intracellular calcium

Calcium is the cell's universal trigger. Normally the cytoplasm keeps calcium astonishingly low — about ten thousand times lower than outside — so that even a small inflow registers as a loud, unmistakable signal. When calcium rises, things happen: muscles contract, glands secrete, neurons release transmitter.

Calcium enters the cytoplasm two ways: from outside through ion channels in the membrane (including voltage-gated calcium channels), and from internal stores in the endoplasmic reticulum when IP3 opens them. Once inside, calcium acts as a second messenger by binding proteins such as calmodulin and troponin, switching on enzymes, kinases, and the contractile machinery. Afterwards, pumps and exchangers quickly clear calcium back out or into stores, resetting the cell.

This central role makes calcium handling a huge drug target. Calcium channel blockers (amlodipine, verapamil) lower blood pressure and treat angina by reducing calcium entry into vessel and heart muscle; cardiac glycosides such as digoxin raise heart-cell calcium indirectly to strengthen contraction. The flip side is that uncontrolled calcium overload is toxic and contributes to cell death in ischaemia, so the system is tightly buffered.