Localization and role of pyruvate kinase isoenzymes in the regulation of carbohydrate metabolism and pyruvate recycling in rat kidney cortex

Abstract

This work was performed to gain more information on the role of pyruvate kinase isoenzymes in the regulation of renal carbohydrate metabolis. Immunohistochemically, pyruvate kinase type L is shown to be localized in the proximal tubule of the nephron and pyruvate kinase type M 2 in the distal tubule and the collecting duct. A tight relationship between gluconeogenesis and pyruvate recycling was found. The rate of gluconeogenesis (8 μmol/g wet wt. per 30 min) was of the same order of magnitude as the ratet of pyruvate recycling (10.92 μmol/g wet wt. per 30 min). Stimulation of gluconeogenesis from 20 mM lactate in kidney cortex slices of 24-h-starved rats by dibutyryl-cAMP, alanine and parathyroid hormone was connected with a decrease in pyruvate recycling; inhibition of gluconeogenesis due to a lack of Ca 2+ in the incubation medium was linked with an increase in pyruvate recycling. The degradation of [6- 14C]glucose to lactate, pyruvate, ketone bodies and CO 2 and of [2- 14C]lactate was unaffected by dibutyryl-cAMP, alanine, epinephrine, vasopressin or the omission of Ca 2+ from the incubation medium. 1 mM dibutyryl-cAMP or 5 mM alanine did not alter the activities of oxaloacetate decarboxylase, ‘malic’ enzyme and malate dehydrogenase from rat kidney cortex. Since aerobic glycolysis in the distabl tubules and the collecting ducts is not influenced by hormones, dibutyryl-cAMP and Ca 2+, pyruvate kinase type M 2 residing in this tissue is unlikely to be a control point of glycolysis. Since this tissue degrades only one-seventh of the glucose formed via gluconeogenesis, it does not contribute significantly to pyruvate recycling. Therefore, the decrease of pyruvate recycling in the presence of dibutyryl-cAMP and alanine in rat kidney cortex slices, leading to increased renal gluconeogenesis, has to be ascribed to the regulation of pyruvate kinase type L.