Serum parathyroid hormone levels and renal handling of phosphorus in patients with chronic renal disease

Renal handling of phosphate in relation to serum parathyroid hormone levels

The relation between the renal handling of phosphate, expressed as the maximal tubular reabsorption of phosphate (TmP)/glomerular filtration rate (GFR) index, and the serum concentration of immunoreactive parathyroid hormone (i-PTH) has been ivestigated in 15 patients with a very wide range of GFR, TmP/GFR and i-PTH. Seven patients had well functioning kidney allografts, with GFR ranging from 43.1 to 64.9 ml/min, while eight had varying degrees of chronic nephropathy, with GFR ranging from 26.7 to 2.3 ml/min. The TmP, the i-PTH concentration, the 51Cr EDTA clearance, the extracellular volume and the serum concentrations of calcium and standard bicarbonate were estimated during conditions where tubular reabsorption of phosphate was maximal. An inverse significant correlation was demonstrated between TmP/GFR and i-PTH (p less than 0.001), while none of the other investigated factors correlated to the TmP/GFR index. It is therefore concluded that the parathyroid hormone has a key role in the regulation of the tubular handling of phosphate in patients with impaired renal function.

Functional profile of the isolated uremic nephron: intrinsic adaptation of phosphate transport in the rabbit proximal tubule

The maintenance of phosphate homeostasis in uremia appears to be governed both by parathyroid hormone (PTH) and by PTH-independent adaptations in renal tubular function. The relative contributions of these two mechanisms that control phosphate excretion by the diseased kidney have been difficult to define in intact animals. The present study was designed to examine the nature of the adaptation of phosphate handling by the proximal tubule of subtotally nephrectomized, uremic rabbits in vitro. Euparathyroid and hyperparathyroid uremic rabbits were studied. Tubular sensitivity to PTH was examined in vitro. The dose-response relationship between bath PTH concentration and inhibition of lumen-to-bath phosphate flux (Jp-lb) in isolated perfused proximal straight tubules (PSTs) revealed that PTH sensitivity was increased in the euparathyroid uremic rabbits and was decreased in hyperparathyroid uremic rabbits. The dose-response to dibutyryl cAMP was the same as normal in both uremic groups. These data strongly suggest the existence of a receptor-mediated adaptation in the effects of PTH on the uremic proximal tubule. In addition to an altered PTH-sensitivity the uremic PST also exhibited an alteration in the basal rate of phosphate transport studied in the presence of normal rabbit serum. Although net volume flux across the PST increased in both uremic groups as a possible consequence of hypertrophy, net phosphate flux per unit length was unchanged. Considering the increase of luminal area in these tubules, net phosphate flux per unit reabsorptive surface area was actually decreased. This dissociation is supportive of the existence of an intrinsic tubular adaptation which is independent of the size of the tubule per se. These studies indicate that there is an intrinsic adaptation of the basal rate of phosphate transport by the uremic rabbit proximal tubule and that the sensitivity of the tubule to PTH is altered. The data are strongly suggestive of an increase in the number of PTH receptors in the proximal tubule of the euparathyroid uremic rabbit and suggest that "down regulation" or persistent occupancy of these receptors occurs when hyperparathyroidism supervenes.

Neonatal hypophosphatasia with elevated serum parathyroid hormone

Two cases of neonatal hypophosphatasia are described. In case 1, hypercalcemia developed at 2 1/2 months of age and continued until death at 10 1/2 months of age. Serum calcium levels decreased transiently in response to phosphate supplementation, prednisolone, and calcitonin. Significantly elevated levels of PTH were detected at 2 1/2 months of age. At autopsy, no parathyroid glands were found. In case 2, hypercalcemia was not detected in his course. Elevated level of serum PTH was recorded on the 17th day of life. A post-mortem examination revealed the presence of one normal parathyroid gland.

The maximal tubular reabsorption of phosphate in relation to serum parathyroid hormone

The relation between the renal handling of phosphate and the serum concentration of immunoreactive parathyroid hormone (i-PTH) was investigated in 15 patients with a very wide range of i-PTH, glomerular filtration rate (GFR), maximal tubular reabsorption of phosphate (TmP) and TmP/GFR-ratio. The latter was used as an index of the renal handling of phosphate. Seven patients had well functioning kidney allografts (GFR 43.1-64.9 ml/min), while 8 had varying degrees of chronic nephropathy (GFR 2.3-26.7 ml/min). The TmP, i-PTH, 51Cr EDTA clearance, the extracellular volume and serum concentrations of calcium and standard bicarbonate were estimated. An inverse significant correlation was demonstrated between TmP/GFR and i-PTH (p less than 0.001), while none of the other investigated factors correlated thyroid hormone has a key role in the regulation of the tubular handling of phosphate in patients with impaired renal function.

Minor influence of parathyroid hormone on fractional tubular reabsorption of phosphate in chronic renal failure

In untreated patients with chronic renal failure and glomerular filtration rates (GFR) between 10 and 25 ml/min the decrease in fractional tubular reabsorption of phosphate was correlated to the degree of secondary hyperparathyroidism. This correlation was not found in patients with GFR below 10 ml/min. During treatment with 1alpha-hydroxycholecalciferol for 11 weeks: a) serum parathyroid hormone concentration decreased in all patients on average 60%, and was normal in 5 patients after treatment b) fractional tubular reabsorption of phosphate decreased significantly (on average 0.21) c) glomerular filtration rate decreased significantly (on average 27.5%). Since the reduction (or normalization) of serum parathyroid hormone concentration did not increase the fractional tubular reabsorption of phosphate, other (GFR-related) factors than parathyroid hormone play the major role for renal handling of phosphate in chronic renal failure when GFR is below 25 ml/min.

[Plasma-cAMP in primary and secondary hyperparathyroidism (author's transl)]

3 patients with operatively proven primary hyperparathyroidism showed normal plasma cAMP concentrations. 5 patients in chronic dialysis treatment independent of the level of parathyroid hormone had increased plasma cAMP concentrations. These observations suggest that there is under these circumstances no regulation of plasma cAMP by the parathyroid hormone.