Development of fasting hyperglycemia in uremic rats

Phosphoenolpyruvate carboxykinase in urine exosomes reflect impairment in renal gluconeogenesis in early insulin resistance and diabetes

Impaired insulin-induced suppression of renal gluconeogenesis could be a risk for hyperglycemia. Diabetes is associated with elevated renal gluconeogenesis; however, its regulation in early insulin resistance is unclear in humans. A noninvasive marker of renal gluconeogenesis would be helpful. Here, we show that human urine exosomes (uE) contain three gluconeogenic enzymes: phosphoenolpyruvate carboxykinase (PEPCK), fructose 1,6-bisphosphatase, and glucose 6-phosphatase. Their protein levels were positively associated with whole body insulin sensitivity. PEPCK protein in uE exhibited a meal-induced suppression. However, subjects with lower insulin sensitivity had blunted meal-induced suppression. Also, uE from subjects with prediabetes and diabetic rats had higher PEPCK relative to nondiabetic controls. Moreover, uE-PEPCK was higher in drug-naïve subjects with diabetes relative to drug-treated subjects with diabetes. To determine whether increased renal gluconeogenesis is associated with hyperglycemia or PEPCK expression in uE, acidosis was induced in rats by 0.28 M NH₄Cl with 0.5% sucrose in drinking water. Control rats were maintained on 0.5% sucrose. At the seventh day posttreatment, gluconeogenic enzyme activity in the kidneys, but not in the liver, was higher in acidotic rats. These rats had elevated PEPCK in their uE and a significant rise in blood glucose relative to controls. The induction of gluconeogenesis in human proximal tubule cells increased PEPCK expression in both human proximal tubules and human proximal tubule-secreted exosomes in the media. Overall, gluconeogenic enzymes are detectable in human uE. Elevated PEPCK and its blunted meal-induced suppression in human urine exosomes are associated with diabetes and early insulin resistance.

Long-term control on renal carbohydrate metabolism--II. Effect of starve-feed cycles on renal tubule glycolysis

1. The effects of different and alternative starve-feed cycles on glycolysis from isolated renal tubules as well as the glycolytic enzymes phosphofructokinase and pyruvate kinase have been studied. Adaptive responses of renal glycolysis under the nutritional conditions mentioned are reported. 2. Renal glucose utilization increased in a linear fashion during the feeding state of the nutritional cycles, becoming twice as much in both feeding and fasting cycles. Conversely, a decrease in this metabolic pathway took place during the starve periods of the cycles. During the feed-starve cycle the decrease reached 70% in 48 hr of fasting after being fed with a high carbohydrate diet. Whereas in the opposite cycle it was almost 35%. 3. The activities of renal glycolytic enzymes, phosphofructokinase and pyruvate kinase are parallel to the glycolytic capacity of renal tubules in different nutritional conditions. These changes only occur at cellular substrate concentration. 4. The behaviour of the kinetic parameters of these enzymes throughout these experimental conditions is reported. In general, variations in Km values without changes in Vmax values take place which reflect an increase in the catalytic efficiency of the glycolytic enzymes during the feeding state and conversely a decrease during the starvation state.

Glucose metabolism in renal tubular function

Heterogeneity of metabolic activity along the nephron points to a very varied relationship between glucose metabolism and ion transport. Glycolysis is linked closely to free-water clearance and possibly to sodium, potassium, and hydrogen ion transport. Glucose oxidation, while not the major source of renal energy, is crucial in sodium, potassium, and phosphate reabsorption. Gluconeogenesis recovers carbon compounds generated during the process of renal ammoniagenesis. Glucose synthesis and active sodium transport appear to compete for renal ATP, although no regulatory function for this competition has been identified. Glucose formed in the proximal tubule may support free-water clearance in adjacent distal tubule, but is not thought to contribute to any medullary function. The complex network of biosynthetic and catabolic pathways of glucose metabolism may have evolved in the kidney to protect the organism against wide variations in glucose demand which would otherwise be unavoidable during the course of rapidly fluctuating renal electrolyte loads.

Glucose turnover in chronic uremia: increased recycling with diminished oxidation of glucose

The effects of chronic renal failure and hemodialysis on the glucose turnover rate, glucose carbon recycling, and glucose oxidation were evaluated in eight chronically uremic subjects. Six normal subjects served as controls. The studies were repeated in seven uremic subjects after they had been established on hemodialysis for 3 to 18 months. Glucose 13C(microliters) was administered by a prime-constant-rate infusion, and the isotopic enrichment of the whole glucose molecule (m + 6), ie, all six carbon atoms of glucose labeled with 13C and that of the C1 atom of glucose in the plasma were measured by mass spectrometry. It was assumed that the C1 atom of the glucose molecule represented the 13C enrichment of the individual glucose carbons. The "true" rate of glucose production was estimated from the dilution of the glucose 13C (microliters) mass (m + 6) in the plasma. In contrast to the whole glucose molecule (m + 6), recycling of tracer carbon resulted in an increased 13C enrichment of the C1 atom of the glucose molecule and an underestimation of glucose turnover ("apparent"). Glucose carbon recycling was estimated from the difference between the "true" and "apparent" rates of glucose turnover. The contribution of glucose to respiratory CO2 was quantified by comparing the 13C enrichment of expired CO2 with that of the plasma glucose carbon. The plasma glucose concentration after an overnight fast was similar in the uremic and control subjects (72.3 +/- 9.5 and 79.0 +/- 9.5 mg/dL, respectively; mean +/- SD).2+ while the "true" rates of glucose production were similar in both groups (normal: 2.02 +/- 0.19 mg/kg X min; uremic: 2.19 +/- 0.53 mg/kg X min).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the kidneys in elimination of glucagon, insulin, secretin and somatostatin in the rat

Immunoreactive plasma glucagon and secretin in the rat was elevated 48 hours after nephrectomy and ureteral ligation. Since kidneys obstructed by ureteral ligation were unable to remove glucagon and secretin from the blood, renal handling of glucagon and secretin must include glomerular filtration. Insulin and somatostatin levels were significantly elevated 48 hours after nephrectomy, but not after ureteral ligation, indicating partial uptake from peritubular capillaries.

Carbohydrate metabolism and uraemia-mechanisms for glycogenolysis and gluconeogenesis

Disturbances of carbohydrate metabolism during acute uraemia are characterized by the degradation of liver and muscle glycogen with a simultaneous activation of hepatic gluconeogenesis. After binephrectomy, the substitution of essential amino acids and keto analogues stimulate liver, but not skeletal muscle glycogen synthesis. Serine proves to be an optimal substrate for liver gluconeogenesis and muscle glycogen generation under acute uraemic conditions. Propranolol does not influence glycogenolysis of skeletal muscle in acutely uraemic rats. During starvation, acute uraemia leads to an increase of total carbohydrate content as well as of glycogen and glucose concentrations in heart muscle Alterations in carbohydrate contents are not observed in the kidney after ureter ligation. Enhanced glycogenolysis of skeletal muscle and liver during acute uraemia may be due to activation of phosphorylase kinase caused by the increased serum concentrations of various hormones (glucagon, catecholamines, parathormone) as well as free proteolytic activity, an increase of intracellular Ca2+-concentration and finally by alterations in the structure of contractile proteins.