Are there pre-synaptic alpha 2 receptors in the post-ganglionic synapses of the heart?
I haven't found any sources that clearly state whether they do or not. I'm also not sure if they exist in most post-ganglionic synapses, or if they do in just specific ones. The reason I ask is wanting to understand the reflex tachycardia non-selective alpha blockers like phenoxybenzamine cause.
One of the side effects of phenoxybenzamine is reflex tachycardia, caused by, from my understanding, partly a reflex to the initially decreased blood pressure and due to the blocking of alpha 2 receptors, increasing nor-adrenaline release. Does this occur by :
- Blocking alpha 2 receptors at other sites, causing increased blood levels of nor-adrenaline, which then cause increased heart rate.
- Blocking alpha 2 receptors in the heart, increasing local increase of nor-adrenaline?