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PubMed1979

[Studies of the half-life of plasma parathyroid hormone: rate of disappearance of immunoreactive fragments of the hormone after surgical removal of the parathyroid adenoma in primary hyperparathyroidism].

The disappearance rate of immunoreactive plasma parathyroid hormone (iPTH) was studied, employing two different antisera, following removal of parathyroid adenoma in patients with primary hyperparathyroidism. One antisera contained antibodies against both the NH2 region and the COOH terminal of the molecule (antiserum 211/32, Wellcome Laboratories), the other contained antibodies against antigenic sites of the terminal COOH portion (Immuno Nuclear Corporation antiserum). The iPTH plasma level dropped in all patients following removal of the adenoma. The half-life was longer than that of the native hormone and shorter than that of the terminal fragment with both antisera, being 38.8 min for the 211/32 and 32.9 min for the I.N.C. antiserum. Whilst this finding might be expected for the 211/32 antiserum, on account of its characteristics, it is difficult to offer an explanation for the observed half-life of the I.N.C. anti serum which is specific for the terminal COOH region. These results appear to suggest that the terminal COOH fragment may be further metabolized and that its longer half-life, observed by other authors, is due to the antisera used recognizing the antigenic sites in a fragment smaller than the terminal COOH portion of the molecule, rather than to the effective half-life of the entire fragment.

Adenoma↗
PubMed1985

Calcium transport in canine renal basolateral membrane vesicles. Effects of parathyroid hormone.

The effects of parathyroid hormone were studied on Ca2+ fluxes in canine renal proximal tubular basolateral membrane vesicles (BLMV). Efflux of Ca2+ from preloaded BLMV was found to be stimulated by an external Na+ gradient, and this was inhibited by the Na+ ionophore, monensin, and enhanced by intravesicular negative electrical potentials, which indicated electrogenic Na+/Ca2+ exchange activity. There was a Na+ gradient independent Ca2+ flux, but membrane binding of Ca2+ was excluded from contributing to the Na+ gradient-dependent efflux. The Na+ gradient-dependent flux of Ca2+ was very rapid, and even 2- and 5-s points may not fully represent absolute initial rates. It was saturable with respect to the interaction of Ca2+ and Na+ with an apparent (5 s) Km for Na+-dependent Ca2+ uptake of 10 microM, and an apparent (5 s) Vmax of 0.33 nmol/mg protein per 5 s. The Na+ concentration that yielded half maximal Ca2+ efflux (2 s) was 11 mM, and the Hill coefficient was two or greater. Both Na+ gradient dependent and independent Ca2+ efflux were decreased in BLMV prepared from kidneys of thyroparathyroidectomized (TPTX) dogs, and both were stimulated by parathyroid hormone (PTH) infusion to TPTX dogs. BLMV from TPTX dogs exhibited significantly reduced maximal stimulation of Na+ gradient-dependent Ca2+ uptake with an apparent (5 s) Vmax of 0.23 nmol/mg protein per 5 s, but the apparent Km was 8 microM, which was unchanged from normal. The Na+ gradient independent Ca2+ uptake was also reduced in BLMV from TPTX dogs compared with normal. Thus, PTH stimulated both Na+/Ca2+ exchange activity and Na+ independent Ca2+ flux. In vivo, the latter could result in an elevation of cytosolic Ca2+ by PTH, and this might contribute to the observed decrease in solute transport in the proximal tubule.

Adenosine Triphosphate↗
PubMed1972

Proparathyroid hormone: identification of a biosynthetic precursor to parathyroid hormone.

Biosynthesis of a precursor to bovine parathyroid hormone has been demonstrated in slices of parathyroid tissue incubated in vitro. The proparathyroid hormone is 15-20 amino acids larger than the bovine hormone, and has a molecular weight of about 11,500 as determined by polyacrylamide gel electrophoresis. Upon incubation of parathyroid slices with [(14)C]aminoacids, radioactivity is detected initially in the precursor. If incorporation of [(14)C]aminoacids is inhibited after a short incubation either by replacement of radioactive amino acids with unlabeled amino acids or by addition of puromycin, the amount of radioactivity in the precursor decreases, while the radioactivity in the hormone continues to increase. The precursor is bound by an antibody that is specific for parathyroid hormone, and its binding can be inhibited by addition of the hormone. Analysis of tryptic digests indicates that the precursor and the hormone have common tryptic peptides, and that there are at least two additional peptides in the precursor.

Amino Acids↗
PubMed2006

Direct in vitro evidence of the suppressive effect of cinacalcet HCl on parathyroid hormone secretion in human parathyroid cells with pathologically reduced calcium-sensing receptor levels.

Clinical studies have been performed to determine the effect of cinacalcet HCl (cinacalcet), an allosteric modulator of the calcium-sensing receptor (CaR), on primary hyperparathyroidism (PHPT) and secondary hyperparathyroidism of uremia (SHPT). However, no in vitro studies on human parathyroid cells have been reported to date. In this study, the inhibitory effect of cinacalcet on PTH secretion was analyzed in primary cultured parathyroid cells obtained from patients. The investigation involved three PHPT and three SHPT patients subjected to therapeutic parathyroidectomy. Notably, all SHPT patients were resistant to intravenous vitamin D analogue therapy. Removed parathyroid tumors were used for immunohistochemistry and parathyroid cell primary culture. Immunohistochemical analyses revealed diminished expression of CaR and vitamin D receptor (VDR) in all parathyroid tumors. PTH secretion from cultured parathyroid cells of PHPT and SHPT patients was suppressed by extracellular Ca2+ and cinacalcet in a dose-dependent manner. Rates of suppression of PTH secretion in PHPT and SHPT by cinacalcet (1000 nmol/l) were 61% +/- 21% and 61% +/- 19%, respectively. Cinacalcet demonstrates significant potency in the suppression of PTH secretion in primary cultured human parathyroid cells in vitro, despite reduced levels of the target protein, CaR. Data from this in vitro analysis support the clinical application of cinacalcet in PHPT and SHPT therapy.

Cinacalcet↗
PubMed1985

1,25-Dihydroxyvitamin D3 suppresses parathyroid hormone secretion from bovine parathyroid cells in tissue culture.

To determine whether 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] regulates PTH secretion, we have tested its effects in both short term incubations (30-120 min) and long term primary cell cultures (24-96 h) of bovine parathyroid cells. In short term incubations, 10(-11)-10(-7) M 1,25-(OH)2D3 had no consistent effect on PTH secretion. In primary cultures of bovine parathyroid cells, significant suppression of PTH secretion occurred, as measured by both N-terminal and C-terminal PTH assays. Suppression of PTH secretion was dose dependent when 10(-11), 10(-9), and 10(-7) M 1,25-(OH)2D3 were tested for 48 h in culture, and the effects of 10(-7) M, 1,25-(OH)2D3 were noted as early as 24 h. Reversal of suppression of PTH secretion was observed after an additional 48 h in the absence of 1,25-(OH)2D3. Other studies from our laboratory have demonstrated that 1,25-(OH)2D3 suppresses levels of pre-pro-PTH mRNA in cultured bovine parathyroid cells, and we found a strong correlation at 48 h between the decrease in PTH release and that in mRNA. We conclude that 1) 1,25-(OH)2D3 suppresses PTH secretion rates in a dose-dependent manner in cells grown for 24-48 h in culture, but does not have a significant effect on short term PTH release (30-120 min); 2) cultured cells exhibiting suppression by 1,25-(OH)2D3 demonstrate nearly full recovery of PTH secretion after an additional 48 h in the absence of added 1,25-(OH)2D3; and 3) PTH secretion closely parallels levels of pre-pro-PTH mRNA in cultured cells, suggesting that the observed effects of PTH secretion reflect, at least in part, suppression of synthesis of PTH by 1,25-(OH)2D3.

Animals↗
PubMed1987

High osmolality: a potent parathyroid hormone secretogogue in dispersed parathyroid cells.

We have examined possible mechanisms by which osmolality might modulate PTH secretion in dispersed bovine parathyroid cells. Increasing medium osmolality by adding sodium chloride causes a marked, dose-dependent increase in PTH release. The maximum effect (4-fold increase) is observed when osmolality is around 650 mosM, with half-maximal stimulation at about 400 mosM. When osmolality is increased to a similar extent with either sucrose or sodium chloride, PTH secretion is enhanced to a comparable degree, suggesting that osmolality itself, rather than ionic strength, is responsible for the increase in PTH secretion. Time course experiments show that the increased secretion of PTH with high osmolality occurs very rapidly (in less than 5 min). In contrast to the suppressive effects of high Ca2+ on PTH release, increasing calcium concentration in the incubation media does not inhibit the stimulation of PTH secretion by high osmolality. Moreover, the effects of dopamine (10(-5) M) and high osmolality on PTH release are additive, further suggesting that high osmolality and cAMP modulate PTH release by different mechanisms. In fact, direct measurement of cellular cAMP in cells exposed to high osmolality shows no change relative to control cells incubated with normal osmolality, 127 +/- 20 vs. 146 +/- 21 fmol/10(5) cells, respectively. Cytosolic Ca2+ increases from 257 +/- 29 nM to 703 +/- 50 nM after 200 mM NaCl is added to the incubation medium at low Ca2+ (0.5mM). Prior removal of extracellular calcium abolished this effect. Increasing the osmolality to a similar level by adding sucrose to the medium does not demonstrate any increase in cytosolic calcium. We conclude that high osmolality is a potent secretogogue in dispersed bovine parathyroid cells. Unlike dopamine and isoproterenol, high osmolality does not act through changes in intracellular cAMP. It also prevents the normal inhibitory effect of high Ca2+ on PTH release. Change of cytosolic Ca2+ is variable and suggests that the effect of high osmolality on PTH release cannot be explained by cytosolic Ca2+ alone. Further understanding of the mechanisms by which osmolality affects PTH release, therefore, may provide clues to the unusual inverse relationship between extracellular and cytosolic calcium and PTH release.

Aminoquinolines↗
PubMed1988

Effects of estradiol and progesterone on parathyroid hormone secretion from human parathyroid tissue.

Estrogens have been used to treat the hypercalcemia in primary hyperparathyroidism. Estrogens and progesterone both stimulate PTH secretion from bovine parathyroid tissue. The effects of these agents on PTH secretion from human parathyroid tissue are not known. In this study, we evaluated the effects of 17 beta-estradiol and progesterone on PTH secretion from abnormal parathyroid tissue from seven patients (four adenomas, three hyperplasia). Both estradiol (10(-9)-10(-6) mol/L) and progesterone (10(-7) and 10(-6) mol/L) significantly stimulated PTH secretion in a time- and dose-dependent manner during a 3-h period. These results indicate the need for a careful evaluation of serum PTH levels and other parameters of parathyroid function in estrogen- and/or progesterone-treated patients with primary hyperparathyroidism.

Adult↗