Immunoextracted calcitonin in milk and plasma from totally thyroidectomized women. Evidence of monomeric calcitonin in plasma during pregnancy and lactation

Bone metabolism in pregnancy and lactation

INTRODUCTION

Different hormonal mechanisms regulate bone metabolism during pregnancy and lactation. In both those periods of life, a fine modulation of calcium metabolism is necessary to meet the needs of foetus and newborn.

METHODS

We review scientific literature on the topic "osteoporosis", "pregnancy" and "lactation", evaluating the most relevant data from original articles, reviews and meta-analyses.

EVIDENCE SYNTHESIS

Pregnancy- and lactation-associated bone loss and related fractures rarely occur and, generally, clinicians have to manage it case to case, since there is not a unique guideline. Fortunately, bone mineral density (BMD) usually tends to recover within 12 months after weaning, thus, it could be reasonable waiting of assessing the effective magnitude of bone mass regain before starting any pharmacological treatment.

CONCLUSIONS

Osteoporosis and/or fragility fractures remain uncommon events associated with gestation and/or breastfeeding. The management of bone loss and/or fractures during those periods is generally conservative and a tailored approach is advisable in the absence of any specific recommendation in this field.

Hypoparathyroidism in Pregnancy and Lactation: Current Approach to Diagnosis and Management

Background: Hypoparathyroidism is an uncommon endocrine disorder. During pregnancy, multiple changes occur in the calcium-regulating hormones, which may affect the requirements of calcium and active vitamin D during pregnancy in patients with hypoparathyroidism. Close monitoring of serum calcium during pregnancy and lactation is ideal in order to optimize maternal and fetal outcomes. In this review, we describe calcium homeostasis during pregnancy in euparathyroid individuals and also review the diagnosis and management of hypoparathyroidism during pregnancy and lactation. Methods: We searched the MEDLINE, CINAHL, EMBASE, and Google scholar databases from 1 January 1990 to 31 December 2020. Case reports, case series, book chapters, and clinical guidelines were included in this review. Conclusions: During pregnancy, rises in 1,25-dihydroxyvitamin D (1,25-(OH)2-D3) and PTH-related peptide result in suppression of PTH and enhanced calcium absorption from the bowel. In individuals with hypoparathyroidism, the requirements for calcium and active vitamin D may decrease. Close monitoring of serum calcium is advised in women with hypoparathyroidism with adjustment of the doses of calcium and active vitamin D to ensure that serum calcium is maintained in the low-normal to mid-normal reference range. Hyper- and hypocalcemia should be avoided in order to reduce the maternal and fetal complications of hypoparathyroidism during pregnancy and lactation. Standard of care therapy consisting of elemental calcium, active vitamin D, and vitamin D is safe during pregnancy.

Morphological, hormonal, and molecular changes in different maternal tissues during lactation and post-lactation

Milk supply and quality during lactation are critical for progeny survival. Maternal tissues and metabolism, influenced by hormonal changes, undergo modification during lactation to sustain breastfeeding. Two organs that suffer essential adjustment are the mammary glands and the bone; however, renal calcium conservation and calcium absorption from the intestine are also modified. Lactation leads to a transient loss of bone minerals to provide adequate amounts of minerals, including calcium for milk production. Physiological, metabolic, and molecular changes in different tissues participate in providing nutrients for milk production. After weaning, the histological, metabolic, and hormonal modifications that take place in lactation are reverted, and bone remineralization is a central function at this time. This study focuses on the hormonal, metabolic, molecular, and tissue modifications that occur in mammary glands, bone, intestine, and kidneys in the mother during lactation and post-weaning periods.

Standards of care for hypoparathyroidism in adults: a Canadian and International Consensus

PURPOSE

To provide practice recommendations for the diagnosis and management of hypoparathyroidism in adults.

METHODS

Key questions pertaining to the diagnosis and management of hypoparathyroidism were addressed following a literature review. We searched PubMed, MEDLINE, EMBASE and Cochrane databases from January 2000 to March 2018 using keywords 'hypoparathyroidism, diagnosis, treatment, calcium, PTH, calcidiol, calcitriol, hydrochlorothiazide and pregnancy'. Only English language papers involving humans were included. We excluded letters, reviews and editorials. The quality of evidence was evaluated based on the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. These standards of care for hypoparathyroidism have been endorsed by the Canadian Society of Endocrinology and Metabolism.

RESULTS

Hypoparathyroidism is a rare disease characterized by hypocalcemia, hyperphosphatemia and a low or inappropriately normal serum parathyroid hormone level (PTH). The majority of cases are post-surgical (75%) with nonsurgical causes accounting for the remaining 25% of cases. A careful review is required to determine the etiology of the hypoparathyroidism in individuals with nonsurgical disease. Hypoparathyroidism is associated with significant morbidity and poor quality of life. Treatment requires close monitoring as well as patient education. Conventional therapy with calcium supplements and active vitamin D analogs is effective in improving serum calcium as well as in controlling the symptoms of hypocalcemia. PTH replacement is of value in lowering the doses of calcium and active vitamin D analogs required and may be of value in lowering long-term complications of hypoparathyroidism. This manuscript addresses acute and chronic management of hypoparathyroidism in adults.

MAIN CONCLUSIONS

Hypoparathyroidism requires careful evaluation and pharmacologic intervention in order to improve serum calcium and control the symptoms of hypocalcemia. Frequent laboratory monitoring of the biochemical profile and patient education is essential to achieving optimal control of serum calcium.

Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

During pregnancy and lactation, female physiology adapts to meet the added nutritional demands of fetuses and neonates. An average full-term fetus contains ∼30 g calcium, 20 g phosphorus, and 0.8 g magnesium. About 80% of mineral is accreted during the third trimester; calcium transfers at 300-350 mg/day during the final 6 wk. The neonate requires 200 mg calcium daily from milk during the first 6 mo, and 120 mg calcium from milk during the second 6 mo (additional calcium comes from solid foods). Calcium transfers can be more than double and triple these values, respectively, in women who nurse twins and triplets. About 25% of dietary calcium is normally absorbed in healthy adults. Average maternal calcium intakes in American and Canadian women are insufficient to meet the fetal and neonatal calcium requirements if normal efficiency of intestinal calcium absorption is relied upon. However, several adaptations are invoked to meet the fetal and neonatal demands for mineral without requiring increased intakes by the mother. During pregnancy the efficiency of intestinal calcium absorption doubles, whereas during lactation the maternal skeleton is resorbed to provide calcium for milk. This review addresses our current knowledge regarding maternal adaptations in mineral and skeletal homeostasis that occur during pregnancy, lactation, and post-weaning recovery. Also considered are the impacts that these adaptations have on biochemical and hormonal parameters of mineral homeostasis, the consequences for long-term skeletal health, and the presentation and management of disorders of mineral and bone metabolism.

Presentation and management of osteoporosis presenting in association with pregnancy or lactation

In this review, we summarize our current understanding of the pathophysiology of fragility fractures that occur for the first time during pregnancy and lactation, and provide guidance on appropriate investigations and treatment strategies. Most affected women will have had no prior bone density reading, and so the extent of bone loss that may have occurred during pregnancy or lactation is uncertain. During pregnancy, intestinal calcium absorption doubles in order to meet the fetal demand for calcium, but if maternal intake of calcium is insufficient to meet the combined needs of the mother and baby, the maternal skeleton will undergo resorption during the third trimester. During lactation, several hormonal changes, independent of maternal calcium intake, program a 5-10 % loss of trabecular mineral content in order to provide calcium to milk. After weaning the baby, the maternal skeleton is normally restored to its prior mineral content and strength. This physiological bone resorption during reproduction does not normally cause fractures; instead, women who do fracture are more likely to have additional secondary causes of bone loss and fragility. Transient osteoporosis of the hip may affect one or both femoral heads during pregnancy but it involves localized edema and not skeletal resorption. Case reports have described the use of calcitonin, bisphosphonates, strontium ranelate, teriparatide, vertebroplasty, and kyphoplasty to treat post-partum vertebral fractures. However, the need for such treatments is uncertain given that a progressive increase in bone mass subsequently occurs in most women who present with a fracture during pregnancy or lactation.

Paracrine signaling by progesterone

Steroid hormones coordinate and control the development and function of many organs and are implicated in many pathological processes. Progesterone signaling, in particular, is essential for several important female reproductive functions. Physiological effects of progesterone are mediated by its cognate receptor, expressed in a subset of cells in target tissues. Experimental evidence has accumulated that progesterone acts through both cell intrinsic as well as paracrine signaling mechanisms. By relegating the hormonal stimulus to paracrine signaling cascades the systemic signal gets amplified locally and signaling reaches different cell types that are devoid of hormone receptors. Interestingly, distinct biological responses to progesterone in different target tissues rely on several tissue-specific and some common paracrine factors that coordinate biological responses in different cell types. Evidence is forthcoming that the intercellular signaling pathways that control development and physiological functions are important in tumorigenesis.

Calcium and bone metabolism disorders during pregnancy and lactation

Pregnancy and lactation cause a substantial increase in demand for calcium that is met by different maternal adaptations within each period. Intestinal calcium absorption more than doubles during pregnancy, whereas the maternal skeleton resorbs to provide most of the calcium content of breast milk during lactation. These maternal adaptations also affect the presentation, diagnosis, and management of disorders of calcium and bone metabolism. Although some women may experience fragility fractures as a consequence of pregnancy or lactation, for most women, parity and lactation do not affect the long-term risks of low bone density, osteoporosis, or fracture.

Severe postpartum osteoporosis with increased PTHrP during lactation in a patient after total thyroidectomy and parathyroidectomy

We present a 27-year-old woman with hypoparathyroidism following total thyroidectomy for papillary carcinoma, who presented postpartum during lactation with several vertebral osteoporotic fractures, increase in bone turnover markers, and measurable parathyroid hormone-related protein (PTHrP) levels. Cessations of lactation led to gradual decrease in bone turnover markers and PTHrP and improvement in bone mineral density. Pregnancy- and postpartum-associated osteoporosis is an uncommon condition characterized by the occurrence of fractures during late pregnancy or the puerperium. The patient presented postpartum with severe back pain and multiple vertebral fractures. Metabolic evaluation performed at presentation revealed hypercalcemia, hypercalciuria, increased alkaline phosphatase, vitamin D insufficiency, normal serum protein immunoelectrophoresis, and a detectable level of PTHrP. Serum levels of bone turnover markers were markedly increased. Bone mineral density at the lumbar spine was severely reduced. After cessation of lactation, the PTHrP level became undetectable. Bone turnover markers gradually decreased to normal and bone mineral density improved. Several factors contributed to the reduced bone mass in this patient, including amenorrhea treated with oral contraceptives, suppressive levothyroxine treatment, and lactation of twins with increased PTHrP. Patients with severely reduced bone mass need surveillance during pregnancy and lactation and should possibly consider avoiding breastfeeding. Patients with hypoparathyroidism should temporarily reduce their alphacalcidiol dose while lactating.

Identification of Calcitonin Expression in the Chicken Ovary: Influence of Follicular Maturation and Ovarian Steroids1

Calcitonin (CALCA), a hormone primarily known for its role in calcium homeostasis, has recently been linked to reproduction, specifically as a marker for embryo implantation in the uterus. Although CALCA expression has been documented in several tissues, there has been no report of production of CALCA in the ovary of any vertebrate species. We hypothesized that the Calca gene is expressed in the chicken ovary, and its expression will be altered by follicular maturation or gonadal steroid administration. Using RT-PCR, we detected Calca mRNA and the calcitonin receptor (Calcr) mRNA in the granulosa and theca layers of preovulatory and prehierarchial follicles. Both CALCA and Calca mRNA were localized in granulosa and thecal cells by confocal microscopy. Using quantitative PCR analysis, F1 follicle granulosa layer was found to contain significantly greater Calca mRNA and Calcr mRNA levels compared with those of any other preovulatory or prehierarchial follicle. The granulosa layer contained relatively greater Calca and Calcr mRNA levels compared with the thecal layer in both prehierarchial and preovulatory follicles. Progesterone (P(4)) treatment of sexually immature chickens resulted in a significantly greater abundance of ovarian Calca mRNA, whereas estradiol (E(2)) or P(4) + E(2) treatment significantly reduced ovarian Calca mRNA quantity. Treatment of prehierarchial follicular granulosa cells in vitro with CALCA significantly decreased FSH-stimulated cellular viability. Collectively, our results indicate that follicular maturation and gonadal steroids influence Calca and Calcr gene expression in the chicken ovary. We conclude that ovarian CALCA is possibly involved in regulating follicular maturation in the chicken ovary.

Rapid inactivation and apoptosis of osteoclasts in the maternal skeleton during the bone remodeling reversal at the end of lactation

There is a rapid reversal in maternal skeletal metabolism and bone remodeling from accelerated bone resorption during lactation to skeletal rebuilding after lactation. The purpose was to determine the changes that occur in maternal osteoclasts during the transition from lactation to postlactation. Skeletal samples were taken from female rats on days 10 and 19 of lactation and 1 and 7 days after lactation. The pups were weaned on day 20. There was a rapid change in the osteoclast population after weaning, resulting in less resorption surface. Osteoclasts detached from bone surfaces, lost their ruffled borders, and became fragmented with immunocytochemical evidence of apoptosis within 24 hr after lactation. Concomitant with the rapid regression in the osteoclast population was an over fivefold increase in maternal calcitonin (CT) levels at 24 hr after weaning. Serum calcium and estrogen (E2) increased, but prolactin (PRL) and PTH decreased after weaning. The hormone changes, particularly that of CT, are consistent with the rapid regression of the osteoclast population at the end of lactation. These changes are similar to a reversal phase of a bone remodeling cycle where bone formation commences when resorption ceases on bone surfaces and suggests that the fate of osteoclasts during bone remodeling is programmed cell death. These results also suggest that bone remodeling is well synchronized prior to, during, and after lactation to accommodate the mineral requirements of the offspring as well as the mother.

Calcitonin is expressed in the chicken pituitary gland: influence of gonadal steroids and sexual maturation

Calcitonin (CT) is primarily produced by the thyroid C cells in mammals or by the ultimobranchial gland in chickens. CT is also expressed by the pituitary gland in rats in which it functions as a paracrine factor causing decreased lactotroph proliferation and prolactin (PRL) secretion. Gonadal steroids influence CT expression in the rat pituitary gland. However, the expression of the CT gene in the pituitary gland of chickens or of any other avian species has not previously been reported. We have tested the hypotheses that CT is expressed in the chicken pituitary gland, and that its expression is influenced by sexual maturation or in response to ovarian steroid administration. We have detected robust expression of CT cDNA in the chicken pituitary gland by reverse transcription/polymerase chain reaction (PCR). The sequence of the pituitary-derived CT cDNA is identical to that of the ultimobranchial gland. CT-immunoreactive (ir) cells have been observed throughout the anterior pituitary gland by confocal microscopy. Many of the PRL-ir cells show co-localization with CT-ir cells. Quantitative real-time PCR analysis has revealed an inverse relationship between the quantities of PRL mRNA and CT mRNA in the pituitary gland: sexually mature hens contain lower amounts of CT mRNA but larger quantities of PRL mRNA compared with sexually immature chickens. Estradiol and/or progesterone treatment of sexually immature chickens leads to a significant decrease in the quantity of pituitary CT mRNA relative to that in the vehicle-treated chickens. We conclude that pituitary CT plays an important paracrine/autocrine role in the control of lactotroph function and PRL secretion in the chicken.

Expression of calcitonin receptor in rat mammary gland during lactation

Calcitonin (CT) and calcitonin receptor (CTR) have been reported to play an important role in mammary tissue during pregnancy, lactation, and involution. In the present study, the expression and distribution of CTR mRNA in rat mammary tissue during pregnancy and lactation were investigated. As measured by real-time RT-PCR, CTR mRNA levels were increased only slightly during pregnancy, but increased markedly immediately postpartum and remained elevated through lactation, with the highest levels observed 14 days postpartum. In situ hybridization analysis showed that intense CTR mRNA signals were detected in the whole mammary gland. We performed immunohistochemistry to determine distribution of CTR in the mammary epithelium. CTR has been reported to act as an amylin receptor when heterodimerized with receptor activity modifying protein-1 (RAMP1) or RAMP3. mRNA expression of RAMP1 and RAMP3 in mammary tissue decreased during pregnancy and lactation, and amylin mRNA was undetectable, suggesting that up-regulated CTR in lactating mammary tissues binds CT rather than amylin. In primary cultures of mammary cells isolated from rat dams 14 days postpartum, CT produced a statistically significant decrease in thymidine incorporation. These results suggest that up-regulation of CTR during lactation may contribute to inhibition of mammary epithelial cell proliferation.

Ablation of calcitonin/calcitonin gene-related peptide-alpha impairs fetal magnesium but not calcium homeostasis

We used the calcitonin/calcitonin gene-related peptide (CGRP)-alpha gene knockout model (Ct/Cgrp null) to determine whether calcitonin and CGRPalpha are required for normal fetal mineral homeostasis and placental calcium transfer. Heterozygous (Ct/Cgrp(+/-)) and Ct/Cgrp null females were mated to Ct/Cgrp(+/-) males. One or two days before term, blood was collected from mothers and fetuses and analyzed for ionized Ca, Mg, P, parathyroid hormone (PTH), and calcitonin. Amniotic fluid was collected for Ca, Mg, and P. To quantify skeletal mineral content, fetuses were reduced to ash, dissolved in nitric acid, and analyzed by atomic absorption spectroscopy for total Ca and Mg. Placental transfer of (45)Ca at 5 min was assessed. Ct/Cgrp null mothers had significantly fewer viable fetuses in utero compared with Ct/Cgrp(+/-) and wild-type mothers. Fetal serum Ca, P, and PTH did not differ by genotype, but serum Mg was significantly reduced in null fetuses. Placental transfer of (45)Ca at 5 min was normal. The calcium content of the fetal skeleton was normal; however, total Mg content was reduced in Ct/Cgrp null skeletons obtained from Ct/Cgrp null mothers. In summary, maternal absence of calcitonin and CGRPalpha reduced the number of viable fetuses. Fetal absence of calcitonin and CGRPalpha selectively reduced serum and skeletal magnesium content but did not alter ionized calcium, placental calcium transfer, and skeletal calcium content. These findings indicate that calcitonin and CGRPalpha are not needed for normal fetal calcium metabolism but may regulate aspects of fetal Mg metabolism.

Calcium and vitamin D metabolism during lactation

Calcium transfer to the fetus in late pregnancy and the subsequent transfer of calcium to milk represent the greatest challenges to calcium homeostasis in adult animals. The adaptation of the maternal calcium homeostatic mechanisms is the result of a complex interplay between calciotropic hormones and the tissues, intestine, bone, and kidney, responsible for providing the large amounts of calcium needed to support fetal skeletal growth and lactation. In this review, we will discuss general calcium homeostasis followed by a review of the specific adaptations required by the human, rat, and cow to meet fetal and lactational demands for calcium. Finally, we will review what is known about the regulation of calcium transfer from the plasma to the milk.

Hormonally active peptides in human milk

Human milk contains many hormone and hormone-like peptides. The gastrointestinal tract of newborn infants exhibits lower proteolytic activity than in adults and higher "permeability" for macromolecules. Studies in experimental animals demonstrate that several peptides (epidermal growth factors, insulin-like growth factor I and bombesin) after orogastric administration exhibit effects on the small intestine and other organs (liver or pancreas). Few studies performed in human neonates suggest a "survival" of epidermal growth factor in their gastric content. Further studies are needed to evaluate the role of milk-borne hormonally active peptides. This need is stressed by the fact that several of those known to be present in human milk were found to be low or not detectable in infant formulae (epidermal growth factor, insulin-like growth factor I, insulin, parathyroid hormone-related peptide.

Parathyroid hormone-related peptide in human milk measured by a mid-molecule radioimmunoassay

Several peptide hormones and growth factors have been found in human milk, and we present here the results of measurements of parathyroid hormone-related peptide (PTHrP). A radioimmunoassay (RIA) using a polyclonal antiserum against the mid-region of the molecule has been developed. In milk collected during the first 6 days after parturition, the PTHrP concentrations showed large interindividual variations ranging from 0.3 to 13.7 nmol-Eq/L (0.5 to 24.4 ng-Eq/mL) (n = 67) and increased between days 3 and five postpartum. PTHrP also increased during the first 4 collecting days when measured in milk from the same mother during a prolonged period. On fast-protein liquid chromatography (FPLC), the bulk of PTHrP eluted with a molecular weight of approximately 10 to 12 kd after treatment with urea. After mid-molecule immunoaffinity extraction of PTHrP from milk, higher levels were obtained by the mid-molecule RIA than by an aminoterminal assay, indicating that all fragments did not contain the aminoterminal. Parts of immunoextracted milk PTHrP stimulated cyclic adenosine monophosphate (cAMP) production in rat osteosarcoma cell line, UMR-106. In conclusion, we have found PTHrP-like immunoreactivity in human milk using a mid-region RIA. Parts of the immunoextracts also contained the aminoterminal and possessed PTH-like bioactivity. Whether PTHrP in human milk plays a physiological role in the maternal breast or in the newborn gastrointestinal tract is unknown, but the present observations demonstrate that a portion of the PTHrP is at least potentially biologically active.

A rapid extraction method for serum calcitonin

A rapid, simple and reliable extraction method for calcitonin from serum has been developed. 2 volumes of acidic ethanol and 1 volume of serum were mixed, centrifuged, and the supernatant was evaporated to dryness. The extracts were reconstituted in assay buffer before radioimmunoassay was performed. Basal concentrations of calcitonin after extraction were 5.4 +/- 3.0 pg-equivalents/ml (mean +/- SD), females, n = 48 and 8.8 +/- 5.5 pg-equivalents/ml, males, n = 42. Calcitonin was detectable in serum from all males and from 90% of the females. The concentrations in males were significantly higher (P less than 0.001). There was a more pronounced calcitonin response in males (n = 12) than in females (n = 12) to a calcium clamp (P less than 0.01). Gel chromatography of serum from patients with medullary thyroid carcinoma on a Sephadex G-75 column and a TSK G 2000 SW column in a fast protein liquid chromatography system, disclosed that the ethanol extraction excluded the high mol mass forms of calcitonin. We propose the acidic ethanol extraction as a convenient method for routine measurements of calcitonin.

Immunoextracted calcitonin in human gastric secretion

Immunoreactive calcitonin (iCT) has been demonstrated in human gastric juice after immunoextraction with immobilized antibodies and subsequent radioimmunoassay. The basal levels were 4.5 +/- 3.1 (mean +/- SD) pg-Eq/ml gastric juice; range 1.2-9.1 pg-Eq/ml; n = 7, and after stimulatory gastric secretion test with pentagastrin 0.3 +/- 0.2 pg-Eq/ml; range 0.1-0.7 pg-Eq/ml; n = 7 (p less than 0.01). The main fraction of iCT from gastric juice eluted in the same region as synthetic human calcitonin (hCT) on Sephadex G-75 gel chromatography. Reverse phase chromatography in a fast protein liquid chromatography (FPLC) system revealed a slightly less hydrophobic character of the iCT from gastric juice compared to synthetic monomeric hCT. The results were further confirmed by using an additional antiserum. In plasma, the calcitonin (CT) levels were after immunoextraction at the basal state 6.6 +/- 1.7 pg-Eq/ml (mean +/- SD); range 5.1-10.1 pg-Eq/ml; n = 7 and after pentagastrin stimulation 9.4 +/- 5.4 pg-Eq/ml; range 6.3-18.5 pg-Eq/ml; n = 7.

Evidence for precursors of calcitonin/PDN 21 in human milk

Large amounts of immunoreactive PDN-21 (iPDN -21) were found in human milk in concentrations similar to those of immunoreactive calcitonin (iCT): 187 +/- 73 pM (mean +/- S.D.) vs 210 +/- 83 pM. (n = 17). Calcitonin (CT) was immunoextracted from milk by means of CT antibodies coupled to Sepharose 4B, and the extracts were gel-chromatographed on Sephadex G-75 after treatment with 6 M guanidine HCl. iCT and iPDN-21 in the fractions were determined with radioimmunoassay. The main part of iCT eluted as high molecular weight forms and these fractions also contained iPDN-21. Enzymatic cleavage of immunoextracted milk CT by trypsin demonstrated that iPDN-21 could be split apart from the high molecular weight forms and be recovered at the position of synthetic human PDN-21 on gel chromatography. iCT was eluted in the region of monomeric CT and as larger forms. Since PDN-21 constitutes the carboxyterminal of preprocalcitonin, our results indicate that human milk contains precursors of calcitonin.

Large forms of immunoreactive calcitonin in human seminal fluid exhibit PDN-21 immunoreactivity

Semen samples were obtained from 4 healthy males on two occasions. One of the samples was liquefied at room temperature for 20 min while the other was diluted immediately with buffer to suppress proteolysis. A pool of immunoextracted calcitonin from liquefied and diluted samples, respectively, was subjected to gel chromatography in a separate experiment. In lyophilized samples the majority of immunoreactive calcitonin (CT) had an approximate molecular weight (Mr) of 10 kilodaltons (Kd) and immunoreactive PDN-21 was also present in the fractions. In diluted ejaculates CT was composed of larger species as well as the 10 Kd form. PDN-21 was present in fractions containing the 10 Kd form but could not be detected in the peaks of larger molecular weight, probably because of lower sensitivity of the PDN-21 assay compared to the CT assay. Our finding of CT and PDN-21 in the same fractions after immunoextraction with CT antibodies suggests the presence of proforms of CT in human seminal fluid.