Effect of extracellular K + on the intracellular free Ca 2+ concentration in leech glial cells and Retzius neurones

Abstract

The effects of extracellular K + on the intracellular free Ca 2+ concentration ([Ca 2+] 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 [Ca 2+] i, which was blocked by Co 2+, Ni 2+ and menthol, whereas nicardipine, flunarizine, ω-conotoxin GVIA and ω-agatoxin IVA were ineffective. In Ca 2+-free solution, the K +-induced [Ca 2+] i increase was largely suppressed in neuropile glial cells and completely abolished in Retzius neurones. The results indicate that the K +-induced [Ca 2+] i increase was mainly due to Ca 2+ influx through voltage-dependent Ca 2+ channels. The Ca 2+ 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 [Ca 2+] i increase was unaltered in Na +-free solution, indicating that active Ca 2+ transport across the plasma membrane is mediated by Na +-independent mechanisms.