抄録
Some biochemical changes in rat kidney following administration of parathyroid ormone (PTH) were examined in vivo and in vitro.
Infusion of 5μg PTH into a 150g rat increased renal parenchyma cyclic AMP by 4 times in a few minutes. Such a high level of cyclic AMP in the kidney was observed also in the kidney from a vitamin D defficient rat which was supposed to have secondary hyperparathyroidism. Infusion of calcium, phosphate, magnesium, acid or lkali could not change the cyclic AMP amount.
Among glycolytic or TCA cycle intermediates assayed, α-ketoglutarate (α-KG) concentration was lowered markedly very soon after infusion of PTH or dibutyryl cyclic AMP (DBcAMP)(3mg). Concentration of α-KG was also low after infusion of CaCl2 and HCl, but increased by infusion of EGTA (ethylene bisoxyethylenenitrilotetraacetate), NaHCO3 or acetazolamide.
In an attempt to know the effector and mechanism of these PTH and cyclic AMP actions on TCA cycle intermediates in the kidney, an in vitro system employing the rat renal tubules prepared by enzymatic digestion with collagenase and hyaluronidase was developed. This preparation had active respiration, and increase in cyclic AMP concentration was observed following addition of 0.2μg/ml PTH. They also formed glucose from lactate or other TCA cycle intermediates, with a higher rate than kidney slices.
When the renal tubules were incubated with lactate substrate, CaCl2 stimulated glucose production and lowered the amount of α-KG formed. Addition of PTH in the medium containing 0.25mM CaCl2 had the same effect, but not in the medium free from calcium. The stimulatory effect of PTH on cyclic AMP formation was not influenced by the presence or absence of calcium in the medium. Addition of DBcAMP 0.5mM stimulated glucose production and lowered α-KG only in the presence of calcium in the same way as PTH did. Thus calcium dependency of PTH or DBcAMP action on glucose production and α-KG metabolism was clear.
Possibility remained that PTH might stimulate renal glucose production through the change in H+ ion concentration, as PTH was known to inhibit renal tubular H+ excretion. The effect of lowering pH to 6.8 by decreasing HCO3 concentration in KRB buffer was compared with that of PTH, DBcAMP and CaCl2. Among the substrates examined, low pH stimulated glucose production only from citrate, α-KG and glutamate, but PTH, DBcAMP and CaCl2 stimulated the production not only from α-KG and glutamate but also from succinate, malate, oxaloacetate (OAA) and pyruvate. Metabolism of α-KG was tested further by analyzing some of TCA cycle and glycolytic intermediates. PTH and DBcAMP stimulated α-KG consumption and glucose production and lowered the amount of OAA in the medium with calcium. This pattern was similar to that of increasing calcium concentration. H+ ion, on the contrary, increased OAA concentration. By using malate as a substrate, the metabolism of malate in the renal tubules was examined in the same way. PTH, DBcAMP and calcium enhanced consumption of malate and production of glucose, and induced the change in metabolites pattern with low OAA and high phosphoenolpyruvate (PEP). Increase in H+ ion concentration did not show any significant change. The typical change in metabolism of α-KG and malate induced by addition of PTH and DBcAMP could not be observed in the tubules incubated in the absence of calcium, but the effect of increasing H+ ion concentration on the metabolism of α-KG was clear even in the absence of calcium.
