Comparative effects of beta-adrenoceptor partial agonists on isolated rat atrium

The chronotropic effect of three beta-1-adrenoceptor partial agonists prenalterol, xamoterol and epanolol has been compared on the right atria of the rat in order to evaluate their intrinsic activity and to place them in rank order of effectiveness. The results show that prenalterol, xamoterol and epanolol are all partial agonists. The intrinsic activities relative to that of isoprenaline are 0.84 for prenalterol, 0.59 for xamoterol and 0.29 for epanolol. This rank order of intrinsic activities should remain the same in different species and in man. Both atenolol and propranolol reversed the chronotropic effects of the three agonists. The KB of the two blockers was similar against prenalterol and xamoterol, which indicates that the two partial agonists are probably competing for the same population of receptors. The EC50 is twice as large than KB for xamoterol, which is consistent with isoprenaline working through both beta-1- and beta-2- receptors and xamoterol finds it more difficult to block the beta-2-receptors.