EFFECT OF EXTRACELLULAR K+ ON THE INTRACELLULAR FREE CA2+ CONCENTRATION IN LEECH GLIAL-CELLS AND RETZIUS NEURONS

The effects of extracellular K+ on the intracellular free Ca2+ concentration ([Ca2+](i)) of neuropile glial cells and Retzius neurones in intact segmental ganglia of the leech Hirudo medicinalis were investigated by using iontophoretically injected fura-2. In both cell types, an elevation of the extracellular K+ concentration ([K+](o)) caused an increase in [Ca2+](i), which was blocked by Co2+, Ni2+ and menthol, whereas nicardipine, flunarizine, omega-conotoxin GVIA and omega-agatoxin IVA were ineffective. In Ca2+-free solution, the K+-induced [Ca2+](i) increase was largely suppressed in neuropile glial cells and completely abolished in Retzius neurones. The results indicate that the K+-induced [Ca2+](i) increase was mainly due to Ca2+ influx through voltage-dependent Ca2+ channels. The Ca2+ channels of the two cell types were activated at different membrane potentials but at the same [K+](o). In both cell types, the recovery from a K+-induced [Ca2+], increase was unaltered in Na+-free solution, indicating that active Ca2+ transport across the plasma membrane is mediated by Na+-independent mechanisms.