The model for studying the mechanism of G protein-mediated signalling cannot account for the observation that high-affinity binding of agonists to many different receptors is not dissociated by the addition of high concentrations of guanine nucleotides. Using the cerebral Ai-adenosine receptor as a model system, we have recently identified a component which is responsible for this phenomenon. This protein, termed the coupling cofactor, can be solubilized from brain membranes and chromatographically resolved from both the G proteins and the receptor. Following reconstitution into appropriate acceptor membranes, the coupling cofactor confers resistance of high-affinity agonist binding to guanine nucleotides. The coupling cofactor acts as a brake and limits receptor-dependent signal amplification; in addition, it is a candidate for participating in the higher level organization of receptors and G proteins in membranes and in the membrane-delimited cross-talk between individual receptors. Here, we present a working hypothesis on the possible biological roles of the coupling cofactor.
This study investigated quantitated expression of dopamine 2 receptor (D2R) and somatostatin receptors of the five types (SSTR1-SSTR5) in a large series of clinically non-functioning pituitary adenomas (CNFAs). Co-expression of these receptors in individual adenomas was studied as well as correlation between receptor types. Adenoma tissue from 198 patients who underwent surgery for CNFAs was analyzed by immunohistochemistry and quantitative real-time PCR. D2R and SSTR1-3 mRNA was expressed in all 198 adenomas. SSTR4 and SSTR5 were detectable in 85 % and 61 % of adenomas, respectively. Expression of D2R was significantly higher than that of the somatostatin receptors. The median relative expressions were as follows from highest D2R >> SSTR3 > SSTR2 > SSTR1 > SSTR5 > SSTR4. High relative expression (ratio to β-glucuronidase mRNA > 1) of D2R was found in 60 % of tumors, high expression of SSTR1 in 7.5 %, SSTR2 in 7 %, SSTR3 in 4 % and SSTR5 in 0.5 %. The quantity of D2R correlated positively with expression of SSTR2 and SSTR3, and negatively with SSTR1 and SSTR5. Among histological adenoma types, SSTR1 was significantly higher in null-cell adenomas and SSTR3 was lower in silent corticotroph adenomas. In conclusions, in CNFAs, high expression of somatostatin receptors is much less common than that of D2R, and co-expression of both these receptors is exceptional. D2R and SSTR3 seem to be the most promising targets for pharmacological treatment., F. Gabalec, M. Drastikova, T. Cesak, D. Netuka, V. Masopust, J. Machac, J. marek, J. Cap, M. Beranek., and Obsahuje bibliografii
The aim of the study was to determine the effect of selective dopaminergic agents [(±)-SKF-81297 (Di agonist), R(-)-2,10,ll-trihydroxy-N-propyl-noraporphine (D2 agonist), pergolid (D agonist), R( + )-SCH-23390 (Di antagonist), S(—)-eticlopride (D2 antagonist) and cis-(Z)-flupenthixol (D antagonist)] on cell-mediated immune response in vivo and in vitro and to verify the presence of dopamine receptors on murine splenocytes. The tested dopaminergic compounds exhibited a pronounced inhibitory effect on T-dependent immunity. They suppressed alloantigen-induced immune response in vivo and in vitro, IL-2 production was also markedly reduced. No substantial difference was found between the effect of dopamine agonists and antagonists or among ligands of subtypes of dopamine receptors. The effect of dopaminergic agents in vitro indicates a direct interaction with immunocompetent cells at the peripheral level. As the binding studies did not confirm the presence of dopamine receptors on splenocytes, the immunosuppressive efficacy of dopaminergic agents does not seem to be mediated via specific dopamine receptors.