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Beta 2 adrenergic and renin


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Hi there,

 

I am a medical student and with my general physiology exam approching, we wondered something with friends! We looked everywhere online but couldn't find the answer! The professor probably won't bother us with that but we still wanna know.

 

So adrenergic beta two receptors increase cAMP production when stimulated, inducing a relaxation of smooth muscle cells (via MLCP activation and all that), correct? This will therefore dilate blood vessels.

 

Adrenergic beta one receptors in kidneys will stimulate renin production, which will activate Angiotensin I then II, producing a vasoconstriction.

 

How can two adrenergic receptors have opposite effects? Am I mistaking somewhere or is it just a question of location of these vessels? But as hormones, Ang I and II should act evrywhere as transported by blood...

 

Imagine epinephrine is released by sympathic nervous system in case of stress, it will activate both receptors. The response from the bronchi and the blood vessels seems logical, but renin goes against all that? Constricting blood vessels will not be beneficient...

 

Sorry for my english mistakes, english isn't my native language so it's not always easy ;)

 

Thanks for your answers!

 

Aurelien

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The reason why the responses are different is that the response of each receptor to the same stimulus is different. Essentially we have a second messenger system in which a receptor recognizes its target molecule and then starts a signaling cascade. Howe the cascade looks like is very dependent on the receptor complexes and can have antagonistic actions, as described (i.e. beta type incrasing cAMP, alpha 2 inhibiting cAMP production).

 

However, the overall physiological response (i.e. whether you see vasodilation or constriction) depends very much on the configuration of the whole system, such as the amount of alpha vs beta receptors. I am a bit rusty on that, but I believe that beta receptors are more sensitive (i.e. react to lower concentrations of epinephrine, whereas alphas are less sensitive but more abundant.

So based on that (and neglecting other mechanisms), at lower concentration the beta response would be dominant (as they sense at levels at which alphas fail to respond), whereas at higher concentrations, the response of the alphas would override that of the much less abundant betas.

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