Energy‐dependent Calcium Uptake by Fibroblast Microsomes

The sarcoplasmic reticulum, isolated as microsomes from skeletal muscle, sequesters calcium with an energy-dependent calcium pump.It is believed that this calcium pump provides both a mechanism for storage of intracellular calcium and a mechanism to remove calcium from the cytoplasm and thus relax contracted muscle.'In microsomal fractions from cardiac and smooth muscle, calcium sequestration is also dependent up?n MgATP.In nonmuscle tissue, studies of intracellular calcium uptake systems have focused primarily in mitochondria.2However, energy-dependent calcium uptake activity has been found in microsomal fractions from a variety of nonmuscle tissues.3-*Recently we described MgATP-dependent calcium uptake activity in microsomes isolated from cultured f i b r ~b l a s t s .~. ' ~ Although much lower activity is found in fibroblast microsomes than in skeletal muscle sarcoplasmic reticulum, cultured fibroblast microsomal activity is comparable to the activity found in microsomal fractions of cultured skeletal muscle cells" or nonmuscleIn nonmuscle cells, microsomal calcium transport may be analogous to the sarcoplasmic reticulum calcium pump of skeletal muscle and function in the regulation of cytoplasmic calcium levels.This report will summarize our recent studies on the regulation of microsomal calcium uptake from cells grown in tissue culture.Cells in tissue cultur'e offer several advantages as a model system to study the regulation of intracellular calcium transport in nonmuscle cells.An established cell line.such as Balbic 3T3 cells, allows one to use a homogenous cell population that can be studied under a variety of culture conditions.Tissue culture also allows one access to uniform populations of cells that are growing or quiescent, mutant cell lines, or cells transformed by a number of agents.Additionally.calcium is an important regulatory ion in the control of both motility and growth of cells in tissue culture.Models of cultured fibroblast motility have emphasiLed the importance of calcium in the regulation of cell motility.The glycerinated fibroblast, like glycerinated skeletal muscle, contracts upon the addition of MgATP.Contraction is inhibited by microsomes isolated from skeletal muscle or from cultured fibroblasts.12The inhibitory effect of these microsomes on glycerinated fibroblast contraction is overcome by calcium.Hoffman-Berling and coworkers12 concluded that both microsomal preparations contain a "relaxing factor" that prevents contraction.We now know that the "relaxing factor" of skeletal muscle