Biochemical and Cellular Effects of Direct Maxacalcitol Injection into Parathyroid Gland in Uremic Rats

Abstract

The most important etiological factors of resistance to medical treatments for secondary hyperparathyroidism are the decreased contents of the vitamin D receptor (VDR) and Ca-sensing receptor (CaSR) in parathyroid cells and a severely swollen parathyroid gland (PTG) as a result of hyperplasia. The effects of direct maxacalcitol (OCT) injection into PTG in terms of these factors were investigated in this study. The PTG of Sprague-Dawley rats that were 5/6 nephrectomized and fed a high-phosphate diet were treated by a direct injection of OCT (DI-OCT) or vehicle (DI-vehicle). The changes in serum intact parathyroid hormone (PTH), Ca 2؉, and phosphorus levels, in VDR and CaSR expression levels in parathyroid cells, and in Ca 2؉ -PTH curves were examined. Apoptosis was analyzed by the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling method and DNA electrophoresis for PTG. DI-OCT markedly decreased serum intact PTH level, and a significant difference in this level between DI-OCT and DI-vehicle was observed. However, serum Ca 2؉ and phosphorus levels did not changed markedly in both groups. The upregulations of both VDR and CaSR, the clear shift to the left downward in the Ca 2؉ -PTH curve, and the induction of apoptosis after DI-OCT were observed. These findings were not observed in the DI-vehicle-treated rats. Moreover, these effects of DI-OCT were confirmed by the DI-OCT into one PTG and DI-vehicle alone into another PTG in the same rat. DI-OCT may introduce simultaneous VDR and CaSR upregulations and the regression of hyperplastic PTG, and these effects may provide a strategy for strongly suppressing PTH levels in very severe secondary hyperparathyroidism. S econdary hyperparathyroidism (SHPT) resulting from ESRD causes not only renal osteodystrophy but also cardiovascular disorders because of ectopic calcification in vascular tissues. A low level of parathyroid hormone (PTH) causes adynamic bone disease; easily increases serum calcium (Ca), phosphorus (P), and CaϫP levels; and may contribute to a poor prognosis in patients with ESRD. Thus, the maintenance of appropriate levels of PTH, Ca, and P is required for the improvement of the prognosis and quality of life of these patients (1-3).SHPT develops in conditions of P retention, low Ca level, and the inactivation of vitamin D (VD) (4 -6); therefore, the control of P level and supplementations of Ca and VD are necessary in patients with SHPT as preventive or medical treatment. However, advanced SHPT with severely swollen parathyroid glands (PTG) as a result of hyperplasia is resistant to these medical treatments because of the low contents of VD receptor (VDR) and Ca-sensing receptor (CaSR) in parathyroid cells (PTC) (7,8). When SHPT progresses into such status, it is considered irreversible. Thus, patients with very severe SHPT require parathyroidectomy-autotransplantation (PTx-AT), which may be complicated by some problems, such as the necessity of general anesthesia, the hyperor hypofunction of autotransplanted PTG, and mental suffering.The ...

Direct Injection of Calcitriol or Its Analog Improves Abnormal Gene Expression in the Hyperplastic Parathyroid Gland in Uremia

“…In this study we confirmed from the laboratory data that this animal model produces uremia-induced SHPT, agreeing with a previous study involving a model of advanced SHPT, in which PTH synthesis and secretion, and PTC proliferation activity, were very high, and levels of VDR and CaSR in the PTG were very low [6]. Moreover, it should be noted that significant differences in terms of changes in the levels of serum intact PTH, Ca 2+ and P between the CAL and OCT groups were not observed, indicating that differences in gene expression levels were independent of these biochemical changes.…”

“…This procedure has been described in detail elsewhere [6]. The Animal Studies Committee of Wakayama Medical University approved all experimental animal protocols.…”

“…The development of animal models of advanced SHPT and a method of DI into the PTG have made it possible to investigate the cellular effects of these treatments in detail. In advanced SHPT, DI of maxacalcitol (OCT) simultaneously ameliorates the important etiological factors of resistance to medical treatment: marked suppression of PTH synthesis and secretion, upregulation of both the vitamin D receptor (VDR) and the Ca-sensing receptor (CaSR), and induction of PTC apoptosis [6]. We recently showed that these effects were specifically caused by local administration of the agents at high concentrations, thereby enabling their specific interaction with nuclear VD binding sites, and that the improvement in clinical status of SHPT was maintained with subsequent conventional treatment [7, 8].…”

Can Hyperparathyroid Bone Disease Be Arrested or Reversed?

“…In rat models, several studies have been unable to find evidence for programmed PC death in normal or hyperplastic parathyroid tissue (19 -21). There are no known stimuli for apoptosis in the PC (22), although an effect of vitamin D has not been excluded completely (23)(24)(25)(26). The evidence for induction of massive apoptosis by calcitriol still is contradictory, however.…”

“…The changes were detectable within 4 d and could have resulted only from apoptosis. Recently, a significant induction of apoptosis was demonstrated by 22-oxa-1,25-dihydroxyvitamin D 3 , maxacalcitol injection directly into the parathyroid glands of uremic rats by Shiizaki et al (26). The process of apoptosis was assessed directly by the transferase-mediated dUTP nick-end labeling method for identification of DNA fragmentation.…”