Contractions can occur in striated muscle as long as Ca2+ is present in the cytosol. As cytosolic levels of free Ca2+decrease, so does the ability for myosin to form cross-bridges with actin

Question

This leads to relaxation of the muscle. You have found a novel muscle and are trying to determine if its relaxation processes are regulated in a manner analogous to vertebrate striated muscle. Describe some of the features of Ca2+regulation that you would expect to see in this novel muscle.

What will be an ideal response?

Answer 1

Answer: One of the most common ways to decrease cytosolic Ca2+levels is to remove the Ca2+. This typically means that Ca2+will have to move from an area of lower concentration to an area of higher concentration. Under these circumstances, it is necessary to have some sort of energy source. The energy (such as ATP) can be used to pump Ca2+ back into a storage organelle (similar to the sarcoplasmic reticulum) or to the extracellular fluid. These types of pumps are referred to as Ca2+ATPases in vertebrate skeletal muscle. Alternatively, energy from another concentration gradient could be used to drive the transport of Ca2+. Again, vertebrate muscle exchanges Ca2+for Na+ (one ion traveling down its gradient and the other traveling up). In both of these cases, Ca2+would be returned to its origin. In other words, if most of the Ca2+has come from extracellular sources, most of it will be pumped back out of the cell. The last analogous mechanism I might expect to find would be some type of cytosolic buffer that binds free Ca2+. Once the Ca2+is bound, then it can no longer interact with the troponin, which allows the muscle to begin its relaxation.