obiwan

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FA says that you use non-selective alpha antagonists to treat pheochromocytomas... wouldn't that remove the normal inhibition of catecholamine release that occurs when NE/epi bind to alpha 2 receptors which i would assume exacerbates the excess catecholamines floating around from the tumor itself...
 

lord_jeebus

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I would assume that the pheo is not being inhibited by normal mechanisms, is secreting catecholamines far in excess of baseline levels anyway, and that small amount of increase in "normal" catecholamines you're worried about will also be blocked at alpha receptors.
 

ASDIC

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In Pheo, the catechols are resistant to the actions of alpha 2 adrenergic receptors. This forms the basis of the clonidine supression test. The goal to treatment is to control blood pressure via phentolamine or phenoxybenzamine which are non-selective alpha adrenergic antagonists. To completely cure the condition, you need surgery.
 

mercaptovizadeh

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FA says that you use non-selective alpha antagonists to treat pheochromocytomas... wouldn't that remove the normal inhibition of catecholamine release that occurs when NE/epi bind to alpha 2 receptors which i would assume exacerbates the excess catecholamines floating around from the tumor itself...
alpha2s are not responsible solely for presynaptic inhibition. They are also located post-synaptically on blood vessels and the GI tract and can cause vasoconstriction and venoconstriction, and also inhibit GI motility. By non-selectively blocking out both alpha1s and alpha2s, you're removing essentially all vasoconstrictive adrenergic inputs into the systemic vasculature. The loss of the presynaptic inhibition is irrelevant since the system is overloaded with (nor)epinephrine anyway, and anyway, I suspect that there is also probably a downregulation of inhibitory alpha2s by the presynaptic terminal in response to the high (nor)epinephrine.