A lifetime aging study of human CD19 transgenic mice.

Mice transgenic for human CD19 have been an important animal model to help understand the role of this molecule in B lymphocyte function. Previously, no lifetime studies had been performed to understand the effects of this CD19 over expression on the survival or spontaneous pathology within the C57BL/6J background strain. We conducted a lifetime study with interim sacrifices to understand the transgenic effects on clinical signs, body weight, survival, and spontaneous pathology. Blood and urine samples were collected from select animals at various time points during the study for measurement of clinical pathology parameters and groups of animals were euthanized and examined at predetermined intervals. There was fair survival with some animals living to 108 weeks of age. Clinical pathology evaluations revealed a declining red cell mass with a regenerative anemia, increasing total white blood cell counts and decreasing glucose level. Total protein, albumin, and globulin levels increased to 52 weeks of age and then declined to or below baseline with advancing age. Increased urinary microalbumin levels correlated with the severity of a glomerulopathy at 76 and 84 weeks of age. Mean body weight increased through 70 weeks and then declined to weights similar to week 28 at 108 weeks. Macroscopic observations included pale kidneys, enlarged seminal vesicles, and enlarged spleens (at 108 weeks of age). The most common neoplasms in this study were bronchiolar alveolar adenomas in the lung, histiocytic sarcoma in several different tissues, and hepatocellular adenomas. The most common non-neoplastic lesions were renal glomerulopathy, and pulmonary lymphocytic infiltrates with increased numbers of alveolar macrophages.

Robust anti-obesity and metabolic effects of a dual GLP-1/glucagon receptor peptide agonist in rodents and non-human primates.

AIMS: To characterize the pharmacology of MEDI0382, a peptide dual agonist of glucagon-like peptide-1 (GLP-1) and glucagon receptors. MATERIALS AND METHODS: MEDI0382 was evaluated in vitro for its ability to stimulate cAMP accumulation in cell lines expressing transfected recombinant or endogenous GLP-1 or glucagon receptors, to potentiate glucose-stimulated insulin secretion (GSIS) in pancreatic ß-cell lines and stimulate hepatic glucose output (HGO) by primary hepatocytes. The ability of MEDI0382 to reduce body weight and improve energy balance (i.e. food intake and energy expenditure), as well as control blood glucose, was evaluated in mouse models of obesity and healthy cynomolgus monkeys following single and repeated daily subcutaneous administration for up to 2 months. RESULTS: MEDI0382 potently activated rodent, cynomolgus and human GLP-1 and glucagon receptors and exhibited a fivefold bias for activation of GLP-1 receptor versus the glucagon receptor. MEDI0382 produced superior weight loss and comparable glucose lowering to the GLP-1 peptide analogue liraglutide when administered daily at comparable doses in DIO mice. The additional fat mass reduction elicited by MEDI0382 probably results from a glucagon receptor-mediated increase in energy expenditure, whereas food intake suppression results from activation of the GLP-1 receptor. Notably, the significant weight loss elicited by MEDI0382 in DIO mice was recapitulated in cynomolgus monkeys. CONCLUSIONS: Repeated administration of MEDI0382 elicits profound weight loss in DIO mice and non-human primates, produces robust glucose control and reduces hepatic fat content and fasting insulin and glucose levels. The balance of activities at the GLP-1 and glucagon receptors is considered to be optimal for achieving weight and glucose control in overweight or obese Type 2 diabetic patients.

CT imaging of complications of catheter ablation for atrial fibrillation.

The complication rate following radiofrequency catheter ablation for atrial fibrillation is low (<5%). Complications include pericardial effusion, cardiac tamponade, pulmonary vein stenosis, oesophageal ulceration or perforation, atrio-oesophageal fistula formation, stroke/transient ischaemic attack, phrenic nerve injury, haematoma at the puncture site, and femoral arteriovenous fistula. Among available imaging tools, computed tomography (CT) can be very useful in diagnosing complications of the procedure, particularly in the subacute and delayed stages after ablation. This review illustrates CT imaging of several of the common and uncommon complications of radiofrequency catheter ablation.

Molecular signatures of cortical expansion in the human fetal brain.

The third trimester of human gestation is characterised by rapid increases in brain volume and cortical surface area. A growing catalogue of cells in the prenatal brain has revealed remarkable molecular diversity across cortical areas.1,2 Despite this, little is known about how this translates into the patterns of differential cortical expansion observed in humans during the latter stages of gestation. Here we present a new resource, µBrain, to facilitate knowledge translation between molecular and anatomical descriptions of the prenatal developing brain. Built using generative artificial intelligence, µBrain is a three-dimensional cellular-resolution digital atlas combining publicly-available serial sections of the postmortem human brain at 21 weeks gestation3 with bulk tissue microarray data, sampled across 29 cortical regions and 5 transient tissue zones.4 Using µBrain, we evaluate the molecular signatures of preferentially-expanded cortical regions during human gestation, quantified in utero using magnetic resonance imaging (MRI). We find that differences in the rates of expansion across cortical areas during gestation respect anatomical and evolutionary boundaries between cortical types5 and are founded upon extended periods of upper-layer cortical neuron migration that continue beyond mid-gestation. We identify a set of genes that are upregulated from mid-gestation and highly expressed in rapidly expanding neocortex, which are implicated in genetic disorders with cognitive sequelae. Our findings demonstrate a spatial coupling between areal differences in the timing of neurogenesis and rates of expansion across the neocortical sheet during the prenatal epoch. The µBrain atlas is available from: https://garedaba.github.io/micro-brain/ and provides a new tool to comprehensively map early brain development across domains, model systems and resolution scales.

Extension of life-span by loss of CHICO, a Drosophila insulin receptor substrate protein.

The Drosophila melanogaster gene chico encodes an insulin receptor substrate that functions in an insulin/insulin-like growth factor (IGF) signaling pathway. In the nematode Caenorhabditis elegans, insulin/IGF signaling regulates adult longevity. We found that mutation of chico extends fruit fly median life-span by up to 48% in homozygotes and 36% in heterozygotes. Extension of life-span was not a result of impaired oogenesis in chico females, nor was it consistently correlated with increased stress resistance. The dwarf phenotype of chico homozygotes was also unnecessary for extension of life-span. The role of insulin/IGF signaling in regulating animal aging is therefore evolutionarily conserved.

Pharmacologic effects in man of a specific serotonin-reuptake inhibitor.

Fluoxetine (Li-ly 110140) caused a 63 percent inhibition of [3H]serotonin uptake into platelets obtained from normal volunteers to whom the drug was administered daily for 7 days. This dose had no effect on the usual pressor response produced by injections of norepinephrine or tyramine.

Benchmarking a novel ultrasound-CT fusion system for respiratory motion management in radiotherapy: assessment of spatio-temporal characteristics and comparison to 4DCT.

Management of respiratory motion during radiation therapy requires treatment planning and simulation using imaging modalities that possess sufficient spatio-temporal accuracy and precision. An investigation into the use of a novel ultrasound (US) imaging system for assessment of respiratory motion is presented, exploiting its good soft tissue contrast and temporal precision. The system dynamically superimposes the appropriate image plane sampled from a reference CT data set with the corresponding US B-mode image. An articulating arm is used for spatial registration. While the focus of the study was to quantify the system's ability to track respiratory motion, certain unique spatial calibration procedures were devised that render the software potentially valuable to the general research community. These include direct access to all transformation matrix elements and image scaling factors, a manual latency correction function, and a three-point spatial registration procedure that allows the system to be used in any room possessing a traditional radiotherapy laser localization system. Counter-intuitively, it was discovered that a manual procedure for calibrating certain transformation matrix elements produced superior accuracy to that of an algorithmic Levenberg-Marquardt optimization method. The absolute spatial accuracy was verified by comparing the physical locations of phantom test objects measured using the spatially registered US system, and using data from a 3DCT scan of the phantom as a reference. The spatial accuracy of the display superposition was also tested in a similar manner. The system's dynamic properties were then assessed using three methods. First, the overall system response time was studied using a programmable motion phantom. This included US video update, articulating arm update, CT data set resampling, and image display. The next investigation verified the system's ability to measure the range of motion of a moving anatomical test phantom that possessed both high and low contrast test objects. Finally, the system's performance was compared to that of a four-dimensional CT (4DCT) data set. The absolute spatial and display superposition accuracy was found to be better than 2 mm and typically 1 mm. Overall dynamic system response was adequate to produce a mean relative positional error of less than 1 mm if an empiric latency correction of 3 video frames was incorporated. The dynamic CT/US display mode was able to assess phantom motion for both high and low contrast test objects to within 1 mm, and compared favorably to the 4DCT data. The 4DCT movie loop accurately assessed the target motion for both of the high and low contrast objects tested, but the minimum intensity and average intensity reconstructions did not. This investigation demonstrated that this US system possesses sufficient spatio-temporal accuracy to properly assess respiratory motion. Future work will seek to demonstrate efficacy in its clinical application to respiratory motion assessment, particularly for sites in the upper abdomen, where low tissue contrast is evident.

Urinary cyclic AMP analyzed as a function of the serum calcium and parathyroid hormone in the idfferential diagnosis of hypercalcemia.

Urinary cyclic AMP (UcAMP) appropriate for the serum calcium concentration was determined in normal subjects during the base-line state and during alteration in their serum calcium concentrations by saline and calcium infusions. This was compared to the UcAMP in 76 patients with hypercalcemia and 5 patients with hypocalcemia. In 54 of 56 patients with primary hyperparathyroidism, the UcAMP was inappropriately high for their serum calcium concentration, the 2 exceptions having renal failure. In four patients with vitamin D intoxication, sarcoidosis, milkalkali syndrome, and thiazide-induced hypercalcemia and in five patients with hypocalcemia due to hypoparathyroidism, the UcAMP was appropriately low for their serum calcium concentration. In 16 patients with nonparathyroid neoplasms, 10 had UcAMP levels that were inappropriately high suggesting ectopic parathyroid hormone (PTH)-mediated hypercalcemia and 6 had UcAMP levels that were appropriately low suggesting that their hypercalcemia was due to osteolytic factors other than PTH. Correlations between UcAMP, serum calcium concentration, and carboxyl-terminal immunoreactive PTH suggest that random UcAMP is a sensitive accurate reflection of circulating biologically active PTH. If there is adequate renal function (serum creatinine concentration less than 2.0 mg/dl), a random UcAMP expressed as mumol/g creatinine and analyzed as a function of the serum calcium concentration completely separates patients with PTH and non-PTH-mediated hypercalcemia.

TC21 and Ras share indistinguishable transforming and differentiating activities.

Constitutively activated mutants of the Ras-related protein TC21/R-Ras2 cause tumorigenic transformation of NIH3T3 cells. However, unlike Ras, TC21 fails to bind to and activate the Raf-1 serine-threonine kinase. Thus, whereas Ras transformation is critically dependent on Raf-1 TC21 activity is promoted by activation of Raf-independent signaling pathways. In the present study, we have further compared the functions of Ras and TC21. First we determined the basis for the inability of TC21 to activate Raf-1. Whereas Ras can interact with the two distinct Ras-binding sequences in NH2-terminus of Raf-1, designated RBS1 and Raf-Cys, TC21 could only bind Raf-Cys. Thus, the inability of TC21 to bind to RBS1 may prevent it from promoting the translocation of Raf-1 to the plasma membrane. Second, we found that TC21 is an activator of the JNK and p38, but not ERK, mitogen-activated protein kinase cascades and that TC21 transforming activity was dependent on Rac function. Thus, like Ras, TC21 may activate a Rac/JNK pathway. Third, we determined if TC21 could cause the same biological consequences as Ras in three distinct cell types. Like Ras, activated TC21 caused transformation of RIE-1 rat intestinal epithelial cells and terminal differentiation of PC12 pheochromocytoma cells. Finally, activated TC21 blocked serum starvation-induced differentiation of C2 myoblasts, whereas dominant negative TC21 greatly accelerated this differentiation process. Therefore, TC21 and Ras share indistinguishable biological activities in all cell types that we have evaluated. These results support the importance of Raf-independent pathways in mediating the actions of Ras and TC21.

Ras, but not Src, transformation of RIE-1 epithelial cells is dependent on activation of the mitogen-activated protein kinase cascade.

Src transformation of NIH3T3 mouse fibroblasts has been shown to be dependent on Ras function. Since we recently showed that the signaling pathways that mediate Ras transformation of RIE-1 rat intestinal epithelial cells are distinct from those that cause Ras transformation of fibroblasts, we utilized three approaches to determine if Src transformation of RIE-1 cells is dependent on Ras. First, although both Ras and Src cause upregulation of an epidermal growth factor (EGF) receptor-dependent autocrine growth loop, only Ras transformation required this activity. Second, whereas both Src and Ras caused upregulation of the p42 and p44 mitogen-activated protein kinases (MAPKs), only Ras transformation was blocked by the inhibition of MAPK activation by treatment with the PD 98059 MEK inhibitor. Third, treatment with the farnesyltransferase inhibitor FTI-277 blocked Ras, but not Src, transformation. Taken together, these observations suggest that Src transformation of RIE-1 cells is not dependent on Ras. Finally, we determined that Ras activation of Raf-independent pathways alone is sufficient to cause growth transformation of RIE-1 cells. Thus, both Ras and Src cause transformation of RIE-1 cells via pathways distinct from those required to cause transformation of NIH3T3 cells.

The dynamic insulin secretory response of isolated pancreatic islets of the diabetic mouse. Evidence for a gene dosage effect on insulin secretion.

Expression of the autosomal recessive (db) gene in homozygous (db/db) C57BL/KsJ mice results in a severe and eventually fatal diabetic syndrome. Many studies of the diabetic mouse have used lean littermates (+/?) as controls despite evidence suggesting a gene dosage effect in heterozygous animals. In order to study the gene dosage effect of the diabetes (db) gene on insulin release in the heterozygote, perifusion experiments were performed on isolated islets of Langerhans of diabetic (db/db), heterozygous (+/db), and normal (+/+) control mice. Islets of normal controls exhibited a fivefold greater increase in insulin release than did those of diabetics in response to 16.7 mM D-glucose. The insulin secretory response of islets of heterozygotes to glucose was intermediate, being two-fold greater than that of diabetics but only about half of that of normal controls. Biphasic insulin release in response to glucose was observed only in islets of normal controls. Islets of all three genotypes exhibited biphasic insulin release in response to 10 mM D-glyceraldehyde; however, overall insulin release in both heterozygotes and diabetics remained diminished as compared with the response of normal controls. This is in contrast to the situation we have previously reported in islets of fasted or aging rats in which, though manifesting defects in glucose-stimulated insulin release, the islets are able to respond normally to 10 mM D-glyceraldehyde in respect to both the dynamic secretory pattern and quantity of insulin released. Our data suggest a gene dosage effect of the (db) gene on glucose-stimulated insulin release in heterozygous (+/db) C57BL/KsJ mice.

Ras-mediated suppression of TGFbetaRII expression in intestinal epithelial cells involves Raf-independent signaling.

Ras-transformed intestinal epithelial cells are resistant to the growth inhibitory actions of TGFbeta and have a marked decrease in expression of the TGFbeta type II receptor (TGFbetaRII). Rat intestinal epithelial cells (RIE) were stably transfected with activated Ras, Sos and Raf constructs and tested for expression of TGFbetaRII and sensitivity to growth inhibition by TGFbeta. The parental RIE line and the RIE-Raf cells were non-transformed in morphology and were sensitive to TGFbeta (70-90% inhibited). In contrast, the RIE-Ras and RIE-Sos lines were transformed, resistant to TGFbeta and expressed 5- to 10-fold decreased levels of the TGFbetaRII mRNA and protein. Cyclin D1 protein expression was repressed by TGFbeta treatment in parental RIE and RIE-Raf cells, whereas levels of cyclin D1 in RIE-Ras and RIE-Sos cells remained unchanged. Treatment of RIE-Ras cells with 25 microM farnesyl transferase inhibitor, FTI L739,749, for 48 hours restored expression of TGFbetaRII to levels equivalent to control cells. In addition, treatment of RIE-Ras cells for 48 hours with PD-98059, a specific MAPKK inhibitor, also increased expression of TGFbetaRII to control levels. Collectively these results suggest that downregulation of TGFbetaRII and loss of sensitivity to growth inhibition by TGFbeta in Ras-transformed intestinal epithelial cells is not mediated exclusively by the conventional Ras/Raf/MAPKK/MAPK pathway. However, activation of MAPK, perhaps by an alternate Ras effector pathway, appears to be necessary for Ras-mediated downregulation of TGFbetaRII.

Calcium inhibition of parathyroid adenylate cyclase.

Parathyroid adenylate cyclase activity has previously been reported to be inhibited by calcium. However, the concentrations of calcium required to inhibit this enzyme were, in most instances, several orders of magnitude higher than cytoplasmic calcium concentrations. We undertook this study to determine the effects of calcium at submicromolar, as well as higher concentrations on parathyroid adenylate cyclase activity. A partially purified membrane fraction was obtained from normal porcine parathyroid glands using buffers free of divalent cation chelators. Relatively high concentrations of EGTA were then used in the incubation mixture to achieve the desired low ionic calcium concentrations. Addition of 0.5 mM EGTA to the reaction mixture resulted in an approximate 30% increase in basal adenylate cyclase activity and a similar percentage increase in the activity measured in the presence of guanosine triphosphate or NaF, known activators of parathyroid adenylate cyclase. EGTA also stimulated the activity measured in the presence of forskolin, which itself markedly stimulated this enzyme in a concentration-dependent manner. The effects of calcium added in amounts calculated to achieve ionic concentrations of 0.5 X 10(-7) M to 1 X 10(-3) M on the forskolin-activated enzyme activity was investigated. The resultant calcium inhibition curve was found to be stepwise in appearance. Analysis of the data using a mathematical model which assumes multiple, independent calcium-binding sites indicated the presence of two inhibitory sites. One had a high affinity for calcium, Kdiss 0.4-1.5 X 10(-6) M. This site accounted for 50-70% of the calcium-inhibitable activity. The second had a considerably lower affinity, Kdiss = 1.0-5.0 X 10(-4) M. Similar affinities were obtained from experiments in which guanosine triphosphate was used to activate the enzyme instead of forskolin. It is suggested that the high affinity calcium-binding site may be involved in the physiological regulation of parathyroid adenylate cyclase activity and that the previously reported estimates of the calcium sensitivity of parathyroid adenylate cyclase represent composites of the high and low affinity calcium-inhibitable processes.

The effects of magnesium on calcium inhibition of parathyroid adenylate cyclase.

cAMP has been shown to mediate the response of the parathyroid gland to a number of agonists and appears to take part in the regulation of this gland by divalent cations as well. We have studied the effects of the concentrations of free magnesium (Mg+2) and ionic calcium (Ca+2) on the kinetic properties of normal porcine parathyroid adenylate cyclase. In a previous study we obtained evidence for two calcium inhibition sites in this enzyme complex. In the present study we observed that the Mg+2 concentration influences the relative contribution of these sites to the overall calcium inhibition. At a high Mg+2 concentration (10 mM), the high affinity site contributes less than 50% of the total calcium inhibitable activity, whereas at a Mg+2 concentration in the low physiological range (0.5 mM), the high affinity site accounts for all the calcium inhibitable activity. Mg+2 was found to be a potent activator of porcine parathyroid adenylate cyclase, with a Ka of Mg of 0.8-2 mM. Ionic calcium at low concentrations (0.2-5 microM) acts as a competitive inhibitor with respect to Mg+2 activation. The calcium inhibition constant was estimated to be 2-3 microM. The Km for ATPMg-2 was 0.3 mM, which is similar to that found in other studies of adenylate cyclase activity in parathyroid tissue. The effects of Ca+2 on enzymatic activity with respect to the ATPMg-2 concentration showed noncompetitive inhibition. The calcium inhibition constant with respect to its effect on Vmax (KIv) was 3 microM; the calcium inhibition constant with respect to its effect on the binding of ATPMg-2 (KIs) was 10 microM. It is concluded from these results that the concentrations of intracellular Ca+2 reported to be present in parathyroid cells could inhibit adenylate cyclase activity. The mode of calcium inhibition that involves competition with magnesium would be particularly significant at low intracellular Mg+2 concentrations, and this phenomenon may account for the parathyroid secretory defect which is a characteristic feature of the magnesium-deficient state.

Parathyroid hormone secretion in magnesium deficiency.

The effect of an acute elevation of the serum magnesium concentration on the concentrations of serum immunoreactive parathyroid hormone (IPTH) were studied in hypocalcemic hypomagnesemic patients, hyperparathyroid patients, and normal individuals. Basal serum IPTH concentrations in the hypomagnesemic patients ranged from undetectable to 3 times the upper limit of normal. All hypomagnesemic patients were observed to have an immediate rise in the serum IPTH concentration after magnesium administration regardless of the basal IPTH concentration. In contrast, normal individuals and patients with primary and secondary hyperparathyroidism responded to magnesium administration with either a decrease or little change in the serum IPTH concentration. These date indicate that an acute stimulation of PTH secretion induced by magnesium is characteristic of the magnesium-deficient state. The consistency of this response suggests that impaired PTH secretion is a significant factor contributing to the hypocalcemia of magnesium deficiency.

Vitamin D: the discovery of its metabolites and their therapeutic applications.

Our understanding of the role of vitamin D in calcium-phosphorus metabolism has changed considerably in the last decade. Studies performed in tissue culture, animal, and man have firmly established that the natural compound requires hydroxylation in the liver at the C-25 position and in the kidney at the C-1 position to form the biologically active derivative 1,25-(OH)2D3. These hydroxylation reactions are finely regulated to maintain normal calcium-phosphorus homeostasis: We now regard 1,25-(OH)2D3 as a hormone which is released by the kidney during periods of hypocalcemia. This hormone acts on the intestinal mucosa to facilitate calcium absorption and on bone to increase calcium mobilization. Its function in other tissues is still being evaluated. The active metabolites of vitamin D and several closely related analogs have been synthesized. It has been clearly demonstrated that 1,25-(OH)2D3 and 1alpha-OH-D3 promote healing in uremic bone disease. Administration of small amounts of these compounds has corrected the biochemical disturbances in vitamin D-dependency and hypoparathyroidism. Limited clinical experience with 25-OH-D3 and 1,25-(OH)2D3 in children with familial hypophosphatemia has failed to show convincing evidence of a therapeutic effect. Further clinical studies are needed to fully evaluate the therapeutic potential of this new family of compounds.

Diagnosis of Pneumocystis carinii pneumonia. Roentgenographic-pathologic correlates based on fiberoptic bronchoscopy specimens from patients with the acquired immunodeficiency syndrome.

We analyzed the diagnosis of Pneumocystis carinii pneumonia by fiberoptic bronchoscopy in a large series of patients with the acquired immunodeficiency syndrome (AIDS). Transbronchial biopsy fragments, as opposed to endobronchial specimens, were found to have high diagnostic value. Their optimal number for diagnosis was determined by a simple statistical principle. It varied from a minimum of two in cases of severe pneumonia to a maximum of four when roentgenographic manifestations were altogether absent. The diagnostic yield of the transbronchial biopsy alone was 97 percent and that of "touch" preparations of the biopsies 88 percent; when both techniques were combined, the accuracy rose to 98 percent. By comparison, bronchial "washings" and "brushings" had a much lower yield, 59 percent and 57 percent, respectively. Because their diagnostic contribution was negligible, we conclude that the latter two procedures represent an unnecessary expense and waste of technical and professional effort.

Radiologic-pathologic-clinical correlation in mechanisms of disease course.

The interdisciplinary approach to medical education has often been deficient in medical school curricula. Because of the nonintegrated approach to medical education, problem solving capabilities are not fostered. The clinical presentation of a patient, the imaging modalities used to investigate the patient, and the ultimate pathologic diagnosis are frequently not correlated. At the University of Miami School of Medicine/Jackson Memorial Medical Center, radiology coordinates, along with pathology and the various clinical medicine sections, a Mechanisms of Disease course for sophomore students. This course serves as a "bridge" between the basic sciences (first two years) and the clinical years (last two years). We utilize a Radiologic-Pathologic-Clinical (RPC) correlation approach to problem-solving and algorithms to teach proper problem-solving techniques.

Parathyroid hormone clearance in man.

The role of the kidney, liver, and bone and/or muscle, in the metabolic clearance of parathyroid hormone (PTH), has been examined in man. Serum was obtained from the femoral artery and the renal, hepatic, and femoral veins of nine hyperparathyroid patients undergoing selective venous catheterization. The concentration of immunoreactive parathyroid hormone (IPTH) was measured in the samples by radioimmunoassay using an antiserum predominantly specific for the aminoterminal (N-terminal) portion of the PTH molecule. Gel filtration of hyperparathyroid sera demonstrated this antiserum to measure essentially only intact PTH. The mean arteriovenous (AV) difference measured across the liver was 44%; across the kidney, 34%; and across the leg, 16%. These arteriovenous differences were all statistically significant (p less than 0.005). A significant positive correlation was found between the AV difference in IPTH across the kidney and the serum calcium concentration (r = 0.50, p less than 0.05). These studies suggest that both the liver and the kidney play major roles in the clearance of PTH in man and indicate that PTH is cleared by bone and/or muscle as well. The correlation observed between the serum calcium concentrations and the AV differences in IPTH across the kidney suggest that the rate of clearance of PTH in man may be modulated by changes in the concentration of serum calcium.