[Calcium and the liver]

The concentration of cytosolic free calcium ([Ca2+]i) in living cells is far below than that of plasma and must be extruded against a large electrochemical gradient. Cytosolic Ca2+ is also concentrated by two organelles, the high-affinity, low-capacity endoplasmic reticulum and the low-affinity, high-capacity mitochondria. In liver, different hormones and neurotransmitters mobilize cell glucose by using Ca2+ as an amplifier signal. These agonists promote a fast and large increase in [Ca2+]i as detected by intracellular fluorescent indicators resulting from permeabilization of the endoplasmic reticulum and stimulation of the Ca2+ influx. The hormone action is mediated by rapid hydrolysis of inositol lipids with formation of two messagers inositol [1,4,5]triphosphate and diacylglycerol. Certain bile acids also promote a pronounced release of Ca2+ from the endoplasmic reticulum in liver but independently of the hormone messengers. Mobilization of cell Ca2+ and/or H+ by these cell messengers is involved in a wide variety of other cell functions.