Effects of thapsigargin, an intracellular calcium‐mobilizing agent, on synthesis and secretion of cartilage collagen and proteoglycan

The calcium‐mobilizing agents thapsigargin and 2.5‐di‐(tert‐butyl)‐1,4‐benzo‐hydroquinone were shown to markedly elevate the intracellular calcium concentration of chick embryo chondrocytes in a dose‐dependent manner. Under these conditions, the metabolism of macromolecules was variably affected. The synthesis and secretion of protein in general, and of collagen in particular, were significantly inhibited; in contrast, proteoglycan synthesis (but not glycosaminoglycan synthesis) was inhibited, whereas secretion was unaffected. Flunarizine, which prevented the thapsigargin‐induced intracellular calcium elevation, and EGTA, which caused only a transient thapsigargin‐induced intracellulr calcium elevation, did not reverse these alterations. It was concluded, therefore, that the observed effects of thapsigargin and 2,5‐di‐(tert‐butyl)‐1,4‐benzohydroquinone on chondrocyte macromolecule metabolism were not related to the ability of these drugs to increase the cytosolic free calcium concentration but may have been due to the specific depletion of the calcium sequestered in the endoplasmic reticulum. The differential effect of these drugs on protein and proteoglycan secretion suggests that the intracellular trafficking of these two classes of macromolecules may be controlled independently.