The interaction of activated G protein‐coupled receptors with G proteins is a key event in signal transduction. Here, using a fluorescence resonance energy transfer (FRET)‐based assay, we measure directly and in living cells the interaction of YFP‐labeled α2A‐adrenergic receptors with CFP‐labeled G proteins. Upon agonist stimulation, a small, concentration‐dependent increase in FRET was observed. No specific basal FRET was detected in the absence of agonist. Kinetics of the onset of receptor/G protein interaction were <100 ms and depended on expression levels of Gα. Simultaneously recorded G protein‐regulated inwardly rectifying K+ channel currents revealed a maximal current response already at agonist concentrations producing submaximal FRET amplitudes. By analyzing FRET signals in the presence of a Gα mutant, which dissociates more slowly from activated receptors, it was demonstrated that only a fraction of wild‐type G proteins interacts with the activated receptor at any time. Our data suggest that α2A‐adrenergic receptors and G proteins interact by rapid collision coupling and indicate that there is no significant precoupling between these receptors and G proteins.

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G-protein-coupled receptors (GPCRs) are the largest group of cell surface receptors. They are stimulated by a variety of stimuli and signal to different classes of effectors, including several types of ion channels and second messenger-generating enzymes. Recent technical advances, most importantly in the optical recording with energy transfer techniques--fluorescence and bioluminescence resonance energy transfer, FRET and BRET--, have permitted a detailed kinetic analysis of the individual steps of the signalling chain, ranging from ligand binding to the production of second messengers in intact cells. The transfer of information, which is initiated by ligand binding, triggers a signalling cascade that displays various rate-controlling steps at different levels. This review summarizes recent findings illustrating the speed and the complexity of this signalling system.

We have reexamined the muscarinic receptor subtype mediating carbachol-induced contraction of rat urinary bladder and investigated the role of phospholipase (PL)C, D, and A 2 and of intra-and extracellular Ca 2ϩ sources in this effect. Based on the nonsubtype-selective tolterodine, the highly M 2 receptorselective (R)-4-{2-[3-(4-methoxy-benzoylamino)-benzyl]-piperidin-1-ylmethyl}-piperidine-1-carboxylic acid amide , and the highly M 3 receptor-selective darifenacin and 3-(1-carbamoyl-1,1-diphenylmethyl)-1-(4-methoxyphenylethyl)pyrrolidine (APP), contraction occurs via M 3 receptors. Carbachol stimulated inositol phosphate formation in rat bladder slices, and this was abolished by the phospholipase C inhibitor 1- Carbachol had only little effect on PLD activity in bladder slices, but the PLD inhibitor butan-1-ol, relative to its negative control butan-2-ol (0.3% each), caused detectable inhibition of carbachol-induced bladder contraction. The cytosolic PLA 2 inhibitor arachidonyltrifluoromethyl ketone weakly inhibited carbacholinduced contraction at a concentration of 300 M, but the cyclooxygenase inhibitor indomethacin (1-10 M) remained without effect. The Ca 2ϩ entry blocker nifedipine (10 -100 nM) almost completely inhibited carbachol-induced bladder contraction. In contrast, 1-[␤-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole HCl (SKF 96,365; 10 M), an inhibitor of storeoperated Ca 2ϩ channels, caused little inhibition. We conclude that carbachol-induced contraction of rat bladder largely depends on Ca 2ϩ entry through nifedipine-sensitive channels and, perhaps, PLD, PLA 2 , and store-operated Ca 2ϩ channels, whereas cyclooxygenase and, surprisingly, also PLC are not involved to a relevant extent.Muscarinic acetylcholine receptors are the physiologically most important mechanisms to elicit contraction of the urinary bladder (Andersson, 1993). In the bladder of various mammalian species, including humans, M 2 and M 3 muscarinic receptors coexist, but the expression of M 2 receptors is much greater than that of the M 3 receptors (Wang et al., 1995;Goepel et al., 1998;Yamanishi et al., 2000;Kories et al., 2003). Nevertheless, the contractile response to the exogenous agonist carbachol and to endogenous agonists released by field stimulation have been attributed predominantly, if not exclusively, to M 3 receptors in rats (Longhurst et al., 1995;Hegde et al., 1997;Tong et al., 1997;Braverman et al., 1998;Choppin et al., 1998;Longhurst and Levendusky, 2000;Kories et al., 2003), mice (Choppin and Eglen, 2001b), pigs (Yamanishi et al., 2000), dogs (Choppin and Eglen, 2001a), and humans (Chess-Williams et al., 2001;Fetscher et al., 2002). Moreover, at least male M 3 (but not M 2 ) receptorknockout mice exhibit bladder distension and develop urinary retention (Matsui et al., 2000). On the other hand, it should be considered that hitherto available antagonists have only modest subtype selectivity and/or do not act in a purely competitive manner; hence, they were not well suited for detecting a pot...

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Signaling via G-protein-coupled receptors (GPCRs) is crucial to many physiological and pathophysiological processes in multicellular organisms, and GPCRs themselves are targets for important drugs. Classical cell supplementation experiments suggest a collision coupling model, in which receptors and G proteins diffuse randomly within the cell membrane and interact only if receptors are activated. This model is also backed by kinetic and live cell imaging data. According to the challenging theory, receptors and G proteins are precoupled--meaning they are forming stable complexes in the absence of agonist, which prevail during signaling. This model has been favored on the basis of copurification and coimmunoprecipitation of inactive receptors with G proteins and more recently by some approaches measuring energy transfer between labeled receptors and G proteins. This article reviews key findings regarding the receptor/G protein coupling mode, including most recent findings obtained by optical techniques.

1 We have compared the signalling mechanisms involved in the pertussis toxin-sensitive and -insensitive contraction of rat isolated mesenteric microvessels elicited by sphingosylphosphorylcholine (SPC) and noradrenaline (NA), respectively. 2 The phospholipase D inhibitor butan-1-ol (0.3%), the store-operated Ca 2+ channel inhibitor SK&F 96,365 (10 mM), the tyrosine kinase inhibitor genistein (10 mM), and the src inhibitor PP2 (10 mM) as well as the negative controls (0.3% butan-2-ol and 10 mM diadzein and PP3) had only little e ect against either agonist. 3 Inhibitors of phosphatidylinositol-3-kinase (wortmannin and LY 294,002, 10 mM each) or of mitogen-activated protein kinase kinase (PD 98,059 and U 126, 10 mM each) did not consistently attenuate NA-and SPC-induced contraction as compared to their vehicles or negative controls (LY 303,511 or U 124). 4 The phospholipase C inhibitor U 73,122 (10 mM) markedly inhibited the SPC-and NA-induced contraction (70% and 88% inhibition of the response to the highest NA and SPC concentration, respectively), whereas its negative control U 73,343 (10 mM) caused only less than 30% inhibition. 5 The rho-kinase inhibitors Y 27,632 (10 mM) and fasudil (30 mM) caused a rightward-shift of the NA concentration-response curve by 0.7 ± 0.8 log units and reduced the response to 10 mM SPC by 88% and 83%, respectively. 6 These data suggest that SPC and NA, while acting on di erent receptors coupling to di erent G-protein classes, elicit contraction of rat mesenteric microvessels by similar signalling pathways including phospholipase C and rho-kinase.