Pharmacokinetics and the Cardiovascular Effects of Irbesartan in Aortic Coarctated Rats

Is urethane-chloralose anaesthesia appropriate for pharmacokinetic-pharmacodynamic assessment? Studies with carvedilol

INTRODUCTION

The aim of the work was to establish the impact of urethane-chloralose anaesthesia on pharmacokinetic-pharmacodynamic (PK-PD) properties of carvedilol in control rats and L-NAME hypertensive animals.

METHODS

Male Wistar Rats were randomly divided into: control (n=12) with tap water to drink and L-NAME rats (n=12) with L-NAME solution (40 mg/kg/day) to drink for 2 weeks. Effects of carvedilol (1 mg kg(-1), i.v.) on blood pressure and heart rate were recorded during 3 h in conscious and urethane (500 mg kg(-1), i.p.) - chloralose (50 mg kg(-1), i.p.) anaesthetized rats. Carvedilol plasma pharmacokinetics was studied by means of traditional blood sampling. PK-PD modeling of carvedilol was made by means of an effect compartment model.

RESULTS

Neither urethane-chloralose nor L-NAME modified estimation of pharmacokinetic parameters of carvedilol. Although urethane-chloralose did not modify potency of carvedilol comparing with awake animals in control and hypertensive group, maximal negative chronotropic response was significantly greater in anaesthetized L-NAME rats in comparison to awake animals. Conversely, anaesthesia did not modify maximal chronotropic response to carvedilol in control rats. Whilst no differences were found in the estimated potency of carvedilol hypotensive response comparing control and L-NAME rats in both awake and anaesthetized conditions, maximal hypotensive effect of carvedilol was significantly greater in anaesthetized control and L-NAME animals in comparison to conscious rats. L-NAME rats showed a greater maximal hypotensive response comparing to control group.

DISCUSSION

Urethane-chloralose anaesthesia is an acceptable experimental condition for the evaluation of PK-PD properties of carvedilol, considering that it does not affect the potency of carvedilol for its chronotropic and hypotensive effect. Conclusions obtained from urethane-chloralose anaesthetized animals, regarding the impact of l-NAME treatment on PK-PD properties of carvedilol, did not differ from those obtained from conscious animals. Anaesthesia did not modify pharmacokinetic behaviour of carvedilol in both normotensive and L-NAME hypertensive rats.

Pharmacokinetic-pharmacodynamic (PK-PD) modeling of cardiovascular effects of metoprolol in spontaneously hypertensive rats: a microdialysis study

The present work addressed possible alterations in the pharmacokinetics and the in vivo pharmacodynamic of metoprolol (MET) in spontaneously hypertensive (SH) rats and Wistar Kyoto (WKY) animals by means of the microdialysis technique. The correlation between MET unbound plasma concentrations and its pharmacological effects, such as heart rate and blood pressure change, was also examined in SH and WKY rats by the application of a PK-PD model. MET dialysate concentrations and its chronotropic and blood pressure effect were determined during 3 h after the administration of 3 and 10 mg.kg(-1) of the drug. A PK-PD model with a separate effect compartment was used to analyse the data. A good correlation between plasma MET concentrations and its hypotensive and chronotropic effect was found in all experimental groups. Although a greater maximal effect (E(max)) for the antihypertensive effect of MET was observed in SH rats (WKY: E(max): -17+/-1 mmHg; SH: E(max): -28+/-4 mmHg; P<0.05 versus WKY rats), no differences were found in the concentration yielding half-maximal response (IC(50)) comparing SH (IC(50): 583+/-146 ng x ml(-1)) and WKY animals (IC(50): 639+/-187 ng x ml(-1)). The bradycardic effect of MET was greater in SH rats (E(max): -29+/-1%, P<0.05 versus WKY rats) than in WK animals (E(max): -22+/-2%), but no differences were observed in the IC(50) comparing both experimental groups (WKY: IC(50): 187+/-53 ng x ml(-1); SH: IC(50): 216+/-62 ng x ml(-1)). Pharmacokinetic analysis shows that the volume of distribution of MET was greater in SH rats (Vd: 3.4+/-0.5 l, P<0.05 versus WKY rats) with regard to Wistar Kyoto (WKY) animals (Vd: 1.9+/-0.2 l). The results suggest that the pharmacokinetic behaviour of metoprolol are modified in SH rats, resulting in an increased volume of distribution. A greater maximal efficacy to the hypotensive effect of metoprolol was observed in SH rats, suggesting participation of beta-adrenoceptors in the maintenance of the hypertension. Also, a greater chronotropic response to metoprolol was found in the hypertensive group compared with WKY animals, suggesting that, at least in part, the greater cardiac effect of metoprolol explained the enhanced hypotensive response of the beta blocker in the SH animals.

Hypothalamic Antihypertensive Effect of Irbesartan in Chronic Aortic Coarctated Rats

The aim of the present work was to study the central and plasma pharmacokinetics of irbesartan (IRB) and its possible hypothalamic antihypertensive effect in sham-operated (SO) and aortic-coarctated (ACo) rats at a chronic hypertensive stage using the microdialysis technique. Anesthetized Wistar rats were used 42 days after ACo or SO. For the study of plasma pharmacokinetics, a vascular shunt probe was inserted into the carotid artery. In a separated experiment, a concentric probe was placed into the anterior hypothalamus for the study of IRB distribution in the central nervous system. Based on the hypothalamic concentrations of IRB reached in ACo rats, the anterior hypothalamus of SO and ACo animals was perfused with a Ringer solution containing approximately 6 microg x ml(-1) of the drug. IRB (10 mg x kg(-1) i.v.) induced a late decrease of heart rate (HR) in ACo animals (DeltaHR: -42 +/- 10 bpm, n = 5, p < 0.05 vs. SO rats) but not in SO rats (DeltaHR: 11 +/- 13 bpm, n = 5). Systemic administration of the drug reduced the mean arterial pressure (MAP) of both experimental groups, but the hypotensive effect was greater in ACo (DeltaMAP: -39.9 +/- 5.0 mm Hg, n = 5, p < 0.05 vs. SO rats) than in SO rats (DeltaMAP: -25.4 +/- 2.1 mm Hg, n = 5). A similar pharmacokinetic profile was observed in both experimental groups. Hypothalamic distribution of IRB was greater in ACo (AUC: 730 +/- 130 ng x ml(-1) h(-1), n = 5, p < 0.05 vs. SO rats) than in SO animals (AUC: 283 +/- 87 ng x ml(-1) h(-1), n = 5). The IRB hypothalamic perfusion induced an antihypertensive effect in ACo (DeltaMAP: -15.1 +/- 1.0 mm Hg, n = 5, p < 0.05 vs. Ringer perfusion) but not in SO rats. In conclusion, the chronic aortic coarctation did not modify the plasma pharmacokinetics of IRB, but it increased the distribution of the drug in the central nervous system. The greater hypotensive effect of IRB observed in ACo animals suggests the involvement of AT1 receptors in the maintenance of the hypertensive stage in chronic ACo rats. The hypotensive effect of IRB in ACo animals could be explained, at least in part, due an action on the anterior hypothalamic angiotensin system.