Sympathetics and Vascular function curve

[Enzymes]

I was reviewing some cardiac phys, and I am having trouble with how sympathetics increase venous volume / tone so as to increase the mean systemic pressure. This is seen on p. 269 of FA 2014.

I was under the impression that the main effect of sympathetics would be to decrease total peripheral resistance (dilating arterioles), which would keep the mean systemic pressure the same (as in the example of exercise).

Can anyone explain how sympathetics affect the vascular function curve? Thanks.

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[Handinhand]

Your alpha-1 receptors are Gq receptors, so when they are activated it causes an increase in IP3 within your smooth muscle cells of your arteries, which in turn releases Ca++ from the sarcoplasmic reticulum, leading to smooth muscle contraction.

Also, and I don't mean to sound harsh, but you should really sit down with a physiology book and go over this stuff in fine detail, because thinking sympathetic activity causes vasodilation makes me think you have some large gaps in your knowledge of this subject.

 

[reverieinvictus]

OP might be confusing skeletal muscle autoregulation with adenosine and lactate during exercise causing an overall decrease in systemic vascular resistance/TPR with the sympathetics.

So during exercise, your sympathetics kick in causing an increase in HR, CO, contractility, VR and vasoconstriction in organs like the kidney and GI system. However, your skeletal muscles have increased metabolism which increases things like adenosine and lactate leading to vasodilation which basically causes an overall decrease in TPR.

 

[Handinhand]

Right, but that's not the result of of any sympathetic activity -- local metabolic regulation of tissue perfusion is far stronger than your sympathetic nervous system and wins out in the scenario. Unless you give someone Prazosin though, sympathetic activity will constrict your vasculature.

 

[Enzymes]

Thanks for the responses. I know that alpha 1 constricts, but I was actually thinking of beta 2 receptors on the vasculature leading to vasodilation (also mediated by sympathetics).

EDIT: so sympathetics are just directly increasing the tone of veins?

 

[Handinhand]

Alpha-1 receptors are usually going to "win out" over Beta-2 receptors on arteries, especially with endogenous stimulation from epinephrine and norepinephrine

The smooth muscle relaxation that takes place with Beta-2 stimulation is very important in the lungs however, and beta-2 is extremely important in many metabolic processes.