Modulation of intercellular calcium signaling in astrocytes by extracellular calcium and magnesium.

The extracellular concentrations of Ca(2+) and Mg(2+) are well known to play important roles in the function of the central nervous system. We examined the effects of extracellular Ca(2+) and Mg(2+) on ATP release and intercellular signaling in astrocytes. The extent of propagation of intercellular Ca(2+) waves evoked by mechanical stimulation was increased by reduction of extracellular Ca(2+) (Ca(2+)) or Mg(2+) concentration (Mg(2+)) and was decreased by elevated Mg(2+). Reduction of extracellular Ca(2+) concentration (Ca(2+)) evokes intercellular Ca(2+) signaling in astrocytes; a similar effect was observed in response to change from 5 mM Mg(2+) to 0 Mg(2+). Release of low-molecular-weight dyes and ATP was also activated by low Ca(2+) or Mg(2+) and inhibited by high Ca(2+) or Mg(2+). Astrocytes showed low Ca(2+)-activated whole cell currents consistent with currents through connexin hemichannels. These currents were inhibited by extracellular Mg(2+). We conclude that extracellular divalent cations modulate intercellular Ca(2+) signaling in astrocytes by modulating the release of ATP, possibly via connexin hemichannels.