Lead interferes with calcium entry through membrane pores

Abstract

Calcium is an important intracellular messenger in all cells, represented here by nerve cells and osteoblast-like (OBL) cells. In neurons the intracellular calcium signal is related, e.g., to bioelectric phenomena. In OBL cells the intracellular calcium concentration ([Ca²⁺]_i) plays a role in the intercellular communication via gap junction channels. [Ca²⁺]_i might be affected by lead (Pb²⁺). In the nervous system even low Pb²⁺ concentrations impair learning and memory functions. Considering long-term potentiation (LTP) as a model for learning and memory it has been proven that the generation and maintenance of LTP is reduced by Pb²⁺ (1–10 μM). As the induction of LTP depends on a rise of [Ca²⁺]_i, we examined the effects of Pb²⁺ on [Ca²⁺]_i and on currents through calcium permeable membrane pores in dorsal-root ganglion (DRG) neurons, using calcium measurements (Fura-2/ AM) and whole cell patch clamp techniques. To study the effects of Pb²⁺ on intercellular communication via gap junctions we used rat OBL cells investigating interactions of Pb²⁺ with electric cell coupling. Furthermore, we examined calcium release activated channel currents (CRACCs) of these cells.

Lead (1–10 μM) reduced the stimulated increase of [Ca²⁺]_i in a concentration 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 μM. The effect was quite specific as voltage activated sodium and potassium channel currents were not significantly altered in the same concentration and voltage range. Furthermore, this effect was not voltage dependent and only partly reversible. A 100-fold higher concentration of Pb²⁺ (IC₅₀ of 46 μM) was found for the reduction of NACC currents. A small portion of this 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 in a concentration dependent manner by extracellular Pb²⁺, the concentrations were between 2 and 20 μM. Surprisingly the electric coupling through gap junction channels in OBL cells was not reduced by either extracellular or intracellular Pb²⁺ (5–25 μM).