Calcium mobilization in permeabilized fibroblasts: Effects of inositol trisphosphate, orthovanadate, mitogens, phorbol ester, and guanosine triphosphate

Utilizing a digitonin‐permeabilized cell system, we have studied the release of calcium from a non‐mitochondrial intracellular compartment in cultured human fibroblasts (HSWP cells). Addition of 1 mM MgATP to a monolayer of permeabilized cells in a cytosolic media buffered to 150 nM Ca with EGTA rapidly stimulates ⁴⁵Ca uptake, and the subsequent addition of the putative intracellular messenger inositol trisphosphate (InsP₃) induces rapid release of 85% (±6% n = 6) of the ⁴⁵Ca taken up in response to ATP. Mitogenic peptides (bradykinin, vasopressin, epidermal growth factor [EGF], and insulin) and orthovanadate, which are effective in mobilizing intracellular Ca in intact cells, have little or no effect when added alone to permeabilized cells. However, in the presence of GTP these agents stimulate accumulation of inositol phosphates and release Ca from the InsP₃‐sensitive pool. These data suggest that a GTP binding protein is involved in receptor mediated activation of phospholipase C, which leads to release of inositol phosphates. The GTP‐dependent release of InsP₃ and the mobilization of ⁴⁵Ca from the intracellular compartment are inhibited by pretreatment of cells, prior to permeabilization, with the protrein kinase C activator 12‐O‐tetradecanoyl‐phorbol‐13‐acetate (TPA). TPA pretreatment does not affect the InsP₃ stimulated Ca release. These results suggest that protein kinase C is involved in down‐regulation or inhibition of phospholipase C, or the GTP binding protein responsible for relaying the mitogenic signal from the cell surface receptor to the phospholipase C activity.