Tulobuterol: a full beta-adrenoceptor agonist or a partial beta-agonist with membrane stabilizing activity?

1. The effects of tulobuterol on the atrial rate was compared with those of isoproterenol, pindolol and propranolol in spontaneously beating isolated right atria of the rat. 2. Isoproterenol markedly increased atrial rate. To the contrary, propranolol, pindolol and tulobuterol decreased atrial frequency. The chronotropic effect of tulobuterol was similar to that of pindolol but lower than that of propranolol. 3. Interactions between tulobuterol and isoproterenol showed that tulobuterol potentiated the chronotropic effect of low concentrations of isoproterenol but reduced the positive chronotropic response to higher concentrations of isoproterenol. 4. These results suggest that tulobuterol is a partial agonist on atrial beta-receptor but also seems to have another action (possibly a membrane stabilizing activity) which reduced the originally induced effect on beta-receptors as a non-competitive antagonist.