CALCIUM-TRANSPORT ACROSS EPITHELIA

This chapter discusses mechanisms by which cells transport calcium and modify calcium fluxes in the body. The major chemical functions that are dependent on calcium include: nerve excitation, muscle contraction, extrusion, and export processes. The level and regulation of the calcium content of body fluids involves millimolar concentrations. Food calcium may undergo solubilization and dilution with bile and intestinal fluids; calcium concentrations in the chyme can reach tens of millimoles. Thus, the body's calcium traffic—ingestion, digestion, absorption, plasma-calcium maintenance, cellular uptake and release, bone-calcium deposition and removal, i.e., resorption, urinary and fecal-calcium excretion—involves an unending manipulation of various calcium concentrations, as well as changes in state from liquid to solid and back to liquid. In the balance approach, the amount of calcium excreted in the stool is subtracted from the amount ingested during a comparable period. This is termed “net absorption.” The net amount of calcium absorbed represents the net load of calcium from the intestine. By using radioactive or stable isotopes of calcium, along with a balance procedure, one of the streams of calcium in the intestine can be labeled—either the endogenous or the food calcium—and thereby the true amount absorbed and the quantity of endogenous calcium lost in the stool can be quantitated. © 1991, Elsevier Science & Technology. All rights reserved.