Propranolol is a synthetic β-adrenergic receptor-blocking drug. After oral administration, propranolol has complete and rapid gastrointestinal absorption. The bioavailability of pro-pranolol is approximately 25%. Propranolol may be a substrate for a distributed P-glycoprotein (P-gp), which might lead to the efflux the drug back to the lumen of the intes-tine. The aim of this study was to investigate the influence of Diltiazem as P-gp inhibitor at different concentrations on intestinal absorption of Propranolol, namely on absorption rate constant (Ka). Single Pass Intestinal Perfusion (SPIP) technique in rats was conducted on three groups of Wistar albino rats (n=6 per group). The first group was perfused with Propranolol HCl (75 μg/mL) alone, meanwhile the second and the third groups were perfused with Pro-pranolol HCl (75 μg/mL) in presence of Diltiazem HCl (250 and 1000 μg/mL) respectively. The whole small intestinal segment of anaesthetized rats was cannulated and perfused with Propranolol HCl in normal saline at 37 °C in the absence and the presence of Diltiazem HCl. Intestinal samples were taken from outlet tubing of small intestine at different time intervals and analyzed using a simple, rapid and validated spectrophotometric method. The Ka values of the drug for the rats in the three groups were calculated. The mean Ka values of Proprano-lol HCl in the first group was 0.81±0.014 hr-1, meanwhile, Ka values of Propranolol HCl in the second and third groups (250 μg/mL and 1000 μg/mL) were 0.778±0.012 hr-1 and 0.857±0.030 hr-1 respectively. Statistically, insignificant differences were found when the three groups were compared with (P>0.05). The expected explanation for the lack effect of Diltiazem as P-gp inhibitor at both concentrations on the absorption constant rate of Propran-olol is that P-gp plays a minimal role in the in situ intestinal absorption process of Propranolol HCl.

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