Modelling of α₁‐adrenoceptor‐mediated temporal dynamics of inotropic response in rat heart to assess ligand binding and signal transduction parameters

Abstract

Background and purpose:In order to use the transient response to an antagonist (prazosin) to evaluate properties of agonist interactions with the a1-adrenoceptor system, an integrative mechanistic model of cardiac uptake of prazosin and its competitive interaction with phenylephrine at the receptor site was developed. Based on the operational model of agonism, the aim was to evaluate both the receptor binding and signal transduction process as determinants of the inotropic effect of phenylephrine. Experimental approach: In Langendorff-perfused rat hearts, prazosin outflow concentration and left ventricular developed pressure were measured, first in the presence of 12.3 mmol·L -1 phenylephrine following a 1 min infusion of 1.27 nmol [ ) reduced the receptor occupation by the agonist and, consequently, contractility. This competitive binding process appeared to be the rate-determining step in response generation. The relationship between receptor occupancy and inotropic response was described by an efficacy parameter (t, ratio of receptor density and coupling efficiency) of 4.9. Conclusions and implications: Mechanistic pharmacodynamic modelling of the kinetics of antagonism by prazosin allows quantitative assessment of the a1-adrenoceptor system both at the receptor and post-receptor levels.

Effect of disease states on α1-adrenoceptor binding and signal transduction parameters in isolated perfused heart: quantification by pharmacokinetic-pharmacodynamic modelling

“…Table presents a list of all parameter estimates with interindividual variability. Because our parameter estimates in normal hearts are not much different from those reported earlier, we will restrict the description to those parameters that showed a significant change under hypoxia or endotoxemia. Both hypoxia and sepsis only led to changes in parameters of signal transduction process (Eq.…”

“…Hearts excised from male Wistar rats (250–300 g) were perfused by the nonrecirculating Langendorff technique as previously described . Briefly, the chest was opened, an aortic cannula filled with perfusate was rapidly inserted into the aorta to prevent ischaemia, and retrograde perfusion was started with an oxygenated perfusate.…”

“…When the steady‐state inotropic response in the presence of phenylephrine was attained after 15 min, a 1.27 nmol dose of [ 3 H]‐PRZ was administered as 1‐min infusion. These concentrations (doses) were selected in the previous study . Outflow samples were collected every 5 s for 1.5 min, every 10 s for the next 1.5 min, every 0.5 min for the next 6 min and every minute for the next 6 min (total collection period, 15 min), and the cardiac response was measured.…”

“…Figure illustrates the PK/PD model of the competitive interaction between the α 1 AR‐specific antagonist PRZ (B) and the agonist PE (A) in the perfused rat heart . Model analysis is based on measurement of PRZ outflow concentration and inotropic response to PE after a single dose of PRZ in the presence of PE.…”

“…The importance of such drug‐independent parameters of agonism, as first defined in the context of the Black/Leff operational model, has been stressed only recently . In a previous paper, we have demonstrated how by PK/PD modelling of the competitive interaction between prazosin and phenylephrine in the perfused rat heart, it is possible to estimate α 1 ‐adrenoceptor (α 1 AR) affinity and density, as well as the parameters of the postreceptor transduction mechanism . Here, we apply this approach to assess the effects of experimental disease states in rats namely endotoxemia and hypoxia.…”