Equivalence ratio and constriction effects on RBCC thrust augmentation

Endogenously Expressed Muscarinic Receptors in HEK293 Cells Augment Up-regulation of Stably Expressed α4β2 Nicotinic Receptors

“…It appears, therefore, that activating muscarinic receptors alone does not significantly up-regulate nAChRs but rather requires a nicotinic agonist (e.g. nicotine), probably acting via a chaperone effect (29,33), to observe the muscarinic receptor augmentation of nAChR up-regulation. Thus, from these oxotremorine experiments, we speculate that the activation of muscarinic receptor stimulation increases nAChR subunits so that more are available for incorporation into the nAChR pentamer.…”

“…14, 16, and 55-57). It is possible that the higher concentrations of nicotinic ligands required to up-regulate these receptors compared with their binding affinity to mature receptors reflect, in part, an action at nascent receptors or even individual subunits, such as the microdomain on the ␤2 subunit that appears to be a target for a chaperone effect of nicotine that leads to incorporation of the subunit into the receptor and subsequent receptor maturation and up-regulation (29,30,33,58).…”

“…Previous studies demonstrated that nicotine-induced up-regulation of nAChRs is most likely triggered from the agonist binding site (24,29,41); in fact, this up-regulation apparently does not even require ion channel function per se because the nAChR channel blocker mecamylamine does not prevent nicotine-induced up-regulation in rat brain in vivo (24) or in cells in culture (29). Furthermore, as shown here, as well as previously (14,30,38,41), the competitive antagonist DH␤E also up-regulates these receptors.…”

“…Moreover, nicotine has also been found to promote nAChR subunit assembly by activating protein kinase C-dependent phosphorylation of immature ␣4 subunits (34). In addition to its action to promote subunit assembly, there is strong evidence that, in cell models heterologously expressing nAChRs, nicotine also induces up-regulation by decreasing the degradation rate of the receptor (26,29). Although both increased receptor assembly and decreased receptor degradation are well supported theories regarding the mechanism for nAChR up-regulation, neither has been confirmed in neurons or in vivo.…”

“…The up-regulation is not accompanied by an increase in steady-state levels of subunit mRNA, indicating that the increase in receptors is independent of transcription or mRNA stabilization (25)(26)(27)(28). Studies in cells that heterologously express nAChRs suggest that nicotine acts as a "pharmacological chaperone," enhancing the incorporation of ␤2 subunits into assembling receptors (29,30) and thereby increasing the number of ␣4␤2 nAChRs and possibly changing the stoichiometry from (␣4) 3 (␤2) 2 to (␣4) 2 (␤2) 3 (31,32). The ability of nicotine to chaperone ␤2 subunits into the nAChR assembly appears to be due, in part, to an action of nicotine binding at a microdomain located on the extracellular portion of the ␤2 subunit (33).…”