Heath, Hunter

Abstract Vol. 30 After pp. 513-514 of the April issue were proofed and released to the printer, the text was scrambled in the process of adjusting layout. We are reprinting the pages here as they should have appeared. The printer apologizes for any confusion and inconvenience this may have caused the readers and authors of this article.

Existing radioimmunoassays for parathyroid hormone (PTH) in rat plasma are based on cross-reactivity of rat PTH (rPTH) with heterologous antisera. We used the synthetic NH2-terminal fragment of rPTH [rPTH-(1-34)] to develop a homologous radioimmunoassay for circulating PTH. An antiserum to rPTH-(1-34) was raised in a goat (G-813), and the same peptide was used as radioligand (125I) and standard. Purification of the label by high-performance liquid chromatography (HPLC) increased specific binding greater than twofold and sensitivity by 50-100%. With a final antiserum dilution of 1:70,000, maximum specific binding of 30-33%, nonspecific binding of 1-5%, and 50-microliters sample additions, the assay detection limit was 1.8-2.5 pmol/l. A midregional fragment of human PTH did not displace 125I-labeled rPTH-(1-34). HPLC of extracts of rat parathyroid glands and hyperparathyroid plasma showed only a single peak of immunoreactivity that eluted 2 min after rPTH-(1-34). Dose dilution curves for rat parathyroid gland extracts, rPTH-(1-34) added to rat plasma, and endogenous rat plasma PTH all paralleled the standard curve. Immunoreactive PTH (irPTH) was detectable in greater than 90% of fasting normal rat plasma and changed appropriately in response to hyper- and hypocalcemia induced by low-calcium and vitamin D-deficient diets, injections of calcium and EDTA, and after thyroparathyroidectomy. The normal range for rat plasma irPTH was less than 2.0-12 pmol/l, in general agreement with bioassay results of others. Thus rPTH-(1-34) is an excellent immunogen for raising antisera to rPTH, and assays incorporating it may be of great value in studying rat parathyroid physiology.

A medullary carcinoma of the thyroid in a dog first became evident because of a distant metastasis. The histologic characteristics of the tumor differ from those previously reported in dogs in that this tumor was highly malignant; anaplasia and mitotic figures were common. The diagnosis was substantiated by radioimmunoassay for calcitonin. Serotonin and 5-hydroxytryptophan also were elevated in the neoplastic tissue. The relation of the biochemical alterations to the clinical course was not clear.

Abstract Some data suggest that human calcitonin (CT) secretion is lower in women than in men, decreases with age and the menopause, and is absent in thyroidectomized persons. To further explore CT secretory physiology, we have studied basal and calcium-stimulated plasma immunoreactive CT (iCT) and silica-extractable monomeric CT concentrations in 148 healthy volunteers and 33 patients with a history of thyroid damage (total or subtotal thyroidectomy, radioiodine treatment for thyrotoxicosis). Both whole-plasma iCT and extractable CT levels were lower basally and after calcium infusion in women than in men, basal levels being reduced about 50% and calcium-stimulated values about 75% from those of male subjects. There were no significant changes in basal iCT or extractable CT concentrations with age, and CT secretory capacity (CT response to calcium infusion) likewise did not change with age. Infusion of monomeric CT to constant concentration in 27 persons permitted estimates of CT metabolic clearance rates (MCRs) and secretion rates (SRs). Calculated MCRs of about 9 ml/min · kg−1 (persons aged 21–30 yr) and 6 ml/min · kg−1 (persons aged 54–70 yr) were in good agreement with published data, and did not differ between the sexes. SRs were dependent upon the assay method used to estimate basal plasma CT concentrations, being highest when whole-plasma iCT values were used. Based on estimates of plasma monomeric CT from the silica extraction procedure, the SR of CT was 59 ± 6 (SE) ng/d · kg−1 in men, and 22 ± 3 ng/d · kg−1 in women. Thyroid damage reduced, but did not abolish, apparent CT immunoreactivity, even in silica extracts of plasma. However, all subsets of thyroiddamaged patients had absent-to-markedly-impaired CT secretion in response to calcium infusion. We conclude that CT secretion is substantially lower both basally and after stimulation in women than in men, and that this difference in CT immunoreactivity probably reflects differences in circulating CT bioactivity. The sex difference in plasma CT concentrations probably results from lower rates of CT secretion in women, not increased MCR. There is no age-related decrease of plasma CT concentrations (or CT secretory reserve), calling into question the concept that a progressive deficiency of CT is partly resonsible for age-related (“senile”) osteoporosis. Surgical or radiation damage to the thyroid gland commonly abolishes C-cell response to calcium; such CT-deficient patients form a population suitable for determining whether or not reduced CT secretion can impair skeletal homeostasis.