MCAT Bio assessment #114 PTH hormone

[LuminousTruth]

How was the location of Ca+2 most likely changed by the addition of PTH?

A) It moved into osteoclast cytoplasm from extracellular medium
B) It moved into the osteoclast cytoplasm from intracellularr membranes and organelles
C) It moved out of the osteoclast cytoplasm and into the extracellular medium
D) It moved out of the membranes and organelles of the osteoclasts and into the extracellular medium.

The correct answer is "B"

I know the PTH increases blood calcium concentration. So I thought the Ca+2 is going to leave the cell and move into the extra-cellular medium. The most direct answer I see would be D.

The passage did state that the PTH increases intracellular calcium (from the release of calcium in the organelles) but wouldn't that be contradictory to common knowledge that PTH lowers intracellular calcium and increases blood calcium concentration?

Can anyone explain the reasoning?

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[richkaj]

I see your dilemma, PTH increases and decreases calcium concentrations which is counterproductive. I have a question. Would PTH decrease calcium concentrations in bone vs. increasing calcium concentrations in osteoclasts. I was just thinking if osteoclasts are breaking bone down wouldn't their intracellular concentration of calcium increase and the bone calcium concentration decrease? I do not have the actual question/passage in front of me... Just asking questions...

 

[manohman]

Bumping this to clarify.

When PTH increases the blood calcium, it do so by eating away at the bone, and released calcium int othe blood stream.

By the answer of this question, since intracellular calcium increased even in the absence of extracelular calcium, does that mean that when PTH is applied, osteoclasts can/will also release their own internal calcium stores into their cytoplasm and then into the blood stream? I see no other point for PTH to increase intracellular calcium stores by using their own calcium from their organelles if not to release it into the blood stream.

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Also, that Intracellular concentrations of Ca2+ increased even when there was no extracellular calcium present doesnt necessarily mean that that is the only and most lkely mechanism by which intracellular calcium increases, does it?

The logic is that intracellular calcium increased both when there was extracellular calcium present and when there wasnt. Isn't it a stretch to assume that extracellular calcium does not get imported?

 

[amy_k]

Osteoclasts take calcium from the bone matrix and release it into the bloodstream. Addition of PTH will stimulate this process so Ca2+ concentration will decrease in the bone and increase in the bloodstream. Does the passage state that there is an intracellular increase in Ca2+ for the osteoclasts specifically or just a general intracellular increase? Organelles such as mitochondria and endoplasmic reticulum also store calcium and PTH stimulates the release of calcium from these organelles as well, causing an increase in intracellular Ca2+, without reading the passage, I believe it is referring to this. PTH addition will cause increase/decrease in Ca2+ depending on whether we are talking about bone matrix, cells, or bloodstream.

 

[manohman]

Osteoclasts take calcium from the bone matrix and release it into the bloodstream. Addition of PTH will stimulate this process so Ca2+ concentration will decrease in the bone and increase in the bloodstream. Does the passage state that there is an intracellular increase in Ca2+ for the osteoclasts specifically or just a general intracellular increase? Organelles such as mitochondria and endoplasmic reticulum also store calcium and PTH stimulates the release of calcium from these organelles as well, causing an increase in intracellular Ca2+, without reading the passage, I believe it is referring to this. PTH addition will cause increase/decrease in Ca2+ depending on whether we are talking about bone matrix, cells, or bloodstream.

Yeah in the experiment, they showed that calcium levels in the cell cytoplasm increased even when there was no calcium in the extracellular medium.

however, there were other trials as well where calcium was present in the extracellualr medium. (they had like 3 trials, from no calcium in extracellualr medium to X amount of calcium in teh medium) and calcium was found in the cytoplasm even when there was no calcium in teh extracellualr medium.

So it was indicated as a possibility, but in choosing the best answer, i thought to choose

"Calcium was taken up from the extracellular medium into the cyotplasm" becaue its not like calcium does not go into the cell from the extracellular medium, its both from the medium and organelles.

or is it my interpretation of the experiment itself? in interpreting the results, id think youd have to do another experiment to see whether the calcium came from both or just the organelles. See the dilemna?

 

[amy_k]

So there is an experiment passage linked to this question and calcium levels in the cell cytoplasm increased even when there is no calcium outside the cell...right? Basically as I said PTH causes cell organelles that store calcium to dump it out into the cell cytoplasm, so the Ca2+ in the cytoplasm is coming from within the cell organelles not from outside the cell.

 

[manohman]

So there is an experiment passage linked to this question and calcium levels in the cell cytoplasm increased even when there is no calcium outside the cell...right? Basically as I said PTH causes cell organelles that store calcium to dump it out into the cell cytoplasm, so the Ca2+ in the cytoplasm is coming from within the cell organelles not from outside the cell.

But intracellular calcium also increased when there was extracellular calcium present as well. And we know osteoclasts take up calcium (acutally i'v only learned that they break it down, not sure about taking it up).

It could just be that the cell does both, takes up extracellualr calcium and intracellular calcium into its cytoplasm couldnt it? And wouldnt that make more sense given the effect of PTH? I guess im trying to understand the logic haha or my lack thereof

 

[amy_k]

I might need to see this passage to understand what you are saying about extracellular versus intracellular.

Osteoclasts break down bone using acid, which causes release of calcium and other minerals from the bone. The calcium is then released into the bloodstream not taken up by the osteoclasts.