Cytosolic calcium responses induced by photolytic release of 1,4,5-inositol trisphosphate in single human fibroblasts.

We have used the whole cell technique to microinject human fibroblasts with either 1,4,5-inositol trisphosphate (InsP3) or 'caged' InsP3, in order to study the mechanisms of transmembrane signalling related to mitogenic stimulations. Cytosolic Ca2+ elevations in response to 1,4,5 InsP3 diffusing from the patch pipette were difficult to detect, while 1,4,5 InsP3, photoreleased after loading the cell with its inactive precursor, was capable of generating not only a single cytosolic Ca2+ rise but sometimes triggered an oscillatory calcium response, similar to that often observed under mitogenic stimulation. We estimated that less than 100 nM InsP3 was sufficient to generate Ca2+ responses. The Ca2+ rise produced by the photoreleased InsP3 could fully activate the K+ channels present in the plasma membrane of human fibroblasts.