Aluminum is a weak agonist for the calcium-sensing receptor

Abstract

Background. Aluminum (Al3+) has diverse biological effects mediated through activation of a putative extracellular cation-sensing receptor. A recently identified calcium-sensing receptor (CaSR), which has been identified in target tissues for Al3+, may transduce some of the biological effects of Al3+. Methods. To test this possibility, we transfected human embryonic kidney 293 (HEK 293) cells with a eDNA encoding the rat CaSR and evaluated CaSR expression by Western blot analysis and function by measurement of intracellular calcium ([Ca2+](i)) levels and inositol monophosphate (IP1) generation following stimulation with Al3+ and a panel of CaSR agonists. Results. The CaSR protein was detected by immunoblot analysis in cells transfected with the CaSR eDNA but not in nontransfected HEK 293 cells. In addition, [Ca2+](i) levels and IP1 generation were enhanced in a dose-dependent fashion by additions of the CaSR agonists calcium (Ca2+), magnesium (Mg2+), gadolinium (Gd3+), and neomycin only in cells transfected with CaSR. To determine if Al3+ activated CaSR, we stimulated cells transfected with rat CaSR with 10 μM to 1 mM concentrations of Al3+. Concentrations of Al3+ in the range of 10 μM to 100 μM had no effect on [Ca2+](i) levels or IP1 generation. In contrast, 1 mM Al3+ induced small but significant increases in both parameters. Whereas Gd3+ antagonized calcium-mediated activation of CaSR, pretreatment with Al13+ failed to block subsequent activation of rat CaSR by Ca2+, suggesting a distinct mechanism of Al3+ action. Conclusion. Al3+ is not a potent agonist for CaSR. Because Al3+ affects a variety of target tissues at micromolar concentrations, it seems unlikely that CaSR mediates these cellular actions of Al3+.