Does calcium increase or decrease muscle contractility?

[cameraGEEK]

LOW calcium causes tetany and seizures because it lowers the threshold for the action potential (skeletal muscle). High calcium causes constipation because lower GI muscle contractions.

However, I have also read that HIGH calcium stimulates vasoconstriction (presumably due to greater contraction of the smooth muscle around the vessel).

So, in the first situation, high calcium causes less contraction, but in the second situation it seems calcium causes greater contraction? Can anyone help me reason this out, or point out where my logic is wrong?

Thanks!

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[Chirurg]

LOW calcium causes tetany and seizures because it lowers the threshold for the action potential (skeletal muscle). High calcium causes constipation because lower GI muscle contractions.

However, I have also read that HIGH calcium stimulates vasoconstriction (presumably due to greater contraction of the smooth muscle around the vessel).

So, in the first situation, high calcium causes less contraction, but in the second situation it seems calcium causes greater contraction? Can anyone help me reason this out, or point out where my logic is wrong?

Thanks!

from what I understand, in your first example the effects are due to calcium interaction with the nervous system (low calcium stimulatory and high calcium inhibitory), so indirect effect on the muscles, and in the second situation calcium interacts directly with the smooth muscles, some of which are sensitive to extracellular calcium.
admittedly you could ask why the smooth muscle in the GI is not stimulated, the GI smooth muscle is also highly innervated, not always the case with smooth muscles in the vasculature.

 

[cameraGEEK]

from what I understand, in your first example the effects are due to calcium interaction with the nervous system (low calcium stimulatory and high calcium inhibitory), so indirect effect on the muscles, and in the second situation calcium interacts directly with the smooth muscles, some of which are sensitive to extracellular calcium.
admittedly you could ask why the smooth muscle in the GI is not stimulated, the GI smooth muscle is also highly innervated, not always the case with smooth muscles in the vasculature.

Thanks Chirurg, any other thoughts out there? Does it have anything to do with a stimulatory effect calcium can have on the adernergic system?

 

[Ty Webb]

Calcium is thought to inhibit sodium channels involved in AP conduction. High levels of calcium will therefore provide more of a inhibitory effect on sodium channels, effectively raising the threshold potential (i.e. more of a stimulus is necessary for the AP to propagate due to the Na channels responsible for said propagation being in a more inhibited state). The converse is true when calcium is low--sodium channels are less inhibited than normal, the threshold potential is lower, and tetanic muscle contractions are more prone to occur.