Stereoselective binding in cardiac tissue of the enatiomers of benzetimide, and antimuscarinic drug.

1 Benzetimide, possessing two stable enantiomers, dexetimide and levetimide, has been investigated in guinea-pig atria with respect to its atropine-like action and its tissue distribution. 2 The antagonistic potency of dexetimide was found to be over 6000 times higher than that of levetimide, the pA2 values being 9.82 and 6.0 respectively. 3 The tissue accumulation was investigated for both isomers in the concentration range from 1.5 X 10(-9) M to 10(-6) M yielding tissue to medium ratios (T/M) of between approximately 50 and 10. The highest values were found for the lowest concentrations. At any concentration investigated, dexetimide exhibited a higher uptake than the levoisomer. 4 The rate of uptake and washout of dexetimide was extremely slow, that of levetimide being considerably faster at equimolar concentrations. The same pattern held true for the onset and decline of the antagonistic action. 5 The high accumulation was found to be almost entirely due to unspecific binding. Even in the case of dexetimide the relative size of the receptor compartment could not be determined. The unspecific binding sites displayed a certain stereoselectivity but to a much lesser extent than the specific receptor binding sites.