Abstract

The aims of the study were to characterize the rate and extent of the rebound effect after abrupt cessation of a chronic exposure ofl-propranolol in spontaneous hypertensive rats, using exercise-induced tachycardia as a pharmacodynamic endpoint. Thirty-two spontaneous hypertensive rats were randomized to receive either placebo or 4 or 8 mg/kg/day s.c. infusion of l-propranolol for 11 days using osmotic minipumps. The heart rate was measured after standardized physical exercise before and during drug exposure and over 12 days after cessation, using a computerized tail-cuff method. Blood samples were collected after each effect measurement during the infusion. A similar reduction in exercise tachycardia was registered for the two doses. No apparent tolerance development was found, but both doses showed a clear rebound effect of similar extent and intensity. The maximal rebound effect was observed on the second day after cessation and was found to have a duration of about 6 days. A mechanism-based model was developed to describe the rate and extent of changes in β-adrenoceptor up- and down-regulation with increased sensitivity of the transducer complex. The half-life of disappearance of up-regulated β-adrenoceptors was estimated to be 2.0 days (1.0–3.9 days). The effect-versus-time data was analyzed by nonlinear mixed-effect modeling with the program NONMEM. A dose-dependent reduction in the growth of body weight was observed during drug treatment, which was reversible. A dose- and time-dependent increase in the α₁-acid glycoprotein concentration was also observed.