Distinct patterns of chromogranin A-related species can be demonstrated in endocrine cells

We have studied the pattern of chromogranin A (CgA)-related species in different human endocrine cells that produce CgA and also express the calcitonin gene. Antibodies against CgA peptides that span its linear sequence were used in Western analysis of cell lines derived from medullary thyroid carcinoma (MTC), small cell lung cancers (SCLC), epidermoid cell lung cancer (ECLC) and a pulmonary carcinoid tumor (CRND). Each of the cell lines demonstrated a distinct pattern of CgA-related species. Gel filtration studies also revealed multiple and different forms of immunoreactive CgA in the cell lines. Although proteolysis may contribute to our results, these observations suggest that native CgA is processed to smaller species in a tissue-specific pattern by different endocrine cells. More conclusive studies, however, are necessary to establish that cell processing leads to the specific CgA moieties that we have observed.

Serum osteocalcin (BGP) levels in normal men: a longitudinal evaluation reveals an age-associated increase

Serum levels of bone gla protein (BGP) have been reported to increase with aging and hence to reflect an age-related increase in bone remodeling activity. To evaluate the relationship between aging and serum BGP levels in a study of longitudinal design, we measured BGP concentrations in 77 normal men at 6 month intervals over a 3 year period. Mean BGP levels at the onset (4.95 +/- 1.5 ng/ml) increased significantly during the study (p = 0.004), and the mean of individual BGP slopes was positive (0.38 +/- 0.6 ng/ml per year, p = 0.0001). The rate of change in BGP was not related to serum creatinine levels or dietary calcium intake.

The rate of bone mineral loss in normal men and the effects of calcium and cholecalciferol supplementation

OBJECTIVE

To determine the rate of bone loss in normal men, and to examine the effects of dietary calcium and cholecalciferol supplementation on bone loss in men.

DESIGN

Double-blinded, placebo-controlled 3-year trial of supplementation with calcium (1000 mg/d) and cholecalciferol (25 micrograms/d).

SETTING

Clinical research center at a university medical facility.

SUBJECTS

Normal men 30 to 87 years old, recruited from the Portland community.

MEASUREMENTS AND MAIN RESULTS

Radial bone mineral content (assessed by single-photon absorptiometry) fell by 1.0%/y (95% CI, -1.3% to 0.7%) at a proximal radial site and 1.0%/y (95% CI, -1.4% to -0.6%) at a distal radial site. Vertebral bone mineral content (assessed by dual-energy quantitative computed tomography) declined by 2.3%/y (95% CI, -2.8% to -1.8%). In these healthy men with a high basal dietary calcium intake (1159 mg/d), calcium and cholecalciferol supplementation did not affect bone loss at any site.

CONCLUSIONS

Normal men experience a substantial bone loss at both axial and appendicular sites that is not prevented by calcium and vitamin D supplementation in a well-nourished population.

Heat shock proteins bind calcitonin

We have demonstrated two heat shock proteins (HSP's) in the human placenta that specifically bind calcitonin. Binding specificity was shown by ligand-affinity chromatography and by competitive binding studies. The HSP's were identified by Western analysis and by amino acid sequence. Our observations are consistent with the hypothesis that HSP's may function by binding and interacting with cellular proteins and peptides during their biogenesis. This interaction may both depend upon and produce conformational changes in these ligands during their intracellular processing. Additionally, HSP-peptide hormone interactions may confound studies designed to investigate classical receptor-hormone interactions.

Glucocorticoid regulation of parathyroid hormone-related peptide gene transcription in a human neuroendocrine cell line

A PTH-related peptide (PTHRP) has been identified and its cDNA cloned from tumors associated with the syndrome of humoral hypercalcemia of malignancy. The PTHRP and PTH genes appear to represent members of a gene family. Whereas the PTH gene is expressed exclusively in the parathyroids, the PTHRP gene appears to be widely expressed, but little is known concerning the regulation of its expression in any site. We studied the regulation of PTHRP gene expression in a human carcinoid cell line (NCI-H727) which has neuroendocrine features and also produces calcitonin, calcitonin gene-related peptide, and chromogranin-A. We found that the synthetic glucocorticoid triamcinolone produced time- and dose-dependent decreases in steady state PTHRP and calcitonin mRNA levels in NCI-H727 cells. This effect was blocked by the competitive glucocorticoid inhibitor RU-486. Messenger RNA stability and transcription run-off experiments revealed that triamcinolone decreased PTHRP and calcitonin expression by repressing the transcription rates of both genes.

Rapid, simple identification of individual osteoblastic cells and their specific products by cell blotting assay

Biochemical and molecular biological studies of osteoblastic cell function and hormonal regulation are frequently confounded by the inherent cellular heterogeneity and phenotypic instability of existing in vitro and in vivo model systems. A new technique (derived from Western blotting or antibody-based detection of protein molecules bound to nitrocellulose paper) is described for identification of individual cells which synthesize osteoblast-specific gene products (bone Gla-protein, type I collagen, and alkaline phosphatase) or produce cAMP in response to parathyroid hormone (PTH) or isoproterenol. Dispersed primary neonatal rat calvariae or osteogenic sarcoma cells were "plated" on Immobilon-P (a hydrophobic transfer membrane with very high protein-binding capacity) for 30 minutes to several hours, followed by agonist treatment, formalin fixation, hematoxylin staining, and immunostaining with a battery of antibodies specific for osteoblastic products. Individual cells and their secretory zones were visualized by light microscopy and counted. Treatment with PTH with or without isoproterenol resulted in increases in the percentages of osteoblastic cells elaborating cAMP, as well as the intensity of immunostaining, but had no effects on MCF-7 cells, a nonosteoblastic breast carcinoma control line. The percentage of cells within each primary osteoblastic cell population isolated or rat osteogenic sarcoma cell clone (G2 or C12) that elaborated bone-specific proteins or that generated cAMP in response to PTH varied with time and the individual cellular preparation, reconfirming the cellular heterogeneity of these systems. This method, in conjunction with techniques such as in vitro hybridization, should prove useful in characterizing discrete osteoblastic bone cell subpopulations and in clarifying mechanisms of hormonal regulation by local and systemic agents.

17 beta-estradiol increases plasma calcitonin levels in salmonid fish

The functional role of calcitonin in teleost fish is in question. Data on the role of calcitonin in calcium regulation are inconsistent, and while a participation in some aspects of sexual maturation has been strongly indicated, the exact function is not known. To establish if there exists a functional relationship between 17 beta-estradiol (E2) and calcitonin in salmonid species, rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar), and coho salmon (Oncorhynchus kisutch) were injected ip with a single or repeated doses of E2. It is concluded that E2 treatment increases plasma calcitonin levels directly or indirectly, and that it is possible that E2 is responsible for the rise in calcitonin levels during late sexual maturation of female salmonids. In accord with earlier studies, no correlation was found between changes in calcitonin levels and free plasma calcium levels. It seems clear that changes in free plasma calcium levels are not the primary cause of the plasma calcitonin changes in teleost fish. It is possible that calcitonin is involved in mobilizing calcium or directing its mobilization by protection of certain calcium pools during vitellogenesis. However, the increase in calcitonin occurs so close to ovulation that a reproductive role other than a calcium regulatory one is likely. The possibility of transfer of calcitonin itself to the developing oocytes and a subsequent role in embryonic development must also be considered.

Effects of sandostatin on plasma chromogranin-A levels in neuroendocrine tumors

We have determined the effects of Sandostatin (SMS 201-995, Sandoz) on chromogranin-A (CgA) in the blood of 14 patients with neuroendocrine tumors of the gastroenteropancreatic axis, 7 with carcinoid tumors, 5 with gastrinomas, and 1 each with a glucagonoma and tumor-secreting vasoactive intestinal peptide. Two thirds of the patients had elevated plasma CgA. Sandostatin administration suppressed CgA in 12 of the 14 patients. In 8 of 10, the clinical response to Sandostatin paralleled the reduction in CgA levels. There was a strong correlation between the change in CgA levels and the respective blood concentration of the hormone produced by the tumor. Serial measurement of CgA may provide an additional means of monitoring these tumors and their secretory activity where other measures are not available.

Changes in plasma vitellogenin, sex steroids, calcitonin, and thyroid hormones related to sexual maturation in female brown trout (Salmo trutta)

Female brown trout (Salmo trutta) from a wild strain (Baltic sea trout) and a cultured strain were sampled individually for blood plasma at regular intervals during the period around final sexual maturation. The plasma samples were analyzed for vitellogenin (VTG), estradiol-17 beta, testosterone, 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), calcitonin, tri-iodothyronine (T3), thyroxine (T4), and total and free plasma calcium. In the wild fish, VTG, estradiol-17 beta, and testosterone peaked 30 days before ovulation, while 17,20 beta-P had a sharp peak at ovulation. Both T3 and T4 declined at the beginning of the sampling period, reached minimal levels 30 days before ovulation, and rose sharply at the time of ovulation. Calcitonin levels were elevated during final maturation. Total plasma calcium correlated with plasma VTG levels. In the cultured strain, sampling was started 2 weeks before ovulation. The levels of VTG, estradiol-17 beta, and testosterone decreased throughout the sampling period. 17,20 beta-P and calcitonin concentrations were high during the period close to ovulation. Plasma thyroxine remained at basal levels in cultured trout. The discrepancies observed between wild and cultured females may be due to differences in stress susceptibility, environmental conditions, life cycles, or to genetic divergence between the strains.

PTHrP secretion is stimulated by CT and inhibited by CgA peptides

We studied the regulation of the secretion in vitro of the parathyroid hormone-related protein (PTHrP) associated with the hypercalcemia of malignancy by Chromogranin A (CgA)-derived peptides and by human calcitonin (CT) in the BEN human lung tumor cell line. The amino terminal peptide of CgA, CgA1-40, inhibited the secretion of PTHrP, whereas other peptides had no such effect. Human CT stimulated the secretion of PTHrP, whereas other hormones had no such effect. Both effects occurred in a dose-dependent manner. These studies reveal novel regulatory pathways among peptides and proteins that are commonly associated with each other and can have paracrine interactions. CgA may be processed at its multiple dibasic sites to peptides that regulate the secretion of its co-resident hormones, in this case PTHrP. In addition to a paracrine effect, CT may be clinically useful as a provocative agent for PTHrP secretion. Complex interactions are present among the calcium-regulating hormones and their associated proteins.

Rapid changes of light microscopic indices of osteoclast-bone relationships correlated with electron microscopy

The relationships of tibial endosteal osteoclasts to bone surfaces were quantitatively evaluated during initiation of calcium repletion in calcium-deficient rats. To do this, indices of osteoclast-bone relationships obtained by light microscopy were devised and evaluated by comparing with those obtained by electron microscopy (EM). These indices are the percent of the osteoclast width that (1) exhibits markers indicative of a ruffled border, (2) is in close contact with bone, (3) is isolated from bone by other cell types, and (4) is separated from bone by intercellular material. The indices obtained by light microscopy were strongly correlated with similar indices obtained by EM and were equally sensitive but considerably easier to obtain. The ruffled border and contact index were significantly decreased by 3 hours after beginning the meal whereas cells of other types became interposed between the osteoclasts and the bone.

Smoltification and seawater adaptation in coho salmon (Oncorhynchus kisutch): plasma calcium regulation, osmoregulation, and calcitonin

In order to examine the dynamics of ion regulation, osmoregulation, and plasma calcitonin during the parr-smolt transformation (smoltification), blood and gill tissue were collected from yearling coho salmon, Oncorhynchus kisutch, from February to October. Fish were kept in fresh water (FW) throughout this period. In addition, fish were exposed to seawater (SW) at the peak of smoltification in mid-April, and samples from these fish were collected until July. Plasma osmolality, gill Na+,K+-ATPase activity, plasma levels of calcitonin, and free and total calcium and magnesium were measured. SW adaptability of FW fish was assessed throughout the study by measurements of plasma osmolality following a 24-hr exposure to seawater. The greatest hypoosmoregulatory ability occurred in April-May, although SW-adapted fish had higher plasma osmolality than FW-adapted fish at all times. Gill Na+,K+-ATPase activity in FW-adapted fish increased from April to June and increased rapidly following exposure of fish to SW, and remained elevated in SW-adapted fish. Free plasma calcium and magnesium levels increased following SW exposure, but returned to prior levels within 1 week. Netting and confinement stress during sampling caused an increase in plasma osmolality and free calcium and magnesium levels in both FW- and SW-adapted fish. Changes in hypoosmoregulatory ability during smoltification and SW adaptation were correlated with changes in gill Na+,K+-ATPase activity. A sharp transitory peak in plasma calcitonin levels occurred early in smoltification (March) and in SW-adapted fish in June. Plasma calcitonin levels gradually increased in FW-adapted fish during the period of desmoltification. However, no change in plasma calcitonin levels occurred during SW-induced hypercalcemia, suggesting that the hormone does not play a major role in short-term plasma calcium regulation in coho salmon.

Human pituitary tumors secrete chromogranin-A

Chromogranin-A (CgA) has emerged as a serum and tissue marker for a number of endocrine tumors. We studied 15 patients with pituitary tumors to evaluate the clinical value of CgA as a serum and tissue marker for such tumors. One third of the patients had elevated serum CgA levels; 2 of these patients had nonsecreting pituitary tumors, and the other 3 had corticotroph adenomas. CgA-positive cells were detected in 9 of the 11 anterior pituitary tumors that were immunostained; in these 9 tumors at least half of the cells were CgA positive. We conclude that CgA production is common in pituitary tumors and that immunohistochemical studies for CgA may aid in their classification. In addition, serum CgA measurements may help to identify patients with pituitary tumors, especially those that do not secrete a known pituitary peptide.

Absence of hyperparathyroidism in severe liver disease

Elevated serum levels of intact parathyroid hormone (PTH) have been reported in severe versus mild biliary cirrhosis. The aim of this study was to determine whether hyperparathyroidism was present in severe liver disease on the basis of the inability of the liver to catabolize the hormone. Because biologic activity resides in the amino terminal, and amino terminal PTH determinations have not been routinely made in liver disease, it is possible that hyperparathyroidism was previously missed in these patients. Accordingly, we obtained fasting blood from 11 patients with severe liver disease and 8 age-matched controls. We measured intact, amino terminal, and mid-region PTH, vitamin D metabolites, bone gamma carboxyglutamic acid protein (BGP), ionized calcium, phosphorus, magnesium, and liver function tests. Serum levels of PTH were normal with all assays and 1,25(OH)2D levels were not elevated. These findings argue against the possibility that hyperparathyroidism plays a role in the pathogenesis of hepatic osteodystrophy.

A high-fat diet increases calcitonin secretion in the rat

We have studied the effects of diet-induced obesity on thyroidal calcitonin, plasma calcitonin, calcium and phosphorus in rats. Twelve 9-week-old female rats were randomly divided into two groups. One group was fed a low-fat diet while the other was fed a high-fat diet. Both diets had 0.76% Ca, 0.56% P and 2.2 U/g vitamin D; however, the high-fat diet had hydrogenated vegetable oil added at 405 g/kg. All rats were pair-fed and consumed 11 g/day per rat for 27 weeks at which time the rats were fasted overnight and exsanguinated. The rats on the high-fat diet weighted 406 +/- 21 g (mean +/- SEM) versus 292 +/- 13 g for controls and had higher levels of serum calcitonin (104 +/- 12 versus 57 +/- 9 pg/ml). The obese rats also had increased thyroidal calcitonin by radioimmunoassay and increased thyroidal C-cells by immunohistology. The increased calcitonin levels occurred without a concomitant increase in calcium levels. These data indicate that a high-fat diet in rats stimulates C-cell growth and calcitonin secretion.

The parathyroid hormone-related protein associated with malignancy is secreted by neuroendocrine tumors

We have demonstrated the production of the PTH-related protein (PTHrP) associated with hypercalcemia of malignancy by human neuroendocrine cell lines that also produce calcitonin gene products and chromogranin A. PTHrP was demonstrable in the cells by immunocytochemistry and immunoassay and Northern analysis of the cells revealed the presence of multiple mRNAs for PTHrP. The cell lines also secreted PTHrP in a regulated fashion, with the most potent secretagogue being phorbol. Thus, PTHrP is secreted by neuroendocrine cells and it may have neuroectodermal lineage. The coexpression of calcitonin gene products and chromogranin A, also neuroendocrine, with PTHRP may influence its secretion and ultimate biological effects in vivo.

Medullary thyroid carcinomas express chromogranin A and a novel neuroendocrine protein recognized by monoclonal antibody HISL-19

PURPOSE AND METHODS

A number of endocrine peptides and proteins are expressed by medullary thyroid carcinoma (MTC). The expression of two newly appreciated neuroendocrine tumor markers, chromogranin A (CgA) and the endocrine antigen defined by monoclonal antibody HISL-19, was determined in 14 MTCs by immunohistology to evaluate the clinical utility of these markers in the diagnosis of MTC. Papillary, follicular, and undifferentiated thyroid tumors were also evaluated along with an MTC cell line. The same tissues were evaluated with antibodies to human calcitonin.

RESULTS

All human calcitonin antibodies were found to react with the MTCs. In addition, all MTCs were reactive for CgA and the antigen detected by antibody HISL-19. CgA was generally present in the human calcitonin-containing cells, whereas the HISL-19 antigen had a more distinctive distribution. The other thyroid tumors failed to show reactivity with any of the three antibodies.

CONCLUSION

Our results demonstrate that, in addition to human calcitonin, MTCs commonly express CgA and the antigen defined by antibody HISL-19. Our observations thus add to the repertoire of endocrine substances produced by MTC. These studies also demonstrate the clinical value of immunohistologic procedures for two novel antigens in distinguishing MTCs from other thyroid tumors.

Expression of messenger ribonucleic acids encoding a parathyroid hormone-like peptide in normal human and animal tissues with abnormal expression in human parathyroid adenomas

A novel PTH-like peptide has recently been purified and cloned from human tumors associated with the syndrome of humoral hypercalcemia of malignancy. We surveyed the expression of mRNAs encoding this peptide in normal tissues by Northern analysis. One or more low abundance hybridizing transcripts was identified in poly(A)+ RNA prepared from human keratinocytes, thyroid, bone marrow, and fibroblasts, from bovine hypothalamus, pituitary, parathyroid, adrenal cortex, and adrenal medulla, and from rat brain, stomach mucosa, and fetal but not adult liver. One or more hybridizing transcripts was also identified in poly(A)+ RNA prepared from a number of established lines, including rat pituitary (GH4), rat pheochromocytoma (PC 12), human osteosarcoma (TE-85), and human medullary carcinoma (TT) cells. Northern analysis of mRNAs from abnormal human parathyroid tissue revealed an overexpression of transcripts for the PTH-like peptide which appeared to be specific for adenomatous or autonomous glands. These findings suggest that the PTH-like peptide is expressed in a number of endocrine and nonendocrine tissues, that it is developmentally expressed in at least one tissue (fetal liver), and that the regulation of its expression is abnormal in human parathyroid adenomas.

Effect of different modes of dialysis on serum erythropoietin levels in pediatric patients. A report of the Southwest Pediatric Nephrology Study Group

The relative importance of erythropoietin (Ep) and inhibitors of erythropoiesis in the development of anemia in pediatric patients with end-stage renal disease (ESRD) was assessed in 82 patients: 41 treated with peritoneal dialysis (PD) and 41 with hemodialysis (HD). Serum Ep was determined with a sensitive radioimmunoassay. Potential serum inhibition of erythroid (CFU-E) and granulocytic (CFU-GM) progenitor cell growth was assessed using human bone marrow cell cultures. The mean Ep level for all 82 patients was 33.1 +/- 3.1 mU/ml, which was significantly higher (P less than 0.05) than the values obtained in 29 normal children (26.2 +/- 2.4 mU/ml). Serum Ep in the PD group (41.6 +/- 5.6 mU/ml) was significantly higher (P = 0.007) than that of the HD group (24.6 +/- 2.1 mU/ml). The mean hematocrit in the PD group (25.2 +/- 0.8%) was also significantly higher (P less than 0.002) than that of the HD group (22.2 +/- 0.5%). The mean serum parathyroid hormone (PTH) level as measured by a mid-terminal radioimmunoassay was not significantly different (P = 0.79) in the HD group (17,298 +/- 3,998 pg/ml) from that of the PD group (15,747 +/- 4,227 pg/ml). Neither serum Ep nor PTH concentration correlated with hematocrit or degree of inhibition of erythroid progenitor cell colony (CFU-E) formation in either group of dialysis patients, nor did the hematocrit correlate with the degree of serum inhibition of CFU-E formation. The higher level of Ep in the PD group may indicate more effective removal by PD of some enzymatic substance which reduces the immunologic and biologic activities of Ep.(ABSTRACT TRUNCATED AT 250 WORDS)

Unique subset of thymic epithelial cells defined by the expression of a novel neuroendocrine antigen

Murine monoclonal antibodies (MoAbs) were produced against a rat medullary thyroid carcinoma to identify neuroendocrine differentiation antigens. One of these antibodies (1D4) identified a novel 62- to 69-kDa antigen expressed by subsets of immune system epithelial and neuroendocrine cells. This antigen is expressed by distinct subsets of thymic epithelial and splenic reticular cells and is shared by discrete subsets of anterior pituitary and thyroid neuroendocrine cells. In the thymus, 1D4 expression identified a unique subset of stellate-shaped Ia+ medullary epithelial cells which did not react with thymosin alpha-1 antisera nor with the MoAb A2B5 specific for a GQ ganglioside expressed by thymic hormone-producing cells. The availability of the 1D4 MoAb should facilitate further characterization of 1D4+ immune system epithelial cells and may advance our understanding of neuroendocrine-immune system interactions.

The effects of forskolin and calcium ionophore A23187 on secretion and cytoplasmic RNA levels of Chromogranin-A and calcitonin

We have studied the regulation of the secretion and cytoplasmic RNA levels of calcitonin (CT) and Chromogranin-A (CgA) to determine if the biosynthesis and secretion of these two substances are controlled in a coordinated fashion. The studies were conducted in two cell lines, a medullary thyroid carcinoma (MTC) cell line and a lung tumor (BEN) cell line. Both cell types secrete CT and CgA. Forskolin treatment resulted in a significant increase in the secretion of CT and CgA in each cell line and in CT-specific cytoplasmic RNA in the MTC cell line. Treatment with calcium ionophore A23187 resulted in significantly increased secretion of both substances in the lung tumor cells but not in the medullary thyroid carcinoma cells. A significant increase in CT-specific or CgA-specific cytoplasmic RNA was not seen in either cell line. We conclude that the secretion of CT and CgA are regulated in a coordinated fashion in these cell lines through processes that are calcium-mediated and processes that involve cyclic AMP-dependent protein kinase A. However, each of these regulatory pathways is not always operative in a given tissue. The coordinate regulation of the secretion of CT and CgA supports the hypothesis that CgA participates in the secretory process of its associated hormones.

Calcium and cholecalciferol: effects of small supplements in normal men

The effect of small calcium and vitamin D supplements on mineral metabolism in normal persons is unclear. To investigate the biochemical response to these medications, we administered 1000 mg Ca and 25 micrograms cholecalciferol per day or a placebo to 92 normal men for 1 y. The Ca and cholecalciferol were tolerated well. 25-Hydroxycholecalciferol [25-(OH)D] and 24,25-dihydroxycholecalciferol [24,25-(OH)2D] levels rose in treated subjects; there was no definite change in 1,25-(OH)2D concentrations. The average difference in 25-(OH)D levels between treated and untreated subjects was 30 nmol/L at 1 y. Fasting serum Ca, alkaline phosphatase, creatinine, and parathyroid hormone levels and the fasting urinary excretion of Ca, phosphorus and cAMP, were not affected. However, 24-h urinary Ca excretion was higher in the supplemented group (3.5 +/- 1.9 vs 4.7 +/- 1.7 mmol/d, p = 0.006). Serum P concentrations were slightly higher in the supplemented group at 1 y. In normal men small calcium and cholecalciferol supplements are safe, provide adequate vitamin D nutrition and apparently increase net gastrointestinal Ca absorption.

Demonstration of Chromogranin A in human neuroendocrine cell lines by immunohistology and immunoassay

We have used immunohistology and radioimmunoassay procedures to study Chromogranin A (CgA) in human neuroendocrine tumor cell lines, especially small cell lung cancers (SCLC). By immunohistology, CgA could be detected in 11 of 18 classical SCLC cell lines, in a medullary thyroid carcinoma (MTC) cell line, and in only one of 13 variant- or non-SCLC cell lines. By radioimmunoassay, CgA could be detected in the cells and culture media of all of the classical SCLC cell lines tested. Many of the classical SCLC cell lines also produced calcitonin (CT). These studies demonstrate that CgA production is a common feature of SCLC cell lines, especially those with neuroendocrine characteristics.

Renal metabolism of calcitonin

The kidneys account for approximately two-thirds of the metabolism of calcitonin, but relatively little is known regarding the details thereof. To further characterize this process, we examined the renal handling and metabolism of human calcitonin (hCT) by the isolated perfused rat kidney. We also studied the degradation of radiolabeled salmon calcitonin (sCT) by subcellular fractions prepared from isolated rabbit proximal tubules. The total renal (organ) clearance of immunoreactive hCT by the isolated kidney was 1.96 +/- 0.18 ml/min. This was independent of the perfusate total calcium concentration from 5.5 to 10.2 mg/dl. Total renal clearance exceeded the glomerular filtration rate (GFR, 0.68 +/- 0.05 ml/min), indicating filtration-independent removal. Urinary calcitonin clearance as a fraction of GFR averaged 2.6%. Gel filtration chromatography of medium from isolated kidneys perfused with 125I-labeled sCT showed the principal degradation products to be low molecular weight forms eluting with monoiodotyrosine. Intermediate size products were not detected. In the subcellular fractionation experiments, when carried out at pH 5.0, calcitonin hydrolysis exclusively followed the activities of the lysosomal enzyme N-acetyl-beta-glucosaminidase. Typically, at pH 7.5, 42% of total degradation occurred in the region of the brush-border enzyme alanyl aminopeptidase and 29% occurred in the region of the cytosolic enzyme phosphoglucomutase. Although 9% of the calcitonin-degrading activity was associated with basolateral membrane fractions, most of this activity could be accounted for by the presence of brush-border membranes.(ABSTRACT TRUNCATED AT 250 WORDS)