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Biomedical subjects

J T Potts

Publications and source records attributed to J T Potts.

At least 127 records · Page 7Linked to original sources
PubMed1982

Metabolism of parathyroid hormone by Kupffer cells: analysis by reverse-phase high-performance liquid chromatography.

Parathyroid hormone (PTH) undergoes rapid proteolysis in the liver, which results in the appearance of multiple COOH-terminal fragments in plasma. Using reverse-phase high-performance liquid chromatographic (HPLC) techniques, we have shown that biologically active bovine PTH (bPTH) internally labeled with [3H]tyrosine is, like 125I-labeled bPTH, rapidly metabolized by isolated rat Kupffer cells in vitro to multiple COOH-terminal fragments that are chemically identical with those previously found in plasma after metabolism in vivo. Quantitation of specific carboxyl fragments in crude mixtures is achieved rapidly by direct HPLC analysis and is as precise as that achieved by Edman degradation. In addition, several different carboxyl fragments with identical NH2 termini were resolved, revealing a complexity not apparent in previous studies employing direct Edman degradation of such mixtures. Parallel studies with [[35S]Met]bPTH show the generation, by the Kupffer cells in vitro, of several labeled NH2-terminal fragments which undergo rapid further degradation in vitro. Thus, hepatic metabolism of PTH by Kupffer cells proceeds by an initial endopeptidase cleavage within the hormonal sequence in a manner compatible with the generation of biologically active NH2 fragments.

Animals↗
PubMed1982

Gut-mediated hypercalcemia in rabbits bearing VX2 carcinoma: new mechanism for tumor-induced hypercalcemia.

The VX2 carcinoma-bearing rabbit is an animal model for tumor-induced hypercalcemia, thought to be due to increased bone destruction effected by prostaglandin E2. The present experiments suggest that the pathophysiology of the hypercalcemia differs from that previously proposed. Tumor was transplanted intramuscularly into 2.5- to 3-kg male New Zealand White rabbits, which were conditioned to a 1.5% calcium diet and treated with daily subcutaneous injections of dichloromethane diphosphonate (10 mg . kg-1 . day-1), a potent inhibitor of bone resorption, or 0.9% NaCl (2 ml . kg-1 . day -1). The diphosphonate had no significant effect on plasma Ca2+ in either group. After day 31, half the animals of each group were fed a calcium-free diet. This normalized the plasma Ca2+ in each VX2-bearing rabbit within 3 to 4 days but had little effect in control rabbits. In a second series of experiments, VX2-bearing rabbits maintained on standard rabbit chow were treated for 11 days with parenteral indomethacin (30--60 mg/day) or 0.9% NaCl. Although indomethacin normalized the markedly elevated urinary excretion of prostaglandin E2, both treatment groups became severely hypercalcemic. Dietary calcium restriction promptly restored to normal the plasma Ca2+ concentration. In a third series of experiments, rabbits were fed standard rabbit chow and treated with oral indomethacin (40 mg/day) while control-rabbits were pair fed in identical chow. Transplantation of VX2 tumor into both groups caused hypercalcemia. We conclude that the hypercalcemia produced by this tumor strain is indomethacin resistant and dependent on an increase in gastrointestinal calcium absorption, not on skeletal calcium mobilization.

Animals↗
PubMed1982

In vitro studies on an antagonist of parathyroid hormone [Nle-8, Nle-18, Tyr-34]bPTH-(3-34)amide.

1 The actions of parathyroid hormone (PTH) are antagonized in vitro by the peptide [Nle-8, Nle-18, Tyr-34]-bPTH-(3-34)amide, an analogue of PTH. In this paper, the actions of the inhibitory peptide were investigated in vivo. 2 Native parathyroid hormone (bPTH-(1-84)), administered i.v. (0.17-1.51 nmol in volume of 0.3 ml) to 7 day old chicks produced hypercalcaemia but administration of the analogue in doses up to 173 nmol was ineffective in this respect. 3 The analogue failed to antagonize the hypercalcaemia produced by bPTH-(1-34) when injected, in 10 fold molar excess, 2 min before or simultaneously with bPTH-(1-34). 4 Normocalcaemia was restored in parathyroidectomized rats by intravenous infusion of bPTH-(1-84) at 32 pmol kg-1 h-1. Addition of the analogue to the infusion fluid in a 200 fold molar excess did not affect the concentrations of calcium and phosphate in the plasma, cyclic adenosine 3',5'-monophosphate (cyclic AMP) in the urine or phosphate clearance but produced a significant (P less than 0.05) rise in urinary calcium clearance. 5 The results suggest that the peptide [Nle-8, Nle-18, Tyr-34]-bPTH-(3-34)amide does not antagonize the actions of PTH in vivo and demonstrate an important dichotomy between in vitro and in vivo biological properties of the PTH analogue.

Animals↗
PubMed1982

Model for parathyroid hormone secretion and metabolism in calves.

A model for parathyroid gland response to differing calcium and magnesium concentrations is proposed based on direct determinations of parathyroid hormone (PTH) secretion rate. A proportional control seems adequate for physiological variations of calcium and magnesium levels. At extreme hypocalcemia, nonlinearities are observed: a model involving the depletion of a storage compartment is proposed, the size of which is calculated. PTH distribution was studied in another group of animals; sampling was made from the saphenous artery and the thoracic duct. Concentrations of plasma and lymph-intact PTH and carboxy-terminal fragments were determined by means of two different radioimmunoassays. The analysis of the results leads to the formulation of a five-compartment model, which shows that the metabolism of intact PTH is only partly due to the formation of carboxy-terminal fragments, most being directly secreted or catabolized. Linking these models for secretion and distribution of PTH, simulation studies were undertaken and compared with published data. This model is then discussed in comparison with previous work concerning other polypeptide hormones.

Animals↗
PubMed1982

Parathyroid gland response to epinephrine: a rate-sensitivity mechanism.

A model for parathyroid gland response to an epinephrine infusion is proposed. It is based on direct measurements of the parathyroid hormone (PTH) secretion rate as measured directly after cannulation of the venous drainage of single superior parathyroid glands of calves. When epinephrine was infused into a jugular vein, a large increase of PTH secretion rate was observed at the beginning of the infusion. The PTH secretion rate thereafter fell, but it was still significantly above the control rate. At the end of the epinephrine infusion a sharp decrease was observed when the infusion was terminated. These observations are consistent with a rate-sensitive control mechanism. Calculation of the parameters yielded a model, which was then linked with a kinetic model for PTH distribution. Simulation studies were undertaken to predict plasma PTH concentration in response to an epinephrine infusion. Good agreement was found between predicted results and previously published work. The role of this rate-sensitive mechanism in the parathyroid gland is discussed.

Animals↗
PubMed1982

Secondary hyperparathyroidism associated with dichloromethane diphosphonate treatment of Paget's disease.

Six patients with extensive polyostotic Paget's disease were treated for 3 months with dichloromethane diphosphonate (CL2MDP; 1600 mg/day, orally). Serum alkaline phosphatase and urinary hydroxyproline excretion decreased 60-80% in each patient. Blood ionized calcium (ca++) and immunoreactive PTH (iPTH) were measured weekly during the first month of therapy and monthly thereafter. As an index of parathyroid function, PTH secretory reserve was assessed by EDTA infusions before treatment, at the end of treatment, and 3 months after Cl2 MDP therapy was stopped. Before therapy, iPTH and Ca++ were normal in all patients. During treatment, Ca++ decreased and iPTH increased in all patients; mean iPTH approximately doubled (95% confidence limits, 1.5- to 2.7-fold increase). At the end of 3 months of treatment, EDTA infusion raised iPTH in each patient to a level higher than that in the control infusion, indicating augmented PTH secretory reserve. Ca++, iPTH, and the iPTH response to EDTA-induced hypocalcemia returned toward baseline by 3 months after the end of CL2MDP treatment. The results indicate that secondary hyperparathyroidism developed as a result of Cl2MDP therapy. The cause of the parathyroid-gland adaptation is not known; the hyperparathyroidism is, however, at least partly reversible. Cl2MDP inhibits bone resorption while allowing bone mineralization to continue; this differential effect could lead to the hypocalcemia and parathyroid hyperfunction.

Aged↗
PubMed1981

Sulfur-free parathyroid hormone analogues containing D-amino acids: biological properties in vitro and in vivo.

Three sulfur-free analogues of bovine parathyroid hormone (bPTH) containing D-amino acids were synthesized by the solid-phase method and their biological properties compared in an in vitro bioassay (rat renal adenylate cyclase assay), a receptor assay for parathyroid hormone (PTH) (canine renal membranes), and an in vivo bioassay (chick hypercalcemia assay). The analogue [Nle8,Nle18,D-Tyr34]-bPTH-(1-34)-amide, which was found to be more than 4 times as potent in vitro as unsubstituted PTH, is the most potent analogue of PTH yet synthesized. The enhanced potency was largely attributable to increased affinity for the PTH receptor. In vivo, however, this analogue was only one-third as potent as bPTH-(1-34). Cumulative evidence suggests that the nearly 15-fold decline in the relative potency when the compound was assayed in vivo is due to the substitution of norleucine for methionine. The other analogues, [D-Val2,Nle8,D-Tyr34]bPTH-(1-34)-amide and [D-Val2,Nle8,Nle18,D=Tyr34]bPTH-(2-34)-amide, were only weakly active in vitro and in vivo, indicating that substitution with D-amino acids at the NH2 terminus of PTH causes markedly diminished receptor affinity. In fact, the placement of a D-amino acid at the NH2 terminus is more deleterious to biological activity than is omission of amino acids at positions 1 and 2.

Adenylyl Cyclases↗
PubMed1981

Deficient production of 1,25-dihydroxyvitamin D in elderly osteoporotic patients.

There is uncertainty about the adequacy of renal secretion of 1,25-dihydroxyvitamin D(1,25-(OH)2-D) in elderly patients with osteoporosis. To investigate this uncertainty, we stimulated secretion of 1,25-(OH)2-D with a 24-hour intravenous infusion of synthetic human parathyroid hormone fragment 1-34 and compared the results in normal young adults and elderly patients with untreated osteoporosis. Serum levels of 1,25-(OH)2-D were similar in both groups (49 +/- 10 and 42 +/- 9 pg per milliliter [116 +/- 24 and 99 +/- 21 pmol per liter]) before the infusion. However, during the 24-hour infusion, serum levels nearly doubled (P less than 0.01) in the normal volunteers but did not change significantly in the patients. Serum ionized calcium increased and serum inorganic phosphate decreased similarly in both groups during the infusion (P less than 0.05). Although the present study does not establish whether deficient 1,25-(OH)2-D secretory reserve is an effect of age or of osteoporosis, it is possible that such a deficiency will explain the inability of elderly osteoporotic patients to adapt to the low-calcium diets common in this age group. If so, this phenomenon may play a part in the pathogenesis of age-related osteoporosis.

Age Factors↗
PubMed1981

Calcitonin messenger RNA encodes multiple polypeptides in a single precursor.

Recombinant DNA techniques were used to analyze the structure of the messenger RNA encoding a precursor of calcitonin, a small calcium-regulating hormone of 32 amino acids. Analyses of the nucleotide sequences of cloned complementary DNA's comprising the entire coding sequence of the messenger RNA revealed that calcitonin is flanked at both its amino and carboxyl termini by peptide extensions linked to the hormone by short sequences of basic amino acids. The location of glycine next to the carboxyl terminal prolinamide of calcitonin is consistent with indications that glycine is required for the enzymatic amidation of proline to the prolinamide. During cellular biosynthesis, calcitonin arises from a large precursor protein by cleavages at both amino and carboxyl terminal residues of the hormone. These findings raise questions concerning the regulation of these cleavages and the potential biological functions of the precursor extensions derived from these cleavages.

Amino Acid Sequence↗
PubMed1981

Procalcitonin is a glycoprotein.

Messenger RNA extracted from rat medullary carcinoma of the thyroid directs the synthesis in cell-free translation systems of a precursor of calcitonin, Mr = 15,000, substantially larger than the mature form of the hormone, Mr = 3,500. When translations of the mRNA were carried out in the presence of microsomal membranes prepared from a canine pancreas, a larger product (apparent Mr = 17,000) was observed by electrophoresis of the labeled proteins in the translation mixtures on sodium dodecyl sulfate-polyacrylamide gels. This membrane-processed product of Mr = 17,000 was specifically immunoprecipitated by an antiserum to synthetic calcitonin and bound to concanavalin A-Sepharose. Incubation of the proteins synthesized in the cell-free translations performed in the presence of microsomal membranes with the glycosidase, endo-beta-N-acetylglucosaminidase H, reduced the apparent molecular weight of the membrane-processed precursor from 17,000 to 12,000. In addition, the processed Mr = 17,000 calcitonin-related precursor, but not the initial, unprocessed precursor of Mr = 15,000, was resistant to proteolytic digestion by a mixture of trypsin and chymotrypsin. These results indicate that the biosynthesis of calcitonin involves the glycosylation and proteolytic cleavage of a newly synthesized precursor along with sequestration of the processed precursor within microsomal vesicles. Thus, the calcitonin precursor undergoes extensive co- and post-translational processing to the smaller, unglycosylated hormone that is secreted.

Animals↗
PubMed1981

Analogues of an in vitro parathyroid hormone inhibitor: modifications at the amino terminus.

Four analogues of parathyroid hormone were synthesized, based on a sequence previously shown to yield an in vitro inhibitor of PTH action. Binding properties of the analogues to presumed parathyroid hormone receptors were evaluated. Each of these analogues contains a structural alteration of the amino terminus of the compound [Nle-8, Nle-18, Tyr-34]bPTH-(3-34)amide. Each of the analogues--[desamino-Ser-3, Nle-8, Nle-18, Tyr-34]bPTH-(3-34)amide, [acetyl-Glu-4, Nle-8, Nle-18, Tyr-34]bPTH-(3-34)amide, [D-Ser-3, Nle-8, Nle-18, Tyr-34]bPTH-(3-34)amide, and [pyroGlu-4, Nle-8, Nle-18, Tyr-34]bPTH-3(3-34)amide--contains modifications selected to diminish or eliminate the weak PTH-like agonist properties detected in the parent compound in vivo. To permit valid comparison of receptor-binding properties, all the analogues were derived from a common solid-phase synthesis. When assayed in the renal adenylate cyclase assay, each of the compounds inhibited completely PTH-stimulated adenylate cyclase activity and was devoid of PTH-like agonist activity. To compare the binding affinity of the analogues for parathyroid hormone binding sites, the peptides were evaluated in a parathyroid hormone renal radioreceptor assay. Each peptide demonstrated a binding affinity comparable with one another and only slightly weaker than that of the unmodified inhibitory analogue, [Nle-8, Nle-18, Tyr-34]bPTH-(3-34)amide. These studies demonstrate that it is possible to alter the amino terminus of parathyroid hormone analogues without causing a dramatic decline in affinity for the parathyroid hormone receptor. Preservation of high receptor-binding affinity in these analogues indicates that synthesis of one or several of these peptides on a scale sufficiently large to permit in vivo evaluation of both agonist and antagonist properties is warranted.

Adenylyl Cyclases↗