We examined the role of intracellular Ca2+ in c-Jun NH2-terminal kinase (JNK) activation and DNA fragmentation in the rat myoblast cell line H9c2 using small GTP-binding protein Rad. A constitutively active mutant of Rad (V12-Rac1) increased JNK-responsive gene expression 6-fold, although this increase was attenuated by the intracellular Ca2+ chelator BAPTA-AM. V12-Rac1 also increased the number of DNA fragmentated cells. However, V12-Rac1-mediated JNK activation was not affected by BAPTA-AM as determined by direct measurement of active forms, and V12-Rac1 did not affect intracellular Ca2+ concentration. These results suggest that Rad can activate JNK and induces cell injury, but [Ca2+](i) is necessary for V12-Rac1 to induce DNA fragmentation downstream Of JNK activation. (C) 1999 Academic Press.