Guanosine 5'-O-(3-thiotrisphosphate) potentiates both thrombin- and platelet-derived growth factor-induced inositol phosphate release in permeabilized vascular smooth muscle cells. Signaling mechanisms distinguished by sensitivity to pertussis toxin and phorbol esters

We compared the mechanisms by which thrombin and platelet-derived growth factor (PDGF) activate phospholipase C in cultured vascular smooth muscle cells. Thrombin caused a transient (<5 min) increase in inositol trisphosphate (IP₃) while PDGF caused a sustained (>10 min) increase. Both pertussis toxin and phorbol 12-myristate 13-acetate (PMA) inhibited the thrombin-induced increase in IP₃ but neither agent affected the PDGF-induced increase in IP₃. To examine the role of GTP binding (G) proteins in the activation of phospholipase C by these two hormones, GTP analogues were introduced into saponin-permeabilized cells. In the absence of hormones, guanosine 5'-O-(3-thiotrisphosphate) (GTPγS) caused a progressive increase in IP₃ release which was inhibited 55% by PMA (200 ng/ml). In the presence of thrombin, GTPγS caused synergistic increase in IP₃ release. The synergism between GTPγS and thrombin was virtually eliminated by 10 min prior exposure to PMA (200 ng/ml). When PDGF was the hormonal agonist, GTPγS also caused synergistic increase in IP₃ release and guanosine 5'-O-(2-thiodiphosphate) blunted PDGF-induced IP₃ release. However, in contrast to thrombin, the synergism between GTPγS and PDGF was unaffected by PMA. Thus, thrombin and PDGF activate phospholipase C by signal transduction systems which differ in kinetic properties and in sensitivity to PMA and pertussis toxin. Despite these differences, both systems appear to involve GTP binding proteins at some step.