Calcimimetics with potent and selective activity on the parathyroid calcium receptor.

Parathyroid hormone (PTH) secretion is regulated by a cell surface Ca2+ receptor that detects small changes in the level of plasma Ca2+. Because this G protein-coupled receptor conceivably provides a distinct molecular target for drugs useful in treating bone and mineral-related disorders, we sought to design small organic molecules that act on the Ca2+ receptor. We discovered that certain phenylalkylamine compounds, typified by NPS R-568 and its deschloro derivative NPS R-467, increased the concentration of cytoplasmic Ca2+ ([Ca2+]i) in bovine parathyroid cells and inhibited PTH secretion at nanomolar concentrations. These effects were stereoselective and the R enantiomers were 10- to 100-fold more potent than the S enantiomers. NPS R-568 potentiated the effects of extracellular Ca2+ on [Ca2+]i and PTH secretion but was without effect in the absence of extracellular Ca2+. Both compounds shifted the concentration-response curves for extracellular Ca2+ to the left. Presumably, these compounds act as positive allosteric modulators to increase the sensitivity of the Ca2+ receptor to activation by extracellular Ca2+. Both NPS R-467 and NPS R-568 increased [Ca2+]i in HEK 293 cells expressing the human parathyroid Ca2+ receptor but were without effect in wild-type HEK 293 cells. Neither compound affected the cytoplasmic Ca2+ responses elicited by several other G protein-coupled receptors in HEK 293 cells or in bovine parathyroid cells. Significantly, these compounds did not affect responses elicited by the homologous metabotropic glutamate receptors, mGluR1a, mGluR2, or mGluR8. These compounds therefore act selectively on the Ca2+ receptor. Compounds that mimic or potentiate the effects of extracellular Ca2+ at the Ca2+ receptor are termed calcimimetics. The discovery of calcimimetic compounds with potent and selective activity enables a pharmacological approach to regulating plasma levels of PTH. Calcimimetic compounds could conceivably provide a specific medical therapy for primary hyperparathyroidism.

The parathyroid calcium receptor: a novel therapeutic target for treating hyperparathyroidism.

Parathyroid cells, C-cells, and certain cells in the kidney express a cell surface calcium (Ca2+) receptor which enables these cells to detect and respond to changes in the concentration of extracellular Ca2+. This receptor protein is a member of the G protein-coupled receptor superfamily and shares limited sequence homology only with metabotropic glutamate receptors. The Ca2+ receptor is the primary physiological mechanism regulating the secretion of parathyroid hormone (PTH) and plays a pivotal role in maintaining systemic Ca2+ homeostasis. Compounds that act as Ca2+ receptor agonists are called calcimimetics because they mimic or potentiate the effects of extracellular Ca2+ on parathyroid cell function. NPS R-568 is a small organic calcimimetic compound that acts as a positive allosteric modulator to increase the sensitivity of the Ca2+ receptor to activation by extracellular Ca2+. In normal rats, orally administered NPS R-568 decreases plasma levels of PTH and Ca2+ and, at higher doses, increases plasma levels of calcitonin. The changes in the circulating levels of these two hormones explain the hypocalcemia caused by this compound. NPS R-568 also effectively lowers plasma PTH levels in normal humans and in rat models of secondary hyperparathyroidism. Calcimimetic compounds that target the Ca2+ receptor provide a novel therapeutic approach for treating primary and secondary hyperparathyroidism.

Cloning and characterization of a truncated dopamine D1 receptor from goldfish retina: stimulation of cyclic AMP production and calcium mobilization.

Receptors for dopamine are present on horizontal cells of fish retina that are linked to the activation of adenylate cyclase. In the present study, the goldfish (Carassius auratus) gene that encodes these receptors, referred to as gfD1, was isolated and analyzed. A single open reading frame within the gfD1 gene encodes a protein of 363 amino acids that is highly homologous with dopamine D1 receptors from rats and humans. Interestingly, the carboxyl terminus of gfD1 lacks 80 amino acids that are present in the mammalian receptor sequences. RNA analysis using the polymerase chain reaction demonstrated that the gene is expressed in the goldfish retina and is intronless within the coding region. The fact that gfD1 encodes a dopamine D1 receptor was demonstrated through pharmacological analysis of transfected cells. Both the gfD1 receptor and the human D1 receptor expressed in mammalian cells had high affinity for SCH-23390 and other D1-specific ligands. In addition, the gfD1 receptor and the human D1 receptor were able to stimulate the accumulation of cAMP in response to SKF-38393 or dopamine. Interestingly, stimulation of both the gfD1 and human receptors with dopamine also resulted in an increase in intracellular Ca2+. Finally, long term pretreatment of transfected cells with dopamine resulted in the desensitization and down-regulation of both the goldfish and human receptors.

Further characterization of the D2 dopamine receptor expressed in MMQ cells.

The D2 dopamine receptor expressed in the MMQ cell line was characterized by saturation binding using the D2 dopamine radioligand [3H]spiperone. The KD for spiperone was 41 pM and the Bmax for these sites was 34 fmol/mg protein. Inhibition of forskolin-stimulated cAMP accumulation occurred in response to a variety of D2 agonists, and the agonist effects were reversed by D2 antagonists. Pertussis toxin pretreatment abolished agonist inhibition of cAMP accumulation. In addition, the alpha 2-adrenergic agonist UK 14304 inhibited cAMP accumulation; this effect was reversed by an alpha 2-adrenergic antagonist but not by a D2 antagonist, indicating the presence of alpha 2-adrenergic receptors on these cells. Specific oligonucleotide primers were used in the polymerase chain reaction to determine, by restriction enzyme analysis and Southern blotting, that the long form of the two alternatively spliced variants of the D2 dopamine receptor was the predominant variant expressed in these cells.

A D1/D2 chimeric dopamine receptor mediates a D1 response to a D2-selective agonist.

D1 and D2 dopamine receptors are G-protein coupled receptors and have seven transmembrane spanning regions (TM) typical of this receptor superfamily. Although dopamine binds equally to D1 and D2 receptors, many compounds are highly selective. To probe the receptors for regions that determine subtype specificity, plasmid constructs coding for the D1 or a D1/D2 chimeric receptor were made and transfected into cells to study the binding and agonist properties of non-selective or subtype-selective compounds. The results suggest that the D2-selective agonist, quinpirole, gains much of its selectivity by binding to within TM VI and VII of the D2 receptor.

Serine mutations in transmembrane V of the dopamine D1 receptor affect ligand interactions and receptor activation.

Several serines present in transmembrane domain V are conserved among members of the G-protein-coupled receptor family that bind catecholamines. Two of these serines that are present in the beta-adrenergic receptor were previously shown by site-directed mutagenesis to affect agonist binding and receptor activation (Strader, C. D., Candelore, M. R., Hill, W. S., Sigal, I. S., and Dixon, R. A. F. (1989) J. Biol. Chem. 264, 13572-13578). We investigated the role of the serines present in transmembrane V of another catecholamine receptor, the dopamine D1 receptor, by site-directed mutagenesis, and the results show that mutations at serines 198, 199, and 202 affect dopamine binding. The substitution of serine 198 or serine 199 by an alanine also affects the binding of several other agonist and antagonist dopaminergic compounds while an alanine substitution at serine 202 has no effect on the binding of these compounds. Moreover, each single serine mutation decreased the maximal cAMP accumulation elicited by a dopamine D1 partial agonist. These results suggest that serines present in transmembrane V of the D1 receptor affect ligand interactions and receptor signal transduction, but not entirely in the manner that would be predicted from the model proposed for the beta-adrenergic receptor.

Dopamine D1 receptor stimulation of cyclic AMP accumulation in COS-1 cells.

Dopamine is shown to stimulate cAMP accumulation in COS-1 cells via endogenously expressed dopamine D1 receptors. A dissociation of dopamine and beta-adrenoceptor responses is demonstrated by the use of selective antagonists and different desensitization patterns following exposure of the cells to dopamine or the beta-adrenoceptor agonist, isoproterenol. The dopamine response in COS-1 cells exhibits a pharmacological profile similar to that found in dopamine D1 tissues such as rat striatum and fish retina. The presence of DARPP-32 (dopamine- and cAMP-regulated phosphoprotein, Mr 32,000) immunoreactivity in COS-1 cells is shown by Western blotting and is consistent with the endogenous expression of a dopamine D1 receptor in these cells. It is concluded that a dopamine D1 receptor is expressed in COS-1 cells and the implications of this are discussed.

Type-A cholecystokinin binding sites in cow brain: characterization using (-)-[3H]L364718 membrane binding assays.

(-)-[3H]L364718 membrane binding assays were employed to localize and characterize cholecystokinin (CCK)-A binding sites in rat and cow brain. Specific binding was detected in all brain areas tested, but in all areas of rat brain and most areas of cow brain the level was too low to allow characterization of the ligand binding specificity of these sites. Membranes prepared from cow nucleus accumbens and striatum contained higher levels of (-)-[3H]L364718 specific binding which represented 55-70% of total binding. Characterization of the ligand binding properties of (-)-[3H]L364718 binding sites in cow nucleus accumbens revealed that these sites are similar to CCK-A sites found in pancreatic membranes. Binding of (-)-[3H]L364718 was saturable and had high affinity (Kd = 45 pm). Sites labeled by (-)-[3H]L364718 displayed stereospecificity for the stereoisomers of CR1409. The competition curve for CCK8 was shallow and was steepened and shifted to the right by the presence of the stable GTP analog guanosine 5'-(beta,delta-imido)triphosphate. The potency of CCK8, but not (-)-L36478, was also affected by the buffer in which the assay was conducted. Future use of (-)-[3H]L364718 membrane binding assays using cow nucleus accumbens and/or striatum will help explore the possibility of differences in ligand recognition among CCK-A sites found in brain and peripheral tissues.

Type-A cholecystokinin receptors in CHP212 neuroblastoma cells: evidence for association with G protein and activation of phosphoinositide hydrolysis.

125I-Bolton Hunter-cholecystokinin octapeptide (BH-CCK8) and (-)-[3H]L-364718 membrane binding assays were used to identify and characterize cholecystokinin (CCK) receptors in CHP212 human neuroblastoma cells. The ligand binding properties of CCK receptors in these cells are similar to those found in pancreas (CCK-A sites) and differ from the predominant type of CCK binding site found in brain (CCK-B sites). The specific binding of 125I-BH-CCK8 but not (-)-[3H]L-364718 was reduced by the metabolically stable GTP analog guanosine 5'-(beta-delta-imido)trisphosphate. A substantial difference in the Bmax for the radiolabeled agonist (125I-BH-CCK8) and antagonist [(-)-[3H]L-364718] was noted. These observations are consistent with CCK receptors existing in guanine nucleotide-binding protein-coupled and -uncoupled states. Similar to its action in pancreatic acinar cells, CCK8(S) stimulated the accumulation of [3H]inositol phosphates in cells prelabeled with [3H]myo-inositol (EC50 = 3.2 +/- 0.4 nM; maximum response = 4.5 +/- 0.4 x basal). The intrinsic activity of CCK analogues in stimulating phosphoinositide hydrolysis was substantially less than their reported intrinsic activity in stimulating phosphoinositide hydrolysis in pancreatic acinar cells. The CHP212 neuroblastoma cell may serve as a useful model for the recently reported CCK-A binding site found in the central nervous system.