Liberation of endogenous compounds by tolazoline.

The anomalous cardiac stimulation by tolazoline (2-benzyl-2-imidazoline), and alpha-adrenergic blocking agent, appears to result from the release of histamine and catecholamines. These responses can be blocked by metiamide and propranolol, respectively. It is probably that tolazoline also releases acetylcholine from the guinea pig atria. The direct effect of tolazoline on chronotropic activity of atrial pairs after blockade appears to be depressant at high concentrations, and this decrease in rate, below control levels, is not blocked by atropine.