Negative chronotropic effect of catecholamines on adrenergic receptors in cardiac ganglia in the spinal dog

Abstract

The effects of catecholamines (CAs) on cardiac chronotropism were investigated in the spinal dog. The CAs were administered through the right subclavian artery (i.a.) to reach the cardiac sympathetic ganglia. Without preganglionic stimulation. CAs administered intra-arterially induced a slight negative chronotropic effect, which was reversed to a positive chronotropic effect after neostigmine (200 microgram, i.a.) in many cases. With preganglionic stimulation, intra-arterial injection of norepinephrine (0.5-25 microgram), epinephrine (0.1-10 microgram), or dopamine (0.1-500 microgram) caused dose-dependent bradycardia. The negative chronotropic effect of dopamine was significantly inhibited intra-arterial phentolamine (2 mg), dihydroergotamine (0.4 mg), apomorphine (0.5 mg), haloperidol (0.5 mg), or chlorpromazine (5 mg) but not by propranolol (0.1 mg) or bulbocapnine (1 mg), whereas the same effect of epinephrine was significantly reduced by alpha-blockade but not by propranolol or the dopamine antagonists. These results suggest that CAs exert a negative chronotropic action by inhibiting cardiac ganglionic transmission and that the receptors for dopamine are alpha-adrenergic and dopamine-specific and those for epinephrine are alpha-adrenergic specific.