Changes in inositol polyphosphate‐sensitive calcium exchange in aortic smooth muscle cells in vitro

Cells that expressed the muscle-specific intermediate filament protein desmin were cultured from the aorta of Fischer 344 rats. When the cultured cells were extracted with digitonin, they accumulated 45Ca2+ from the incubation medium in a manner that was stimulated by ATP and released subsequently by exposure to the Ca2+ ionophore A23187. Ca2+ bound in the presence of ATP was also released by exposure to inositol 1,4,5-trisphosphate (IP3). Like contraction in some kinds of smooth muscle, IP3 released Ca2+ in either the absence or the presence of the ATPase-inhibitor ruthenium red. When the responsiveness of digitonin-extracted cells cultured from 3-, 12-, and 24-month-old rats was compared, cells from the youngest group released only about one-half as much Ca2+ as cells from the 12- or 24-month-old rats. The results suggest that in the rat there are changes during maturation in the responsiveness to inositol polyphosphates of intracellular compartments that sequester Ca2+ for stimulus-contraction coupling in the aortic smooth muscle cell. These changes, characterized in smooth muscle cells in vitro, might contribute to the way vascular responsiveness is regulated in vivo.