THE EFFECT OF ACTIVE SERUM-ALBUMIN ON PC12 CELLS .2. INTRACELLULAR CA2+ TRANSIENTS AND THEIR ROLE IN NEURITE RETRACTION

In the preceding paper it was shown that an isoform of serum albumin (ASA; active serum albumin) causes a rapid retraction of neurites and increases intracellular content of Ins1,4,5P3 in PC12 cells. Here we examined whether ASA's effects in nerve growth factor-differentiated PC12 cells were mediated through the Ins1,4.5P3/Ca2+ second messenger pathway by monitoring intracellular Ca2+(Ca2+i) with Fura2. It was found that ASA caused a dose-dependent increase in Ca2+i. In Ca2+-free medium, the increase in Ca2+i elicited by ASA was smaller, but the rise in Ins1,4,5P3 content was not appreciably changed. The small Ca2+i increase seen in Ca2+-free medium was probably due to the release of Ca2+ from Ins1,4,5P3-sensitive intracellular stores. In Ca2+-containing medium the Ca2+ transient induced by ASA was not affected by organic Ca2+ Channel blockers, but decreased when Co2+, Mn2+ or Zn2+ were present in the extracellular medium. The effect of other ligands, such as carbachol and bradykinin, whose receptors are coupled to the phosphoinositide system was also investigated. Carbachol at concentrations from 2 to 200-mu-M, and bradykinin at a concentration of 2-mu-M did not cause neurite retraction, whereas 200-mu-M bradykinin caused an approximately 40% decrease in neurite length. Thapsigargin, a Ca2+-ATPase inhibitor, caused a sustained elevation of Ca2+i and retraction of neurites, whereas depolarization of the cells by high K+ gave only a transient elevation of Ca2+i, and no neurite retraction. Therefore, a sustained elevation in Ca2+i might be a sufficient trigger to induce neurite retraction in differentiated PC12 cells. Since ASA induces only a transient rise in Ca2+i in most celts, the Ca2+i transient evoked by ASA alone may not be sufficient to cause neurite retraction in PC12 cells.