These results suggest that U73122 is inhibiting, at least in part, an active component in SF This conclusion is also supported by reports using sea urchin egg homogenates in which Ca2+ responses to SF were abrogated by preincubation of SF with U73122, although the concentrations of the inhibitor used in those studies were significantly higher than dosages previously reported. Preincubation of sea urchin egg homogenates with up to 400 ^M U73122 failed to show complete inhibition of SF-induced Ca2+ release.
These discrepancies in effectiveness and dosages of U73122 in sea urchin egg homogenates may be related to the concentration of SF required to trigger Ca2+ release or to the fact that the inhibitor may be partially inactivated when incubated with the homogenates.
U73122 could be inhibiting multiple steps of the PI pathway. Initially, this inhibitor was thought to be specific for PLC. However, additional studies revealed that U73122 was more effective in blocking the activation of PLC by receptor agonists or by GTP7S than activation of PLC by [Ca2+]i increases. Similarly, other studies demonstrated inhibition of Gaq subunits by U73122, suggesting that the site of action of this inhibitor may be at the level of G-proteins or at the link between G-proteins and PLCs. Thus, the inhibition of SF-induced oscillations by U73122 implicates the PI pathway; however, the nature of the active molecule(s) in SF cannot be deduced from these studies.