Regulation of Ketogenesis, Gluconeogenesis, and Glycogen Synthesis by Insulin and Proinsulin in Rat Hepatocyte Monolayer Cultures

Abstract

The metabolic actions of porcine insulin and biosynthetic human proinsulin on fatty acid and glucose metabolism were studied in rat hepatocytes cultured in monolayer for 24 h. Our aim was to establish whether proinsulin action in the liver is similar to insulin action and whether the relative potencies of the two hormones are the same for different metabolic processes. Proinsulin and insulin exerted a similar maximal inhibitory effect on ketone body formation from palmitate and on gluconeogenesis from pyruvate. The half-maximal effective concentration of proinsulin was 11-13 times that of insulin. The antiketogenic effects of insulin and proinsulin were associated with an increased glycerol 3-phosphate content and a decreased affinity of carnitine palmitoyltransferase for its substrate palmitoyl-CoA. When the basal rate of ketogenesis was increased with isobutyl methylxanthine, the half-maximal effective concentrations of both proinsulin and insulin were decreased, but the relative potency of the two hormones was unchanged. Proinsulin and insulin exerted similar maximal stimulatory effects on glycogen synthesis and on the activities of pyruvate kinase, glucose 6-phosphate dehydrogenase, phosphogluconate dehydrogenase, and malic enzyme. The half-maximal effective concentration of proinsulin was 10-30 times that of insulin. These findings are consistent with receptor binding studies on liver membranes that suggest that proinsulin interacts with insulin-specific and not proinsulin-specific receptors. Our findings also suggest that proinsulin action does not differ from insulin action at a postreceptor site.

A Role for Astrocytes in Glucose Delivery to Neurons?

“…This picture was not altered by changes in the concentration of glucose used, or the pres ence or absence of fetal calf serum in the medium, for the 24 h prior to an experiment. In experiments of a sim ilar design, cultured hepatocytes show clear increases in extracellular glucose content after transfer to glucosefree conditions, that account for significant proportions of the induced fall in glycogen content [53].…”

“…Cells incu bated with pyruvate showed no, or inconsistent glucose production [54], Pretreatment with dexamethasone (which stimulates gluconeogenesis in hepatocytes [53]) stimulated low rates of gluconeogenesis of the order of 1 nmol-mg protein1-h ', only at concentrations of 100 or 1,000 nM dexamethasone in serum-free medium. There was no clear dependency of gluconeogenic flux upon pyruvate concentration [54].…”

Etomoxir, sodium 2‐[6‐(4‐chlorophenoxy)hexyl]oxirane‐2‐carboxylate, inhibits triacylglycerol depletion in hepatocytes and lipolysis in adipocytes

“…Medium was deproteinized with perchloric acid for determination of acetoacetate and D-3-hydroxybutyrate [17], Protein was determined as in [18]. …”

Fatty acid uptake and metabolism to ketone bodies and triacylglycerol in rat and human hepatocyte cultures is dependent on chain‐length and degree of saturation Effects of carnitine and glucagon

“…The medium was then replaced by serum-free MEM containing dexamethasone phosphate (10 nM) either rinsed three times with 150 mM NaCl and frozen for later extraction [21] …”

“…These solutions were diluted in MEM containing 5 mM L-lactate to a final fatty acid acetoacetate and D-3-hydroxybutyrate [21]. Fatty acid [20] and concentration of 1.0 mM.…”