The homeostasis of calcium in heart cells.

The signalling function of Ca2+ demands a very low ionic concentration of the cation within heart cells. This is achieved by two mechanisms, the reversible complexation of calcium by non membranous (protein) ligands, and its binding and transport by transmembrane proteins. The second mechanism is more efficient, since the complexation by soluble proteins is limited by their amount in heart sarcoplasm or within heart organelles, whereas membrane proteins can regulate calcium efficiently, even if present in low amounts, if they 'return' rapidly in the uncomplexed from after each binding and transport cycle. Seven systems for the transport of calcium have been documented in heart membranes. There are located in sarcolemma, two in mitochondria, two in sarcoplasmic reticulum. These seven systems can be simplified to four basic transport modes: ATPases, Na+/Ca2+ exchangers, channels, electrophoretic uniporters. They have either low or high calcium affinity, thus serving different purposes in the various phases of the functional cycle of heart cells. On an integrated level, sarcoplasmic reticulum can be considered as the organelle presiding over the rapid and fine regulation of Ca2+ linked to the contraction/relaxation cycle. Sarcolemma regulates Ca2+ with both low and high affinity, but handles a quantitatively minor amount of Ca2+ (trigger Ca2+). Mitochondria are low-affinity organelles, whose primary role probably is the regulation of Ca2+ in their matrix, rather than in the sarcoplasm.