L-Amino Acids Regulate Parathyroid Hormone Secretion

Abstract

Molecular analysis of the CaR indicates that it is composed of several functional domains that are conserved across related members of subgroup C. The N-terminal Venus Fly Trap domains of many of these receptors recognize amino acids (especially glutamate) or the amino acid analog, ␥-aminobutyric acid. More recent work indicates that several cloned members of the family are also broad-spectrum amino acid sensors. In mammals, these include the CaR, which is allosterically activated by aromatic, aliphatic, and polar amino acids as well as plasma-like amino acid mixtures (6) and a heterodimeric amino acid taste receptor, which has broad selectivity for aliphatic, polar, and charged amino acids but not aromatic amino acids (7). In human embryonic kidney (HEK)293 cells that stably express the human CaR, L-amino acids markedly enhanced the sensitivity of the receptor to Ca 2ϩ and other cationic agonists including spermine and Gd 3ϩ (6). Because the CaR mediates the acute control of PTH secretion, the finding that it is allosterically activated by L-amino acids raises the possibility that PTH secretion is acutely regulated not only by adjustments in extracellular Ca 2ϩ o but also by physiological changes in amino acid concentration. We have tested and confirmed this hypothesis in the current study on normal human parathyroid cells. The data indicate that Lamino acids and plasma-like mixtures of L-amino acids allosterically activate endogenous parathyroid CaRs contributing to the control of intracellular signaling pathways and PTH secretion. In the presence of physiological concentrations of extracellular Ca 2ϩ , L-amino acids (also at physiological concentrations) stereoselectively activated Ca 2ϩ mobilization and inhibited PTH secretion. The data support the view that fluctuations in serum amino acid levels acting via the CaR acutely regulate PTH secretion and thus whole body calcium metabolism.

Calcium-sensing receptor is a physiologic multimodal chemosensor regulating gastric G-cell growth and gastrin secretion

“…Unlike the mGluR 1 and GABA B R but similar to the taste receptor type 1 members 1 and 3 amino acid sensing taste receptor heterodimer (25) and the less well-characterized nutrient-sensing receptor, GPRC6A (26), CaR recognizes a broad array of L-amino acids interacting within the Venus Fly Trap domain (27). However, CaR has a preference for aromatic amino acids, such as Phe and Trp, in vitro in transfected HEK293 cells (11) and acutely isolated human parathyroid cells (14). Consistent with these studies, L-Phe, gavaged at a concentration expected during a meal, elicited a gastrin response that could account for most of the stimulus observed with peptone in WT and heterozygous littermates.…”

“…Studies in isolated parathyroid cells show that CaR can be allosterically activated by most L-amino acids in the presence of extracellular Ca 2+ (14). Mice were gavaged with peptone to assess the role of CaR in protein-stimulated gastrin secretion.…”

“…Although there have been several in vitro-based studies in either a variety of heterologously transfected cell systems (11)(12)(13) or acutely isolated primary cell cultures (14) that support the role of CaR as a multimodal sensor regulating both Ca 2+ and non-Ca 2+ homeostatic cell systems, there has not been any in vivo physiologic evidence confirming this proposed role.…”

Allosteric Modulation of the Calcium-sensing Receptor by γ-Glutamyl Peptides

“…Upon arrival in the laboratory, it was either used immediately or, more typically, stored overnight in MEM at 4°C as described previously (23). Prior to collagenase digestion, parathyroid tissue retains viability under these conditions (24).…”

“…Determination of PTH Secretion from Perifused Human Parathyroid Cells-Perifusion of normal human parathyroid cells was undertaken in low molecular mass (4 -5 kDa) cut-off gel filtration micro beads so that intact PTH (ϳ9 kDa) would appear in the void volume as described previously (23,25). Gel filtration media were pre-equilibrated with physiological saline: 125 mM NaCl, 4.0 mM KCl, 1.25 mM CaCl 2 , 1.0 mM MgCl 2 , 0.8 mM Na 2 HPO 4 , 20 mM HEPES (NaOH), 0.1% D-glucose (pH 7.4) that contained 1ϫ basal amino acid mixture (total concentration 2.8 mM; (26)), and 1 mg/ml bovine serum albumin.…”