Calcium sensitivity of the parathyroid in renal failure: another look with new methodology

Claus P. Schmitt and Franz Schaefer

Division of Pediatric Nephrology, University Children's Hospital, Heidelberg, Germany

Correspondence and offprint requests to: PD Dr. Franz Schaefer, Division of Pediatric Nephrology, University Children's Hospital, Im Neuenheimer Feld 150, D-69120 Heidelberg, Germany.

Abnormal calcium sensitivity in uraemia?

The parathyroid's sensitivity to changes in ionized calcium has been a matter of controversy for more than two decades. In 1978 Brown introduced the concept of a four-parameter sigmoidal model to characterize the calcium dependency of parathyroid hormone (PTH) release from parathyroid cells in vitro. He demonstrated an increased calcium `set-point', i.e. a decreased calcium sensitivity of cells derived from patients with primary and uraemic hyperparathyroidism [1]. Several clinical studies adapting the in-vitro model to the clinical setting are in line with a reduced sensing of the extracellular calcium level in patients with parathyroid adenoma [2,3]. The reduced expresFion of the calcium receptor on the parathyroid cell surface of patients with primary and uraemic hyperparathyroidism is also compatible with the notion of a calcium sensing deficiency [4]. On the other hand, many investigators failed to demonstrate a rightward shift of the calcium set-point of uraemic patients, and the role of vitamin D in the regulation of calcium sensitivity is still unclear. Calcitriol administration has been reported to lower [5], increase [6] or have no effect [7] on the calcium set-point. More confusion has arisen from the introduction of modifications of the mathematical model originally used by Brown, where the calcium set-point had been defined as the serum calcium concentration at mid-range between the maximal and minimal PTH concentration. In follow-up studies the set-point was also defined as the calcium concentration at which maximal PTH is reduced by 50% [8], or as the concentration required to suppress basal PTH levels by 50% [6]. The validity of these calculations has been discussed extensively, however a direct comparison of in-vivo and ex-vivo data has not been made.

Moreover, recent studies demonstrated that the calcium set-point is not an intrinsically stable feature, but depends on the basal calcium concentration: an increased basal ionized calcium increases the calcium set-point independently of the set-point definition used [9]. Consequently, doubts have been raised concerning the physiological and clinical relevance of the concept [10].

A new methodological approach

Further, hitherto undiscussed methodological aspects are the short half-life of intact PTH, which is only 2–3 min in healthy humans, and the pulsatile mode of endogenous PTH secretion [11]. Rapid fluctuations of plasma PTH occur physiologically and may remain undetected when blood is sampled at low frequency. Moreover, the parathyroid secretory response depends not only on the absolute change, but also on the rate of change in ionized calcium: the faster calcium levels decline the more PTH is released [12,13]. Using a high-frequency (1 min) sampling protocol during acute hypo- and hypercalcaemia induced by the citrate–calcium clamp technique, we were able to demonstrate that the relationship between ionized calcium and PTH is not simply sigmoidal [11] (Fig. 1Go). In fact PTH secretion is maximally stimulated at a time when ionized calcium levels start to fall and steady-state hypocalcaemia is not yet achieved. This peculiar biphasic behaviour, which has escaped detection in previous studies, using a slower lowering of ionized calcium and less frequent blood sampling, is explained by a short-lasting volley of high-frequency and high-amplitude PTH pulses, settling into a more regular continuous secretion pattern on a lower, although still elevated, level at the time of maximal hypocalcaemia (Fig. 2Go).



View larger version (18K):
[in this window]
[in a new window]
 
Fig. 1. Plasma PTH concentrations versus serumionized calciumlevels in seven healthy adults during hypocalcaemic clamp investigations using a 1-min sampling paradigm [11]. There is no sigmoidalrelationship but a brisk initial rise in PTH plasma levels, subsequentlydecreasing to a lower but still elevated steady state despite anongoing decline in ionized calcium.

 


View larger version (32K):
[in this window]
[in a new window]
 
Fig. 2. Instantaneous PTH secretion profiles in a healthy adult ascalculated by deconvolution analysis during hypocalcaemic (experiment I) and hypercalcaemic clamp studies (experiment II) performed1 week apart. Induction of hypocalcaemia induced an initial volleyof pulsatile secretory bursts, followed by a proportionate increase inthe pulsatile and tonic secretion components during steady-statehypocalcaemia. Hypercalcaemia elicited a sharp, proportionatedecline of pulsatile and tonic PTH secretion rates.

 
Several further confounding factors have to be taken into account when comparing the secretory behaviour of the parathyroids in response to calcium changes in normal controls and patients with uraemic hyperparathyroidism. Whilst administering equal doses of citrate in a clamp infusion study, we observed a more rapid decrease in ionized calcium in uraemic patients than in healthy controls. A possible mechanism for this difference is suggested by the observed relationship between the rate of calcium change and serum albumin [14]. As albumin serves as an immediate buffer for blood ionized calcium, diminished serum albumin concentrations in uraemic patients may compromise their ability to compensate for acute changes in ionized calcium. In addition, the uraemic state is associated with skeletal demineralization and reduced sensitivity to the calcaemic action of PTH [15], which may further limit the patients' capacity to compensate for acute alterations of ionized calcium. The more rapid rate of change in ionized calcium in uraemic patients introduces a systematic bias by providing a more powerful stimulating signal for PTH release than in healthy subjects.

A novel approach to characterize inherent functional properties of an endocrine gland delivering pulsatile hormone signals is to look at the true secretion kinetics and not merely at the hormone plasma levels. A major breakthrough in the study of hormone signalling was the advent of the multiparameter deconvolution technique, a sophisticated mathematical algorithm that permits the calculation of instantaneous glandular secretion rates underlying a plasma concentration pattern by separating the secretory processes from the continuously acting elimination of hormone from the bloodstream (Fig. 3Go) [16]. The hormone secretion rate can be further differentiated into the pulsatile, e.g. episodic, secretory events and a basal, non-pulsatile (tonic) secretion component. Further analysis of the frequency, duration, amplitude, mass and orderliness of the pulsatile events gives a detailed insight into the secretory behaviour of endocrine glands.



View larger version (9K):
[in this window]
[in a new window]
 
Fig. 3. Schematic illustration of the multiparameter deconvolution technique. Plasma hormone concentrations at any given time areassumed to reflect the simultaneous operation of four determinable parameters: the temporal positions, the standard deviations, andamplitudes of all underlying secretory impulses, acted upon continuously by endogenous metabolic clearance of the hormone. Simultaneousvalues of multiple secretion and clearance parameters with statistical confidence limits are determined by nonlinear least-squares analysis,in which the combined influences of all plasma hormone concentrations and theirvariances are considered (adapted from [16]).

 
The normal secretory pattern of PTH

In the first application of this methodology to plasma PTH concentration profiles in healthy adults, we observed a pulsatile secretion mode consisting of seven pulses per hour, accounting for around 30% of total PTH secretion [11]. The remaining 70% were attributable to continuous (tonic) PTH release. In patients with uraemic hyperparathyroidism, total PTH secretion rate was increased by 8-fold, resulting from a proportionate increase in pulsatile and tonic secretion rates [14]. The prolonged PTH half-life in uraemic patients accounted only for a 2–3-fold increase in basal plasma PTH levels.

The PTH secretory pattern in uraemia

Modulation of serum ionized calcium by infusion of sodium citrate elicited an immediate, frequency- and amplitude-mediated, selective increase in the pulsatile secretion component, that was diminished by more than 60% in uraemic patients vs controls [14]. With the more rapid decline in ionized calcium levels in the patients, one can speculate that the observed differences might have been even more pronounced if an equally strong hypocalcaemic stimulus had been provided in patients and controls. Interestingly, we observed that the frequency of the PTH bursts, while elevated at baseline, was less up-regulated in the patients as compared to controls.

The complementary experiment (the induction of hypercalcaemia) suppressed total PTH secretion in patients and controls, however, the relative change was again much weaker in the patient group. PTH pulse frequency remained unchanged in the patients in contrast to a 30% decrease in controls [14].

The reduced capability of the uraemic parathyroid glands to adapt to changes in ionized calcium by modulation of pulse frequency and pulse amplitude was even more pronounced in patients on haemodialysis compared to subjects with pre-terminal renal failure. These profound alterations provide in-vivo evidence for a reduced sensitivity of the parathyroid glands to ionized calcium, and may represent a functional correlate to the observed reduction in calcium receptor density of the parathyroid cell surface in uraemia [4].

Issues in interpretation

The remarkable advances in the description of minute-to-minute PTH secretion achieved by the deconvolution approach adds a new level of complexity to the regulation of parathyroid function, and introduces new questions to be addressed by further research. First of all, what is the morphological substrate of the dual, e.g. pulsatile and tonic mode of PTH release? Do two types of secretory granules, one constitutive and one responsive to calcium signals coexist within the parathyroidal cells? Is there differential recruitment of specific cell subpopulations in response to endocrine, neuronal and metabolic signals? Secondly, what are the intracellular signalling pathways linking the binding of calcium to its membrane receptor directly to the exocytosis of preformed PTH granules, permitting an instantaneous secretory response to minute changes in ambient calcium concentrations? Thirdly, what mechanisms underlie the parathyroid quadruplets ability to secrete PTH in a synchronous, pulsatile fashion? Is neuronal input a prerequisite for PTH pulsatility? Alternatively, are PTH pulses part of a spontaneous non-linear feedback system including reciprocal oscillations of ionized calcium? The study of patients with denervated single parathyroid glands after parathyroidectomy and autotransplantation should be useful to differentiate these possibilities. Finally, what is the biological function of the endogenous oscillations of plasma PTH? Do target tissues respond differently to tonic, pulsatile and combined PTH signals? Recent evidence in animals and humans suggests that bone metabolism is indeed affected by the temporal pattern of PTH administration [17,18]. Cell studies suggest that different second messenger pathways may be involved depending on the duration of exposure to PTH [19]. Extensive further in-vitro research will be required to reveal whether specific information is transmitted to the target cells via modulations of the temporal pattern of plasma PTH concentrations. The full understanding of the mechanism and biological relevance of PTH pulsatility may eventually lead to new therapeutical strategies in the treatment of various disorders of PTH action or secretion.

References

  1. Brown EM. Four-parameter model of the sigmoidal relationship between parathyroid hormone release and extracellular calcium concentration in normal and abnormal parathyroid tissue. J Clin Endocrinol Metab 1983; 56: 572–558[Abstract]
  2. Brown EM, Broadus AE, Brennan MF et al. Direct comparison in vivo and in vitro of suppressibility of parathyroid function by calcium in primary hyperparathyroidism. J Clin Endocrinol Metab 1979; 48: 604–610[ISI][Medline]
  3. Felsenfeld AJ, Rodriguez M, Dunlay R, Llach F. A comparison of parathyroid-gland function in haemodialysis patients with different forms of renal osteodystrophy. Nephrol Dial Transplant 1991; 6: 244–251[Abstract]
  4. Gogusev J, Duchambon P, Hory B et al. Depressed expression of calcium receptor in parathyroid gland tissue of patients with hyperparathyroidism. Kidney Int 1997; 51: 328–336[ISI][Medline]
  5. Delmez JA, Tindira C, Grooms P, Dusso A, Windus DW, Slatopolsky E. Parathyroid hormone suppression by intravenous 1,25-dihydroxyvitamin D. A role for increased sensitivity to calcium. J Clin Invest 1989; 83: 1349–1355[ISI][Medline]
  6. Ali AA, Varghese Z, Moorhead JF, Baillod RA, Sweny P. Calcium set point progressively worsens in hemodialysis patients despite conventional oral 1-alpha hydroxycholecalciferol supplementation. Clin Nephrol 1993; 39: 205–209[ISI][Medline]
  7. Ramirez JA, Goodman WG, Belin TR, Gales B, Segre GV, Salusky IB. Calcitriol therapy and calcium-regulated PTH secretion in patients with secondary hyperparathyroidism. Am J Physiol 1994; 267: E961–967[Abstract/Free Full Text]
  8. Felsenfeld AJ, Rodriguez M. The set point of calcium—another view. Nephrol Dial Transplant 1996; 11: 1722–1725[ISI][Medline]
  9. Borrego MJ, Felsenfeld AJ, Martin-Malo A et al. Evidence for adaptation of the entire PTH-calcium curve to sustained changes in the serum calcium in haemodialysis patients. Nephrol Dial Transplant 1997; 12: 505–513[Abstract]
  10. Fournier A, Hardy P, Yverneau, Moriniere P, Achard JM. The set point of calcium-regulated PTH secretion as assessed in vivo: a physiological or artificial concept? Nephrol Dial Transplant 1997; 12: 1302–1303[Free Full Text]
  11. Schmitt CP, Schaefer F, Bruch A et al. Control of pulsatile and tonic parathyroid hormone secretion by ionized calcium. J Clin Endocrinol Metab 1996; 81: 4236–4243[Abstract]
  12. Grant FD, Conlin PR, Brown EM. Rate and concentration dependence of parathyroid hormone dynamics during stepwise changes in serum ionized calcium in normal humans. J Clin Endocrinol Metab 1990; 71: 370–378[Abstract]
  13. Schmitt CP, Schaefer F, Huber D et al. 1,25(OH)2-vitamin D3 reduces spontaneous and hypocalcemia-stimulated pulsatile component of parathyroid hormone secretion. J Am Soc Nephrol 1998; 9: 54–62[Abstract]
  14. Schmitt CP, Huber D, Mehls O et al. Altered instantaneous and calcium-modulated oscillatory PTH secretion patterns in patients with secondary hyperparathyroidism. J Am Soc Nephrol 1998; 9: 1832–1844[Abstract]
  15. Llach F, Massry SG, Singer FR, Kurokawa K, Kaye JH, Coburn JW. Skeletal resistance to endogenous parathyroid hormone in patients with early renal failure. A possible cause for secondary hyperparathyroidism. J Clin Endocrinol Metab 1975; 41: 339–345[Abstract]
  16. Veldhuis JD, Carlson ML, Johnson ML. The pituitary gland secretes in bursts: appraising the nature of glandular secretory impulses by simultaneous multiple-parameter deconvolution of plasma hormone concentrations. Proc Natl Acad Sci USA 1987; 84: 7686–7690[Abstract]
  17. Tam CS, Heersche JN, Murray TM, Parsons JA. Parathyroid hormone stimulates the bone apposition rate independently of its resorptive action: differential effects of intermittent and continuous administration. Endocrinology 19982; 110: 506–512[Abstract]
  18. Lindsay R, Nieves J, Formica C et al. Randomised controlled study of effect of parathyroid hormone on vertebral bone mass and fracture incidence among postmenopausal women on oestrogen with osteoporosis. Lancet 1997; 350: 550–555[ISI][Medline]
  19. Ishizuya T, Yokose S, Hori M et al. Parathyroid hormone exerts disparate effects on osteoblast differentiation depending on exposure time in rat osteoblastic cells. J Clin Invest 1997; 99: 2961–2970[Abstract/Free Full Text]




This Article
Extract
FREE Full Text (PDF)
Alert me when this article is cited
Alert me if a correction is posted
Services
Email this article to a friend
Similar articles in this journal
Similar articles in ISI Web of Science
Similar articles in PubMed
Alert me to new issues of the journal
Add to My Personal Archive
Download to citation manager
Disclaimer
Request Permissions
Google Scholar
Articles by Schmitt, C. P.
Articles by Schaefer, F.
PubMed
PubMed Citation
Articles by Schmitt, C. P.
Articles by Schaefer, F.