METHYLMERCURY MOBILIZES CA++ FROM INTRACELLULAR STORES SENSITIVE TO INOSITOL 1,4,5-TRISPHOSPHATE IN NG108-15 CELLS

  • 1 March 1995
    • journal article
    • research article
    • Vol. 272  (3) , 1016-1023
Abstract
Fluorescence intensity was monitored from individual NG108-15 cells loaded with the Ca++-selective probe fura-2,, and exposed to 2 mu M methylmercury (MeHg). The initial effect of 2 mu M MeHg was an elevation in intracellular Ca++ concentration ([Ca++](i)), which was not blocked by lowering extracellular Ca++ (Ca-e(++)), nifedipine (0.1 mu M) or by Ni++ (1 mM). Addition of 100 mu M Mn++ to Ca++-containing medium did not alter fluorescence intensity at either the Ca++-insensitive excitation wavelength of 360 nm or the Ca++-sensitive wavelength of 380 nm. Depolarization with K+ decreased the intensity at both wavelengths, indicating Mn++ entry. In the presence of Mn++, MeHg decreased the 380 nm, but not the 360 nm signal. Bradykinin (Bk) caused a transient increase in the fluorescence ratio, which was blocked by the endoplasmic reticulum Ca++-adenosine triphosphatase inhibitor thapsigargin. Pretreatment with Bk and thapsigargin reduced significantly the increase in ratio induced by MeHg from 21.9 +/- 3.4 to 6.9 +/- 1.8% of base line. Bk had no effect when applied after MeHg. Caffeine reduced the Bk-induced increase in [Ca++](i) and the MeHg-induced increase in ratio from 21.9 +/- 3.4 to 9.0 +/- 2.1%. Thus, Bk, caffeine and MeHg all appear to release a common pool of intracellular calcium (Ca-i(2+)). When applied after MeHg, Bk increased inositol 1,4,5-trisphosphate (IP3) by 305 +/- 27% compared to 270 +/- 29% in controls. Thus, MeHg did not induce Ca++ release by IP3 generation, nor did it block the effects of Bk by interfering with IP3 synthesis. These results suggest that, in NG108-15 cells, the initial effect of MeHg was to elevate [Ca++](i) which originates almost exclusively from one or more IP3-sensitive pools of Ca-i(++).