NIH 3T3 fibroblasts expressing the Ha-ras oncogene respond to bradykinin (100 nmol/l) with sustained oscillations of cell membrane potential. Otherwise identical cells not expressing the oncogene respond to bradykinin with a single, transient hyperpolarization of the cell membrane. The oscillations of cell membrane potential in ras oncogene-expressing cells are the result of intracellular calcium oscillations with subsequent activation of calcium-sensitive K+ channels. As shown here, oscillations of cell membrane potential are elicited by acute addition of 0.5% fetal calf serum. Acute addition of cAMP (1 mmol/l), on the other hand, did not elicit oscillations of cell membrane potential. The involvement of calmodulin was excluded by the use of trifluoperazine (10 mumol/l), pimozide (1 mumol/l) and compound 48/80 (3 mumol/l) and the involvement of cycloxygenase products by the use of indomethacin (1 mumol/l). Neither substance modified the oscillations significantly. Similarly, dantrolene (10 mumol/l), an inhibitor of calcium-induced calcium release remained without significant effect on the oscillations. On the other hand, thapsigargin (1 mumol/l), an inhibitor of endoplasmatic reticulum Ca2+-ATPase, abolished the oscillations in 7 out of 10 cells. Furthermore, phorbolester TPA (10 mumol/l) acutely inhibited the oscillations. In cells not expressing the ras oncogene but pretreated for 24 h with lithium (10 mmol/l), oscillations of cell membrane potential similar to those in ras oncogene-expressing cells are observed following the addition of bradykinin. HPLC reveals markedly increased formation of 1,3,4,5-InSP4 and 1,4,5-InSP3 prior to the addition of bradykinin as well as an enhanced stimulatory effect of bradykinin on the inositol phosphates in ras oncogene-expressing cells. In conclusion, the calcium oscillations probably involve thapsigargin-sensitive intracellular calcium stores and may be secondary to enhanced formation of 1,3,4,5-InsP4 and/or 1,4,5-InSP3. Phorbolesters abolish the oscillations presumably by inhibiting phospholipase C, and lithium triggers the oscillations possibly by enhancing cellular 1,3,4,5-InSP4 and/or 1,4,5-InsP3.
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