Guanine nucleotide binding proteins, interchangeably called N or G proteins, seem to be the primary signal-transducing components of various agonist-induced cell membrane functions. In the heart, G proteins have been implicated in beta-adrenergic modulation of the slow inward Ca2+ current. We have investigated the role of G proteins in muscarinic activation of an inwardly rectifying, acetylcholine (ACh)-induced K+ current (IACh), and beta-adrenergic activation of an (isoprenaline)-induced Ca2+ current (Isi). Here we report that intracellular application of the non-hydrolysable GTP analogue 5'-guanylylimidodiphosphate (GppNHp) brought about an agonist-induced, antagonist-resistant, persistent activation of IACh and Isi. This functional uncoupling of channel from receptor suggests that the muscarinic receptor and the IACh channel are separate molecular structures. Membrane conductance responses to sequential activation of muscarinic and beta-adrenergic receptors demonstrate that in contrast to the muscarinic inhibition of Isi, muscarinic stimulation of IACh is mediated by a G protein via a pathway that does not involve adenylate cyclase. Taken together, the results support the notion that agonist is required to induce GppNHp binding and/or activation of the G proteins. Once triggered by agonist, the control system remains maximally activated, thereby transforming the cell so that it no longer responds to subsequent homologous receptor-mediated signals.

A homology model for the human calcium sensing receptor (hCaR) transmembrane domain utilizing bovine rhodopsin (bRho) structural information was derived and tested by docking the allosteric antagonist, NPS 2143, followed by mutagenesis of predicted contact sites. Mutation of residues Phe-668 (helix II), Arg-680, or Phe-684 (helix III) to Ala (or Val or Leu) and Glu-837 (helix VII) to Ile (or Gln) reduced the inhibitory effects of NPS 2143 on [Ca 2؉ ] i responses. The calcimimetic NPS R-568 increases the potency of Ca 2؉ in functional assays of CaR. Mutations at Phe-668, Phe-684, or Glu-837 attenuated the effects of this compound, but mutations at Arg-680 had no effect. In all cases, mutant CaRs responded normally to Ca 2؉ or phenylalanine, which act at distinct site(s). Discrimination by the Arg-680 mutant is consistent with the structural differences between NPS 2143, which contains an alkyl bridge hydroxyl group, and NPS R-568, which does not. The homology model of the CaR transmembrane domain robustly accounts for binding of both an allosteric antagonist and agonist, which share a common site, and provides a basis for the development of more specific and/or potent allosteric modulators of CaR. These studies suggest that the bRho backbone can be used as a starting point for homology modeling of even distantly related G protein-coupled receptors and provide a rational framework for investigation of the contributions of the transmembrane domain to CaR function.

A B ST RACT The role of a guanine nucleotide-binding protein (Gk) in the coupling between muscarinic receptor activation and opening of an inwardly rectifying K + channel [IK(M)] was examined in cardiac atrial myocytes, using hydrolysis-resistant GTP analogues. In the absence of muscarinic agonist, GTP analogues produced a membrane current characteristic of IK(M). The initial rate of appearance of this receptor-independent IK(M) was measured for the various analogues in order to explore the kinetic properties of IK(M) activation. We found that Ir~(M) activation is controlled solely by the intracellular analogue/GTP ratio and not by the absolute concentrations of the nucleotides. Analogues competed with GTP for binding to Gk with the following relative affinities: GTP'yS > GTP > GppNHp > GppCH2p. At sufficiently high intracellular concentrations, however, all GTP analogues produced the same rate of Is(M) activation. This analogue-independent limiting rate is likely to correspond to the rate of GDP release from inactive, GDP-bound Gk. Muscarinic receptor stimulation by nanomolar concentrations of acetylcholine (ACh), which do not elicit IS(M) under control conditions, catalyzed /V.(M) activation in the presence of GTP analogues. The rate of Gk activation by ACh (kAch) was found to be described by the simple relationship kAch = 8.4 X l0 s min-lM -~'[ACh] + 0.44 rain -~, the first term of which presumably reflects the agonist-catalyzed rate of GDP release from the Gk'GDP complex, while the second term corresponds to the basal rate of receptor-independent GDP release. Combined with the estimated K0.5 of the IK(M)-[ACh] dose-effect relationship, 160 nM, this result also allowed us to estimate the rate of Gk" GTP hydrolysis, k ..... ,to be near 135 min -j. These results provide, for the first time, a quantitative description of the salient features of G-protein function in vivo.

Expression of Ca2+-sensing receptors (CaR) was demonstrated in several human intestinal epithelial cell lines (T84, HT-29, and Caco-2) and in rat intestinal epithelium by both reverse transcriptase-polymerase chain reaction (PCR) and Northern blotting of RNA. Restriction patterns of the PCR products were of the sizes predicted by the human and rat sequences. CaR agonists (Ca2+, poly-l-arginine, protamine) mediated an increase in intracellular Ca2+ in HT-29–18-C1 cells (monitored by changes in fura 2 fluorescence), which was dependent on release from thapsigargin-sensitive stores. U-73122, an inhibitor of phosphatidylinositol-phospholipase C, eliminated the CaR agonist-mediated rise in intracellular Ca2+, whereas its inactive analog, U-73343, had no effect. Pertussis toxin pretreatment had no effect on CaR agonist-mediated modulation of intracellular Ca2+. Taken together, these studies demonstrate that CaR are expressed in intestinal epithelial cells and couple to mobilization of intracellular Ca2+. The presence of CaR in intestinal epithelial cells presents a new locus for investigations into the role(s) of extracellular Ca2+ in modulating intestinal epithelial cell differentiation and transepithelial Ca2+ transport.

Calcium-sensing receptors (CaSRs) regulate systemic calcium homeostasis in the parathyroid gland, kidney, intestine, and bone and translate fluctuations in serum calcium into peptide hormone secretion, cell signaling, and regulation of gene expression. The CaSR is a G protein (heterotrimeric guanosine triphosphate-binding protein)-coupled receptor that operates in the constant presence of agonist, sensing small changes with high cooperativity and minimal functional desensitization. Here, we used multiwavelength total internal reflection fluorescence microscopy to demonstrate that the signaling properties of the CaSR result from agonist-driven maturation and insertion of CaSRs into the plasma membrane. Plasma membrane CaSRs underwent constitutive endocytosis without substantial recycling, indicating that signaling was determined by the rate of insertion of CaSRs into the plasma membrane. Intracellular CaSRs colocalized with calnexin in the perinuclear endoplasmic reticulum and formed complexes with 14-3-3 proteins. Ongoing CaSR signaling resulted from agonist-driven trafficking of CaSR through the secretory pathway. The intracellular reservoir of CaSRs that were mobilized by agonist was depleted when glycosylation of newly synthesized receptors was blocked, suggesting that receptor biosynthesis was coupled to signaling. The continuous, signaling-dependent insertion of CaSRs into the plasma membrane ensured a rapid response to alterations in the concentrations of extracellular calcium or allosteric agonist despite ongoing desensitization and endocytosis. Regulation of CaSR plasma membrane abundance represents a previously unknown mechanism of regulation that may be relevant to other receptors that operate in the chronic presence of agonist.