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?

The heart of course does have Alpha 2 pre synaptic receptors , the definition states that alpha 2 receptors are present on sympathetic postganglionic endings , so I don't see why heart should be an exception . I will explain to you how reflex tachycardia occurs . When you give non selective blockers , presynaptic Alpha 2 are blocked , therefore the pre synaptic neurons cannot sense NE amount in synaptic cleft , so they release large amounts of NE . BUT , post synaptic alpha1 is also blocked , so this large amount of NE cannot act BUT THE HEART DOES NOT HAVE alpha 1 it has BETA 1 receptors which are perfectly capable of being stimulated by this large amount of NE that is released , hence causing tachycardia.


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