Hypocalcemic emergencies

Chronic hypocalcemia occurs frequently, although emergent hypocalcemia does not. When hypocalcemia is suspected, verification of ionized hypocalcemia is required and an etiopathologic search warranted. Etiology-specific therapy is suggested, although at times emergent intravenous calcium is indicated. Long-term nonspecific therapy includes oral calcium and vitamin D supplementation.

Chromosomal localization of the parathyroid hormone/parathyroid hormone-related protein receptor gene to human chromosome 3p21.1-p24.2

The human PTH/PTH-related peptide (PTH/PTHrP) receptor could be involved in hereditary disorders of PTH or PTHrP action. Knowledge of the gene's chromosomal location would allow studies linking it to specific disease traits. Therefore, we mapped the human PTH/PTHrP receptor gene by polymerase chain reaction of human/rodent somatic cell hybrid panels using oligonucleotide primers designed to amplify a portion of the gene from genomic DNA. The PTH/PTHrP gene was unambiguously assigned to the short arm of human chromosome 3, in the region designated 3p21.1-p24.2. Analysis of a second chromosome 3-specific mapping panel suggests that the gene is located near the 3p21.2-p21.3 boundary. The availability of highly polymorphic markers located in this region will permit exploration of the PTH/PTHrP receptor locus in genetic linkage searches for the causes of bone, calcium, and other potential disorders.

Familial benign hypercalcemia--from clinical description to molecular genetics

Familial benign hypercalcemia (or familial hypocalciuric hypercalcemia), a syndrome of lifelong hypercalcemia inherited as an autosomal dominant trait, is distinct from the multiple endocrine neoplasia syndromes and other forms of inherited parathyroid disease. Familial benign hypercalcemia results from the inappropriate secretion of parathyroid hormone despite hypercalcemia, enhanced renal tubular reabsorption of calcium (independent of parathyroid hormone), and apparent tissue resistance to adverse effects of hypercalcemia. Heterozygosity for the familial hypercalcemia trait is benign, although homozygosity for the trait may lead to severe neonatal primary hyperparathyroidism. Genetic linkage studies show that most persons affected with familial hypercalcemia have a mutation on the long arm of chromosome 3 (3cen-q21), although one phenotypically indistinguishable family appears to have a mutation on the short arm of chromosome 19 (19p), and another family has neither 3q nor 19p mutations. One group has recently shown mutations in a putative parathyroid cell-surface calcium receptor that are plausible causes for the chromosome 3q variant of the familial hypercalcemia syndrome. Perhaps the other genes for this syndrome encode proteins representing hitherto-unknown regulators of systemic calcium metabolism independent of parathyroid cell calcium sensing or proteins involved in signal transduction from the calcium receptor.

Osteoporosis: pathophysiology, prevention, diagnosis, and treatment

Bone is a living tissue; throughout life, new bone formation coexists with bone resorption. Although a large number of hormones and cytokines modulate osteoblast and osteoclast function, osteoporosis results from any disorder in which bone formation becomes uncoupled from bone resorption. Many disorders are associated with the uncoupling of bone formation and resorption. The most common is loss of gonadal steroid action on bone, as occurs in menopause or in male and female hypogonadism not associated with menopause. Other relatively common causes include primary hyperparathyroidism and endogenous or exogenous hypercortisolism and thyrotoxicosis. A large number of other, less frequent disorders also cause osteoporosis. Treatment of osteoporosis consists first of removing the cause if possible, for example, abolishing hypercortisolism, thyrotoxicosis, or hyperparathyroidism. In menopausal women or hypogonadal men or women, replacement of estrogens or androgens represents effective therapy. Estrogens and androgens given to hypogonadal subjects strikingly reduce bone resorption. For patients with established osteoporosis who either cannot take gonadal steroids or who are not hypogonadal, calcitonin decreases bone resorption and may stabilize bone mass. Estrogen replacement and calcitonin are approved by the Food and Drug Administration for treatment of osteoporosis. Experimental therapies presently include 1,25-dihydroxyvitamin D (calcitriol), bisphosphonates in intermittent treatment regimes, and fluoride in lower dosages than were used in previous studies. The use of fluoride is controversial, and to some extent it has fallen into disrepute. Effective use of any treatment is predicated on understanding the pathophysiology in any particular disease setting.

Genetic linkage analysis in familial benign (hypocalciuric) hypercalcemia: evidence for locus heterogeneity

Familial benign hypercalcemia (FBH, or hypocalciuric hypercalcemia) is characterized by inheritance, in an autosomal dominant pattern, of lifelong hypercalcemia without hypercalciuria, which is often mistaken for classical primary hyperparathyroidism. Recently, the FBH trait was linked, in four families, to chromosome 3q. We report genetic linkage analysis in 140 persons from five additional families having FBH (65 affected, 67 unaffected, and 8 unclassifiable). In four families, FBH mapped to chromosome 3q, between D3S1215 and D3S20, maximum multipoint lod score 12.9. By contrast, in the fifth kindred FBH mapped to chromosome 19p13.3, tightly linked to the marker loci D19S20 and D19S266 (two-point lod score at recombination fraction = .001 is 3.44 and 3.70, respectively). Thus, the FBH phenotype results from mutations at two separate loci on chromosomes 3q and 19p.

Calcium infusion suggests a "set-point" abnormality of parathyroid gland function in familial benign hypercalcemia and more complex disturbances in primary hyperparathyroidism

PTH clearly plays a role in maintaining the hypercalcemia of familial benign hypercalcemia (FBH or familial hypocalciuric hypocalcemia). To better define the abnormalities of parathyroid function in FBH and primary hyperparathyroidism (1 degree HPT), we used a two-site immunochemiluminometric assay for intact PTH to examine PTH suppressibility in normal individuals and patients having FBH or 1 degree HPT. Twelve normal, 11 FBH, and 7 1 degree HPT subjects were given calcium (Ca) iv with frequent sampling for ionized Ca and intact PTH. In normal and FBH subjects, plasma PTH levels decreased essentially identically in response to iv Ca. In the 1 degree HPT group, PTH was not normally suppressible. However, there was a spectrum of responsiveness in 1 degree HPT patients, with a significant correlation between tumor mass and degree of PTH nonsuppressibility (r = 0.87, P = 0.01). Analysis of the relationship between plasma PTH and ionized Ca values in the three groups demonstrated a shift to the right in the FBH curve, with no difference of slope, consistent with the notion of a simple "set-point" error in FBH. In contrast, the curve in 1 degree HPT was not only shifted to the right but also differed from normal in slope (normal, -8.92; 1 degree HPT, -3.92, P = 0.04). Thus, we propose that the parathyroid functional abnormality in FBH represents a simple set-point error, whereas the defect in 1 degree HPT consists of a set-point error combined with varying degrees of Ca nonsuppressible PTH secretion that may be related to tumor mass.

Risk of age-related fractures in patients with primary hyperparathyroidism

BACKGROUND

Bone mass is reduced, but the influence of primary hyperparathyroidism (HPT) on fracture risk is controversial. We addressed this issue in a population-based retrospective cohort study.

METHODS

Ninety residents of Rochester, Minn, were first diagnosed with HPT in 1965 through 1976 and an equal number of age- and sex-matched control subjects from the community were identified. Fractures were assessed through review of each subject's complete (inpatient and outpatient) medical records in the community.

RESULTS

Prior to the date of diagnosis, Rochester residents with HPT were more likely to have a history of fractures than were matched control subjects from the same population (30% vs 18%). Subsequently, 36% of cases and 31% of control subjects experienced one or more new fractures during 1072 person-years of follow-up; survival free of a new fracture was almost the same in the two groups. Women had more fractures than men, and fracture rates increased with age. Fractures appeared to be somewhat more frequent in those with baseline serum calcium levels of 2.74 mmol/L or more, in those with comorbid conditions possibly due to HPT and in those who did not undergo parathyroidectomy, but these differences were not statistically significant. In a multivariate analysis, only age at diagnosis was an independent predictor of fracture risk, with a 36% increase in risk per 10-year increase in age.

CONCLUSIONS

Overall fracture risk was increased prior to diagnosis of HPT but not afterward. Because the numbers involved were small, however, we cannot exclude an increased likelihood of fractures in certain subgroups of HPT patients.

Humoral hypercalcemia associated with a dysgerminoma

A 16-year-old girl sought medical attention at the Mayo Clinic because of a 4.5-kg weight loss, hypercalcemia, and a pelvic mass. Preoperatively, the level of the beta-subunit of human chorionic gonadotropin was 147 IU/liter. After a brief period for observation and hydration, abdominal exploration revealed a stage III dysgerminoma; total abdominal hysterectomy and bilateral salpingo-oophorectomy were performed. Within the dysgerminoma, syncytial giant cells expressed human chorionic gonadotropin-positive immunostaining in the cytoplasm. Postoperatively, the value of the beta-subunit of human chorionic gonadotropin decreased rapidly. The patient received whole-abdomen irradiation 4 weeks postoperatively, after which the level of calcium returned to normal. The patient has been free of disease for more than 7 years.

Genetic linkage analysis in familial benign hypercalcemia using a candidate gene strategy. I. Studies in four families

Despite extensive study since the first report of familial benign hypercalcemia (FBH, or hypocalciuric hypercalcemia) in 1972, there is no evidence of the specific abnormal gene product. FBH is highly suitable for either a candidate gene or a reverse genetics approach to localizing the genetic abnormality, because it is inherited in an autosomal dominant pattern, is highly penetrant, does not affect survival, and can be diagnosed in families with readily available measurements. Importantly, several candidate genes have been cloned and mapped. Therefore, we collected blood samples and extracted leukocyte DNA from 94 members of 4 families with well documented FBH (44 affected, 45 unaffected, and 5 unclassifiable). We digested the DNA samples with various restriction endonucleases, conducted standard Southern blotting, and searched for restriction fragment length polymorphisms for the following candidate genes (probe names in parentheses): multiple endocrine neoplasia (MEN) type 1 (pMCMP.1, pHBI59, p3C7, and pTHH26), MEN 2a (MCK2 and cTB14.34), basic fibroblast growth factor (pHFL1-7), (Ca2+,Mg2+)ATPase isoform 4 (hPMCA4), membrane Na/Ca exchanger (cNC28 M-A), PTH (pPTH-LF), and calbindin-D28K (pSKCalb). In addition, we used the anonymous variable number tandem repeat marker pYNH24 to verify pedigree structures by excluding misinheritances. Data were analyzed using the Linkage program. For none of the genes was there significant linkage with the FBH trait; logarithm of odds scores ranged from -1.3 to -26.0 at a recombination fraction of 0.001, and from 0.6 to -5.6 at a recombination fraction of 0.10. We conclude that FBH is unrelated to the MEN syndromes and is not caused by mutations in any of the calcium-regulating or -binding proteins or growth factors studied thus far.

Inhibition by human interleukin-1 alpha of parathyroid hormone-related peptide effects on renal calcium and phosphorus metabolism in the rat

Humoral hypercalcemia of malignancy (HHM) is at least partly caused by tumor secretion of PTH-related peptide (PTHrP), but there is growing evidence for cosecretion with PTHrP of other bone-resorbing peptides, such as the cytokine interleukin-1 alpha (IL-1 alpha). Administration of PTHrP in vivo and in vitro generally mimics the actions of PTH itself, with increases in both resorption and formation of bone. However, bone in HHM is characterized by uncoupling of bone turnover, with increased resorption and decreased formation. We performed experiments to determine whether IL-1 alpha might alter the effects of PTHrP and produce uncoupling. Thus, we administered to 100-g male rats by sc osmotic minipumps synthetic PTHrP-(1-34) alone (2 micrograms/100 g/day), recombinant IL-1 alpha alone (1.5 micrograms/100 g/day), both peptides together at the previous doses, or vehicle only. We infused 5 groups of 12 rats each (PTHrP, IL-1 alpha, PTHrP plus IL-1 alpha, ad libitum fed control, and controls pair-fed to the PTHrP plus IL-1 alpha group) for 14 days. At the end of the study, blood and urine were taken for chemical measurements, and tibias and femurs were harvested for histomorphometry and extraction of RNA from periosteal cells. As expected, PTHrP induced hypercalcemia, relative hypophosphatemia, phosphaturia, and reduced bone mass. Osteoblast number was increased, but osteoclast number was not. Indices of bone formation were unchanged or reduced. The dose of IL-1 alpha chosen had no statistically significant effect, except for reduced longitudinal bone growth, but when combined with PTHrP, IL-1 alpha reduced hypercalcemia, hypophosphatemia, and phosphaturia. In contrast to the blood and urine effects, IL-1 alpha did not interact significantly with PTHrP's effect on bone measurements. Northern analysis of periosteal cell mRNA showed that PTHrP reduced expression of osteocalcin, but not glyceraldehyde-3-phosphate dehydrogenase; IL-1 alpha had no additional effect. These data suggest that 1) continuously administered PTHrP alone may induce uncoupled bone turnover with decreased cortical bone formation; 2) IL-1 alpha appears to inhibit strongly the renal effects of PTHrP and weakly (if at all) its actions on bone and, thus, to decrease its hypercalcemic, phosphaturic, and hypophosphatemic actions; and 3) cosecretion of IL-1 alpha, and possibly other peptide cytokines, with PTHrP may modify the clinical expression of HHM.

Visual acuity after successful treatment of large macular retinoblastoma

Recent trends in the treatment of retinoblastoma have favored radiation therapy as opposed to enucleation. A major determining factor in selecting radiation therapy is the possibility of useful posttreatment visual function. While the treatment of nonmacular tumors seems reasonable, little information is available about the posttreatment visual outcome of large posterior pole tumors. We treated 17 patients (20 eyes) with group III-V retinoblastoma and large posterior pole tumors with external beam radiation. Visual acuity after treatment ranged from 5/200 to 20/50. Potential posttreatment visual function was difficult to predict using such pretreatment factors as age at diagnosis, funduscopic appearance, and the number, size, and location of the tumors. Surprising visual function was obtained in some patients with multiple large macular tumors. Follow up ranged from 1 to 8 years. No patient developed metastatic disease; however, four patients required subsequent cataract extraction. This study supports the consideration of radiation therapy as the primary treatment in eyes previously felt to have a poor visual outcome.

Parathyroid hormone (PTH)-induced intracellular Ca2+ signalling in naive and PTH-desensitized osteoblast-like cells (ROS 17/2.8): pharmacological characterization and evidence for synchronous oscillation of intracellular Ca2+

We showed recently that the initial peak cytosolic ionized calcium ([Ca2+]i) response to PTH (2-min exposure) is preserved relative to the cAMP response in osteoblast-like rat osteosarcoma cells (ROS 17/2.8) desensitized by 72-h exposure to PTH. We attempted in the present studies to determine the mechanisms for preservation of the [Ca2+]i response and to explore the effects of longer PTH rechallenges. The [Ca2+]i response to a 20-min perifusion with rat PTH [rPTH-(1-34)] was monitored by aequorin luminescence in both naive and PTH-desensitized ROS 17/2.8 cells. The responses of both naive and desensitized cells consisted of two phases: an initial peak, followed by an intermediate plateau that was sustained in the presence of PTH. We observed in the naive cell populations synchronous oscillations in [Ca2+]i concentration during this second phase (amplitude, 10-60 nM; frequency, 1-3/100 sec). These oscillations were maintained through extracellular calcium (EC Ca2+) entry; the initial peak was the result of Ca2+ release from intracellular stores. In desensitized cells, these two phases could not be clearly separated with respect to Ca2+ source, but, as we showed before, exhibited an enhanced dependence on EC Ca2+ entry for the response to PTH. Nevertheless, in the desensitized cells, the sustained [Ca2+]i response was diminished in magnitude and showed little oscillatory behavior. Brief exposure to neomycin sulfate, an inhibitor of phosphoinositide turnover, attenuated the PTH-induced [Ca2+]i rise in both naive and desensitized cells. Protein kinase-C activity did not appear to be required for either phase of the PTH-induced [Ca2+]i response. Exposure to cholera toxin attenuated the [Ca2+]i response to hormone in both naive and desensitized cells, more markedly in the latter. Cholera toxin treatment dramatically increased basal cAMP levels in both cell preparations; PTH-stimulated cAMP production was unchanged in naive cells, but increased nearly 4-fold in desensitized cells. We propose that the preserved PTH-induced peak [Ca2+]i rise in desensitized cells results primarily from the diminished regulation of EC Ca2+ entry by the cAMP response limb. The attenuated sustained oscillatory behavior observed in desensitized cells upon rechallenge with hormone may be the result of reduced phosphoinositide turnover and reduced Ca2+-stimulated Ca2+ release. Thus, the [Ca2+]i response to PTH in osteoblast-like cells is complex and modulable and seems to provide a number of ways to regulate intracellular metabolism under various conditions. We speculate that this plasticity of the [Ca2+]i response to PTH is related to the pleiotropic actions of the hormone on cells of the osteoblast lineage.

Dihydroergotamine and sumatriptan attenuate levels of CGRP in plasma in rat superior sagittal sinus during electrical stimulation of the trigeminal ganglion

Vasoactive neuropeptides, present in unmyelinated C-fibers, can be released from perivascular sensory axons by antidromic stimulation, to mediate vasodilation and extravasation of plasma protein (neurogenic inflammation). In this report, the effects of antidromic trigeminal stimulation on levels of calcitonin gene-related peptide (CGRP) in plasma were examined in the superior sagittal sinus and the effects of drugs that have been shown previously to block extravasation of neurogenic plasma determined. The levels of immunoreactive CGRP in plasma were measured both before and during electrical stimulation of the trigeminal ganglion (0.1-1.0 mA, 5 msec, 5 Hz, 3-5 min), using a highly specific and sensitive immunochemiluminometric assay. Levels of CGRP increased and became maximal within the first minute of stimulation. The increases were detectable at intensities of current as small as 0.1 mA. Peak levels related to the intensity of the stimulus. Samples from femoral arterial blood did not show concomitant increases at 1 min. Pretreatment with dihydroergotamine (DHE) (50 micrograms/kg i.v.) did not change the baseline levels but decreased levels of CGRP during stimulation (0.3 mA), by 55% at 1 min and 50% at 3 min. Sumatriptan (GR43175) (300 micrograms/kg) attenuated the increase by 57% at 3 min (0.1 mA, 5 msec, 5 Hz) but not after 1 min of stimulation, although decreases were observed at the latter time during an individual experiment. Drug-induced attenuation of levels of CGRP in plasma may reflect inhibition of release, to thereby provide evidence to explain blockade of neurogenic extravasation of plasma.

Clinical spectrum of primary hyperparathyroidism: evolution with changes in medical practice and technology

Over the last 25 years, the perceived clinical spectrum of primary hyperparathyroidism (HPT) has changed dramatically from a disorder characterized by severe bone and renal disease to one typically manifested by few or mild symptoms and little evidence of organ damage. Reasons for this change in spectrum include changing demographics (primary HPT is primarily a disease of the middle-aged and elderly), diffusion of medical knowledge leading to a higher index of suspicion, and improved clinical laboratory technology (especially inexpensive and accurate determination of serum calcium and parathyroid hormone). In the first 343 cases of primary HPT seen at the Massachusetts General Hospital, 57% had renal stones, 23% had hyperparathyroid bone disease, and less than 1% had no symptoms. By contrast, studies dating from the availability of automated serum calcium measurement found renal stones and hyperparathyroid bone disease in less than 5% of cases, and about half of cases had few or no symptoms. Most patients with primary HPT today have mild, nonspecific symptoms, such as weakness, fatigue, and mental depression, and such signs as arterial hypertension and osteopenia, and detection of their hypercalcemia is generally serendipitous. The mildness and slow progression seen in many cases of primary HPT has resulted in much controversy about appropriate management.

Homologous amino-terminal radioimmunoassay for rat parathyroid hormone

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.

Cyclic adenosine 3',5'-monophosphate responses to parathyroid hormone, prostaglandin E2, and isoproterenol in dermal fibroblasts from patients with familial benign hypercalcemia

Plasma concentrations of PTH are much lower for a given calcium or phosphorus level in patients with familial benign hypercalcemia (FBH, or familial hypocalciuric hypercalcemia) than in those with primary hyperparathyroidism; these and other data suggest that there might be tissue hypersensitivity to PTH in FBH. To test this hypothesis, we have used cultured dermal fibroblasts from abdominal skin biopsies of six patients with FBH and six age- and sex-matched controls as surrogate PTH-responsive tissues. Cells in 24-well plastic plates were exposed to vehicle, human PTH-(1-34) (10(-10)-10(-7) M), prostaglandin E2 (10(-6) M), or isoproterenol (10(-4) M) for 10 min in the presence of isobutylmethylxanthine, and cellular cAMP was determined by RIA. All cells responded to PTH with dose-dependent increases in cAMP, and all responded strongly to prostaglandin E2 and isoproterenol. There were no consistent or significant differences between control and FBH fibroblasts in maximal responses to the three agonists, and half-maximal stimulation was achieved with about 10(-9) M PTH in both normal and FBH cells. These data are not consistent with increased tissue sensitivity to PTH in FBH.

Plasma intact parathyroid hormone (PTH) and PTH-related peptide in familial benign hypercalcemia: greater responsiveness to endogenous PTH than in primary hyperparathyroidism

The cause of hypercalcemia in familial benign hypercalcemia (FBH; also called familial hypocalciuric hypercalcemia) is unclear, although it is PTH dependent. It is also uncertain how plasma PTH levels are related to the severity of biochemical abnormalities in FBH. Because the PTH-related peptide (PTHrP) has many PTH-like actions, it might have a role in the hypercalcemia of FBH. Thus, we studied 29 patients with FBH from 11 families, 29 age- and sex-matched controls, and 42 patients with primary hyperparathyroidism (1 degree HPT), measuring PTH with a highly sensitive two-site immunochemiluminometric assay and the hypercalcemic tumor factor PTH-related peptide (PTHrP) with an extraction/concentration RIA. Plasma PTH values were elevated in 86% of 1 degree HPT patients (36 of 42), but in only 20% of FBH patients, (6 of 29). Plasma PTHrP was elevated in 1 FBH patient, and the group mean value was normal. Plasma PTH was positively correlated with calcium (Ca) in 1 degree HPT (r = 0.66; P less than 0.0001) and in FBH (r = 0.53; P less than 0.004), but the slopes of the regressions were markedly different: 1 degree HPT, 6.72; FBH, 1.61 (P less than 0.0001). There was a negative correlation between PTH and phosphorus (P) in 1 degree HPT (r = -0.39; P less than 0.01) and in FBH (r = -0.41; P less than 0.03), but, again, the slopes differed greatly: 1 degree HPT, -6.57; FBH, -1.95 (P less than 0.0001). There were no correlations between PTHrP and Ca or between PTH and PTHrP. The sums and products of PTH and PTHrP were not better correlated with Ca than PTH alone. Thus, PTH values are lower at given Ca and P levels in patients with FBH than in those with 1 degree HPT, suggesting that PTH is more effective in raising Ca and lowering P in FBH than in 1 degree HPT. The enigma of FBH remains: what molecular defect can simultaneously cause parathyroid cell insensitivity to Ca, enhanced renal tubular reabsorption of Ca, increased renal rejection of P, and enhanced or retained sensitivity to PTH?

Desensitization of rat osteoblast-like cells (ROS 17/2.8) to parathyroid hormone uncouples the adenosine 3',5'-monophosphate and cytosolic ionized calcium response limbs

We have investigated the effects of PTH-induced desensitization on second messenger interactions in the rat osteosarcoma cell line ROS 17/2.8. Adenylate cyclase activation was assessed by accumulation of immunoassayable cAMP, and cytosolic calcium ion ([Ca2+]i) concentrations were measured in adherent perifused cells loaded with the Ca2(+)-sensitive bioluminescent protein aequorin. Preexposure to rat PTH-(1-34) [rPTH-(1-34); 10(-8) M for 48 h, then 10(-7) M for 24 h] dramatically reduced (by 85%) the cAMP response to fresh challenge [2 min; 10(-9)-10(-7) M rPTH-(1-34)], but the peak PTH-induced rise of [Ca2+]i was not diminished significantly (0-20%). Nevertheless, we did observe other changes in the PTH-induced [Ca2+]i response. Exposure of treated cells to (Bu)2cAMP nearly abolished the [Ca2+]i response to PTH (greater than 80% reduction), but had much less effect on the PTH-stimulated [Ca2+]i increment of the naive cells (less than 35% reduction). Treated cells also had a blunted [Ca2+]i response to PTH in the presence of low extracellular calcium (greater than 60% reduction), but in the naive cells, low extracellular Ca2+ did not significantly diminish the peak PTH-induced [Ca2+]i rise, although low extracellular Ca2+ dramatically reduced the area under this [Ca2+]i transient (greater than 50%). Low extracellular Ca2+ had no influence on the peak [Ca2+]i responses of treated cells to bradykinin or prostaglandin F2 alpha. Although the peak PTH-stimulated [Ca2+]i rise of treated cells in normal Ca2+ medium was not significantly attenuated, the time to half-maximum [Ca2+]i concentration was significantly increased (greater than 100%), and the area under the [Ca2+]i transient was diminished. These alterations in the [Ca2+]i response of treated cells were not observed upon challenge with bradykinin or prostaglandin F2 alpha. Thus, 1) the cAMP and [Ca2+]i responses of ROS 17/2.8 cells to rPTH-(1-34) are not obligatorily coupled; 2) the response of naive cells to PTH includes both the release of Ca2+ from intracellular stores and the entry of extracellular Ca2+; and 3) pretreatment of these cells with rPTH-(1-34) augments the dependence on Ca2+ entry during hormone rechallenge. We propose that the preserved PTH-stimulated [Ca2+]i rise in treated cells results partly from loss of cAMP-mediated inhibition of extracellular Ca2+ entry.