Question

In skeletal muscle, the sarcoplasmic reticulum regulates the intracellular concentration of calcium. During depolarization in cardiac muscle, calcium enters the intracellular fluid from additional sources. Where, besides the SR, does the calcium come from? (this question asks where the additional calcium is coming from, not how it’s getting through the membrane). In skeletal muscle, the sarcoplasmic reticulum regulates the intracellular concentration of calcium. During depolarization in cardiac muscle, calcium enters the intracellular fluid from additional sources. Where, besides the SR, does the calcium come from? (this question asks where the additional calcium is coming from, not how it’s getting through the membrane).

Answer 1

Calcium comes from the extracellular source through the calcium channels.

Unlike skeletal muscles, cardiac muscles depend upon two source of calcium in order to initiate contraction, the two sources are calcium sequsterd in the sarcoplasmic reticulum of the muscle cell and extracellular calcium that enters the cardiac cells via the calcium channels on the membrane of cardiac muscle.

Most of this calcium enters through the L-type channels, located primarily at sarcolemmal/sarcoplasmic reticulum junctions. The influx of calcium triggers the release of further calcium from the sarcoplasmic reticulum via ryanodine receptors.