The beta-adrenergic receptor kinase: role in homologous desensitization in S49 lymphoma cells

Angiotensin II Type 1 Receptor Tachyphylaxis Is Defined by Agonist Residence Time

Several GPCRs (G-protein-coupled receptors) have been reported to exhibit tachyphylaxis, which is an acute loss of functional receptor response after repeated stimuli with an agonist. GPCRs are important clinical targets for a wide range of disorders. Therefore, elucidation of the ligand features that contribute to receptor tachyphylaxis and signaling events underlying this phenomenon is important for drug discovery and development. In this study, we examined the role of ligand-binding kinetics in the tachyphylaxis of AT₁R (angiotensin II type 1 receptor) using bioluminescence resonance energy transfer assays to monitor signaling events under both kinetic and equilibrium conditions. We investigated AT₁R signal transduction and translocation promoted by the endogenous tachyphylactic agonist Ang II (angiotensin II) and its analogs, described previously for inducing reduced receptor tachyphylaxis. Estimation of binding kinetic parameters of the ligands revealed that the residence time of Ang II was higher than that of the analogs, resulting in more sustained G_q protein activation and recruitment of β-arrestin than that promoted by the analogs. Furthermore, we observed that Ang II led to more sustained internalization of the receptor, thereby retarding its recycling to the plasma membrane and preventing further receptor responses. These results show that the apparent lack of tachyphylaxis in the studied analogs resulted from their short residence time at the AT₁R. In addition, our data highlight the relevance of complete characterization of novel GPCR drug candidates, taking into account their receptor binding kinetics as well.

Decreased dromotropic response to verapamil despite pronounced increased drug concentration in rheumatoid arthritis

AIMS

Inflammation reduces hepatic clearance of many drugs with unknown therapeutic consequences. This study was carried out to examine the effect of rheumatoid arthritis (RA) on the pharmacokinetics and pharmacodynamics of verapamil.

METHODS

Eight RA patients were age- and sex-matched with eight healthy volunteers. The disease severity was assessed, and ECG, blood pressure and verapamil enantiomers concentrations were measured for 12 h post 80 mg oral verapamil. Serum interleukin-6 (IL-6) and nitrite (NO2-) were measured in predose samples.

RESULTS

IL-6 and NO2- concentrations were significantly increased in parallel with disease severity. Oral clearance of both S- and R-verapamil was significantly decreased by RA. While the unbound fraction of S- and R-verapamil decreased by 5 and 7-fold, respectively, the unbound AUC remained unchanged for the more potent enantiomer, S-verapamil. AUC of norverapamil enantiomers was increased 2-3-fold. Despite elevated serum drug concentrations in RA, the potential to prolong the PR-interval was significantly reduced by one fold and the effect on the heart rate and blood pressure did not increase.

CONCLUSIONS

RA results in increased verapamil concentrations due likely to changes in protein binding, decreased clearance and/or altered hepatic blood flow. A significant decrease in dromotropic effect, despite increased serum drug concentrations, may be attributed to receptor down regulation caused by pro-inflammatory cytokines and/or NO.