Relative weight of glucose, insulin and parathyroid hormone in the urinary loss of phosphate by chronically diabetic rats.

This report deals with the relationships between glucose (G) and insulin on the tubular transport of phosphate (P) in chronically diabetic rats with high plasma levels of parathyroid hormone (PTH). Alloxan-induced diabetes leads to phosphorus depletion of the soft tissues. This phenomenon appears associated with weight loss and negative P balances caused by the increased urinary P excretion. Administration of 2 IU of insulin/100 g body weight (bw) to diabetic rats normalized their P balance and body weight. The effect of parathyroid function on the P metabolism of diabetic rats was investigated with balance experiments. Diabetic rats, intact or thyroparathyroidectomized (TPTX), have a greater urinary excretion of P than their controls. However, in control rats, the ratio intact:TPTX for urinary P is 1.0:0.76, showing the antiphosphaturic effect of parathyroid ablation. For diabetic animals, on the other hand, the ratio is 1.0:1.44. The simultaneous deficit of insulin and PTH thus quadruples the urinary P loss, instead of compensating for each other. The contribution of insulin deficit and hyperglycemia to the defect in tubular reabsorption (TRP) was investigated with clearance experiments (done on anesthetized, perfused rats). Five experimental groups were used: Controls (C), diabetics (D), controls + glucose (C + G), diabetics + insulin (D + I) and diabetics + insulin + glucose (D + I + G). All experimental groups showed a linear relationship between the TRP of P and G. The regression equation for C is significantly different (F = 40.1, P < 0.001) from that of D animals. The slope value measure the number of mumoles of P per mumol of G reabsorbed. For C and D rats, the ratio P:G approximates 1:4 and 1:20, respectively. The increase in P:G ratios represents the competition between both substrates for tubular resorption. Glycemias up to 11 mM (C and D + I) exist concurrent with the P:G ratio 1:4 Glycemias above 25 mM (D, C + G and D + I + G) produce a P:G ratio of 1:20. Fractional excretion of P (FEP) increased significantly in untreated, chronically diabetic rats (0.47 +/- 0.12 vs controls = 0.05 +/- 0.01, P < 0.001). After a single intramuscular injection of insulin, the FEP decreased as a function of insulin levels. To normalize the FEP of diabetic rats in short-term experiments, insulin had to be administered in doses that produce plasma insulin levels 25 times greater than normal. The general information afforded by the present experiments shows that in untreated, chronically diabetic rats, insulin deficit plays an indirect role. The absence of PTH enhances the effect of hyperglycemia. The latter and the concurrent tubular overload of glucose are the cause of hyperphosphaturia in these animals.

Growth and development of bone mass in untreated alloxan diabetic rats. Effects of collagen glycosylation and parathyroid activity on bone turnover.

Body and skeletal growth and development were studied in alloxan-treated and age-matched control rats, between 3 and 23 weeks of age. For both groups the growth of the skeletal and body weights were in phase, with a maximum at 7 weeks of age. The growth data was assessed according to Parks' theory of feeding and growth. Alloxan-treated rats showed an important reduction in body and bone mass, with a greater impact on soft tissues. As expected, the asymptotic body and skeletal weights were reduced respect to controls. The time needed to attain 63% of mature food intake (Brody's 'time constant') was also reduced, indicating that maturation occurred at an earlier age than controls. The diabetic state is characterized by a reduced food conversion efficiency. Despite hyperfagia, alloxan-treated rats showed circa one-half the body and skeletal weights of age-matched controls. The following adverse effects of alloxan diabetes on bone tissue were observed: (a) a decrease in trabecular bone volume (femoral metaphyses) and cortical width (femoral diaphyses), (b) increased bone collagen glycosylation as a function of extracellular glucose concentration, (c) increased resistance of bone collagen to collagenase hydrolysis, (d) decreased rate of bone resorption except under strongly stimulated parathyroid function, (d) significantly lower ashes/bone matrix ratio in diabetic rats with more than 10 weeks of diabetes, and (e) no histological evidence of osteomalacia.

Calcium metabolism of rats with varying degrees of insulinopenia.

This paper reports an investigation designed to determine the influence of varying degrees of insulinopenia upon the calcium metabolism of actively growing, alloxan-treated rats fed diets with three levels of calcium. A significant reduction in the skeletal mass (in absolute terms) was observed one month after alloxan administration in rats fed diets with normal or high calcium contents. The impact of insulin deficiency was greater on bone collagen than on the mineral mass, as shown by the increased calcium/hydroxyproline ratio. Alloxan-treated rats showed rather increased levels of PTH which was at variance with respect to control animals and unrelated to the calcium content of the diet. In spite of the high PTH levels, diabetic rats showed significantly diminished rates of bone Ca accretion and resorption. In addition, the animals fed the diet with the normal Ca content, showed significantly reduced areas of osteocytes lacunae and hypocalcemia after 24 h of fasting. The overall information obtained indicates that, in the rat, insulin deficiency more pronouncedly affects organic matrix than mineral turnover. The diabetic state is characterized by an impaired response of bone tissue to physiological stimuli, which is attributed to defective cellular activity caused by insulin deficit. Diminished bone resorption is considered to be an adaptative response to preserve bone mass.

The urinary excretion of ionized and non-ionized calcium by rats treated with 1.25-dihydroxycholecalciferol.

The administration of 1,25-dihydroxycholecalciferol to rats increased their calciuresis as a power function of the dose. Up to 95% of the urinary calcium was excreted as a non-ionized complex, together with equimolar amounts of citrate. The stoichiometry between calcium and citrate, observed at all dose levels, suggests that citrate metabolism is coordinated with the mobilization of calcium.

Respiratory alkalosis and reduced plasmatic concentration of ionized calcium in rats treated with 1,25 dihydroxycholecalciferol.

The daily administration of supraphysiological doses of 1,25 dihydroxycholecalciferol (0.1-2.5 micrograms/d/100 g body weight) to rats, produced respiratory alkalosis. With the doses of 0.1-0.2 micrograms/d/100 g and feeding a diet with 0.7% of calcium, calcemias did not exceed 2.75 mM, and significantly reduced plasma ionized calcium levels were measured. The latter phenomenon was found associated with increased urinary excretion of cAMP, soft tissue calcium content, and polyuria with hypostenuria, all known effects of parathyroid hormone. These effects were absent in thyroparathyroidectomized rats treated in the same fashion. Present results suggest that the stimulus of low levels of plasma ionized calcium overcomes the probably inhibitory effect of the steroid on parathyroid hormone secretion.

The long-term effects of pancreatectomy on the Ca metabolism of the rat.

The Ca metabolism of pancreatectomized (PX) rats was investigated three months after surgery. Most PX animals were in negative Ca balance because of increased endogenous fecal Ca excretion and reduced true Ca absorption. Significant increases were also observed in bone Ca resorption rates, hydroxyproline excretion and size of osteocyte lacunae. Inverse correlations between the rates of bone Ca resorption and Ca balance were observed in the PX and control groups. Significantly, both correlations were found to fit the same function. It was concluded that parathyroid hormone secretion and its metabolic expression were not impaired by pancreatectomy. The coupling between bone Ca accretion and resorption was absent in PX animals: high resorption rates were found associated with normal or decreased accretion rates. These latter rates, in combination with the negative Ca balance, explain the reduction in the skeletal Ca mass.