High affinity quinuclidinyl benzilate binding to rat parotid membranes requires muscarinic receptor. G protein interactions

Thermal imaging of receptor-activated heat production in single cells

Changes in enthalpy (i.e., heat content) occur during the diverse intracellular chemical and biophysical interactions that take place in the life cycle of biological cells. Such changes have previously been measured for cell suspensions or cell-free biochemical extracts by using microcalorimetry, thermocouples, or pyroelectric films, all of which afford minimal spatial or temporal resolution. Here we present a novel thermal imaging method that combines both diffraction-limited spatial (approximately 300 nm) and sampling-rate-limited time resolution, using the temperature-dependent phosphorescence intensity of the rare earth chelate Eu-TTA (europium (III) thenoyltrifluoro-acetonate). With this thermosensitive dye, we imaged intracellular heat waves evoked in Chinese hamster ovary cells after activation of the metabotropic m1-muscarinic receptor. Fast application of acetylcholine onto the cells evoked a biphasic heat wave that was blocked by atropine, and after a brief delay was followed by a calcium wave. Atropine applied by itself produced a monophasic heat wave in the cells, suggesting that its interactions with the receptor activate some intracellular metabolic pathways. The thermal imaging technique introduced here should provide new insights into cellular functions by resolving the location, kinetics, and quantity of intracellular heat production.

A common low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

[3H]PGE2 and [3H]PGF2 alpha were shown to bind with similar binding capacity and dissociation constants to bovine aorta endothelial cells. The similarity in the binding parameters suggests that both agonists may bind to the same binding site. Displacement of [3H]PGE2 performed with PGE2, PGF2 alpha or U-46619, a thromboxane agonist, shows that all three prostanoids displaced the bound [3H]PGE2 with comparable potency (IC50 = 10(-7) M). These results indicated that the three different prostanoids, which serve as specific agonists to different prostanoid receptors, also compete for the same binding site in bovine endothelial cells with similar affinity. Comparison of the displacement of [3H]PGE2 or [3H]PGF2 alpha by a number of prostaglandin agonists and antagonists further supports the notion that the natural prostanoids bind with similar affinities to the same binding site. Thus, sulprostone, an EP1/EP3 agonist, displaced bound [3H]PGE2 and [3H]PGF2 alpha with IC50 of about 10(-7) M. On the other hand, thromboxane antagonists (BAY u-3405 and GR-32191B), EP1 specific antagonist (SC-19220) EP1/DP antagonist (AH-6809) and iloprost, a stable prostacyclin agonist, failed to displace bound [3H]PGE2 or [3H]PGF2 alpha at a concentration range of 10(-9)-10(-6) M. Gradual increase of sodium fluoride (NaF), a general activator of G binding proteins, or incubation of permeabilized cells with GTP gamma S resulted in a decrease in [3H]PGE2 binding, suggesting that the binding site represents a low-affinity common prostanoid receptor which, similar to other prostanoid receptors, is probably coupled with G binding proteins.

G-protein coupling of muscarinic receptors in adult and neonatal rat submandibular cells

The submandibular glands of neonatal and adult rats express muscarinic cholinergic receptors. Receptor occupancy initiates signaling through activation of phospholipase C, hydrolysis of inositol phospholipids, and calcium mobilization. The increased cytoplasmic [Ca2+] activates ion transport pathways, resulting in secretion of primary saliva. We have previously shown that muscarinic receptors are present in the gland of neonates and that they couple effectively to inositol trisphosphate production and Ca2+ mobilization but that the monovalent ion transport paths are poorly activated. To characterize age-related differences in signal transduction further, we examined the coupling of muscarinic receptors to G-proteins by determining the effect of GTP on the IC50 for competition by the muscarinic agonist carbachol with the radiolabeled antagonist, 3H-quinuclidinyl benzylate. Data were fit to one-site and two-site models, and in all cases the two-site model provided the better fit. Using the two-site model, a substantial GTP-induced shift from high affinity to low affinity binding was observed in membranes from adults, whereas more of the receptors were already in the low affinity form in the membranes from neonates, and little additional shift was induced by GTP. These results suggest differences in the G-protein coupling of muscarinic receptors in submandibular cells of adult and early postnatal rats that may be associated with differences in the content, affinity, or properties (i.e., posttranslational modifications) of G-proteins as the cells mature.