Evidence for postsynaptic alpha 1- and alpha 2-adrenoceptors mediating catecholamine-induced negative chronotropic ventricular responses in the conscious dog.

1. Adrenaline (0.25-1 microgram/kg), noradrenaline (0.125-0.5 microgram/kg) and dopamine (25-100 micrograms/kg) given in the conscious dog with chronic atrio-ventricular block after beta-adrenoceptor blockade, increased ventricular cycle length (VCL) and mean blood pressure (MBP). 2. Atropine (muscarinic receptor blocker) reduced the catecholamine-induced effects on VCL without modifying their hypertensive effects. 3. Phenoxybenzamine or phentolamine (alpha-adrenoceptor blockers) only decreased the effects of adrenaline on VCL but suppressed those of noradrenaline and dopamine. They only reduced the effects of adrenaline and noradrenaline on MBP, but reversed that of dopamine. 4. Yohimbine (alpha-adrenoceptor blocker) suppressed the catecholamine-induced effects on VCL, and reduced strongly the hypertensive effects of adrenaline and noradrenaline and reversed that of dopamine. 5. Thus, these results show the existence of negative chronotropic postsynaptic alpha-adrenoceptors in the ventricles.