Heterogeneity among β-adrenoreceptor blockers in the modulation of energy-dependent uptake of the organic cation amantadine by rat renal tubules

Abstract

Eight representative beta-adrenoreceptor blocking drugs, acebutolol, atenolol, labetalol, metoprolol, nadolol, pindolol, propranolol, and timolol, were studied in vitro with respect to their potential to block energy-dependent uptake of [3H]amantadine into proximal and distal rat renal tubule fragments in the presence and absence of bicarbonate. Five of the eight beta-adrenoreceptor blockers showed a dose-dependent inhibition of renal tubule accumulation of amantadine: labetalol, metoprolol, pindolol, propranolol, and timolol. Labetalol was the only beta-adrenoreceptor blocker with greater inhibitory potency in phosphate-based buffer than in bicarbonate-based buffer. Propranolol was the most potent inhibitor of renal tubule amantadine accumulation with IC50 values of 15 +/- 10 and 31 +/- 11 microM for proximal and distal tubule fragments, respectively, in a bicarbonate-based buffer environment. Inhibition in a phosphate-based buffer was less potent only in proximal tubules, with an IC50 of 76 +/- 30 microM. Kinetic studies of propranolol inhibition of amantadine uptake were consistent with both uncompetitive and competitive inhibition mechanisms in bicarbonate-based buffer in both proximal and distal renal tubule segments. There was no chiral preference between the R and S forms of propranolol for the inhibitory effects observed. These data suggest that there is potential for selection among the beta-adrenoreceptor blocking drugs to minimize or restrict the inhibition of amantadine energy-dependent uptake at the organic cation ion uptake sites characterized by amantadine in the presence and absence of bicarbonate.