LEAD INTERFERES WITH CALCIUM-ENTRY THROUGH MEMBRANE PORES

Calcium is an important intracellular messenger in all cells, represen ted here by nerve cells and osteoblast-like (OBL) cells. In neurons th e intracellular calcium signal is related, e.g., to bioelectric phenom ena. In OBL cells the intracellular calcium concentration ([Ca2+](i)) plays a role in the intercellular communication via gap junction chann els. [Ca2+](i) might be affected by lead (Pb2+). In the nervous system even low Pb2+ concentrations impair learning and memory functions. Co nsidering longterm potentiation (LTP) as a model for learning and memo ry it has been proven that the generation and maintenance of LTP is re duced by Pb2+ (1-10 mu M). As the induction of LTP depends on a rise o f [Ca2+](i), we examined the effects of Pb2+ on [Ca2+](i) and on curre nts through calcium permeable membrane pores in dorsal-root ganglion ( DRG) neurons, using calcium measurements (Fura-2/AM) and whole cell pa tch clamp techniques. To study the effects of Pb2+ on intercellular co mmunication via gap junctions we used rat OBL cells investigating inte ractions of Pb2+ with electric cell coupling. Furthermore, we examined calcium release activated channel currents (CRACCs) of these cells. L ead (1-10 mu M) reduced the stimulated increase of [Ca2+](i) in a conc entration dependent manner, by reducing both voltage-activated calcium channels (VACCs) and N-methyl-D-aspartate activated calcium channels (NACCs) in neurons. Voltage-activated calcium channel currents (VACCCs ) were reduced by Pb2+ with an IC50 of 0.46 mu M. The effect was quite specific as voltage activated sodium and potassium channel currents w ere not significantly altered in the same concentration and voltage ra nge. Furthermore, this effect was not voltage dependent and only partl y reversible. A 100-fold higher concentration of Pb2+ (IC50 Of 46 mu M ) was found for the reduction of NACC currents. A small portion of thi s effect was not reversible. Other agonist activated channel currents (kainate and quisqualate) are not affected. In OBL cells, the calcium entry through calcium release activated channels (CRACs) was reduced i n a concentration dependent manner by extracellular Pb2+, the concentr ations were between 2 and 20 mu M. Surprisingly the electric coupling through gap junction channels in OBL cells was not reduced by either e xtracellular or intracellular Pb2+ (5-25 mu M).