Cocaine, phenoxybenzamine (Dibenzyline) and, less effectively, phentolamine (Regitine) block the chronotropic response (beta-receptor) of tyramine on the isolated guinea-pig’s atria. Phenoxybenzamine induces a persistant blockade, the others a transitory one as they can readily be removed by washing the preparation. Cocaine is effective in protecting against the persistent phenoxybenzamine blockade while phentolamine is not.