. The data here reported showing opposing effects of glucagon and insulin, provide a mechanism for the regulation of the glycolysis/gluconeogenesis balance by these hormones.

Substrate cycles (SC) are formed by a forward pathway (FP) and a backward pathway (BP), the difference between FP and BP forming the metabolic flux (MF) through the route of which the cycle is part. SC modulate regulatory effects, i.e. amplify or reduce the % change in MF compared to the % change in FP and BP, thus affecting the sensitivity to regulatory factors, including hormones. A formula is given to calculate (with an approximation of ± 0.5) the flux response index (FRI), i.e. the factor by which the % change in FP plus the % change in BP must be multiplied to obtain the % change in metabolic flux, when FP and BP undergo opposite changes (as is often the case in metabolic regulation). The formula is: FRI = [( FP + BP)/(FP-BP)]/2. By this formula we evaluated the hepatic activities of glucose-6 phosphatase and glucokinase (which roughly reflect hepatic glucose production and uptake, respectively), i.e. the two enzymes that catalyze the cycle between glucose-6-P and glucose. Based on data obtained in normal (n=12), nonobese diabetic (n=12) and obese diabetic (n=5) subjects as well as in normal (n=7), streptozotocin-diabetic (n=5), and obese diabetic (ob/ob) mice (n=7), we found that FRI was reduced in nonobese diabetic humans and animals whereas it was increased in obese-diabetic humans and mice, compared to normal controls. Thus, diabetes without obesity decreases, and obesity with diabetes increases, the sensitivity of the glucose-6-P/glucose cycle to regulatory agents.