Description:
<jats:title>ABSTRACT</jats:title>
<jats:p>The effects of extracellular ATP on intracellular free Ca2+ concentration ([Ca2+]i) and depolarization-induced elevations of [Ca2+]i were investigated in salivary cells of the leech Haementeria ghilianii using the fluorescent Ca2+ indicator Fura-2. Simultaneously, the membrane potential was monitored or controlled by voltage-clamp. The cell membrane was depolarized either by transient elevations of the extracellular K+ concentration ([K+]o) to 90 mmol l−1 or by depolarizing steps under voltage-clamp. The resulting transient elevations of [Ca2+]i (Ca2+ transients) could be repeatedly elicited with little variability in amplitude. Ca2+ transients were completely inhibited by 2 mmol l−1 Ni2+ or in Ca2+-free saline. The transients are, therefore, dependent on Ca2+ influx from the external medium through voltage-gated Ca2+ channels. The Ca2+ influx was rapidly and reversibly inhibited by extracellular application of ATP. The effect was dose-dependent with a threshold concentration below 10−7 mol l−1. A 50 % reduction in the amplitude of Ca2+ transients was obtained by application of 1–2 μmol l−1 ATP or ATP--y-S (apparent IC50, 1.6 μmol l−1 ATP) and Ca2+ transients were almost completely inhibited by 30–100 μmol l−1 ATP. Resting [Ca2+]i, the resting membrane potential and membrane potential changes induced by 90 mmol l−1 [K+]o were not affected by ATP. Adenosine (10 μmol l−1) did not affect resting [Ca2+]i, the resting membrane potential or membrane potential changes induced by 90 mmol l−1 [K+]o and had little effect on Ca2+ transients. Suramin, an antagonist of vertebrate P2 receptors, was without effect on the inhibitory actions of ATP. We conclude that activation of a suramin-insensitive purinoceptor by ATP inhibits Ca2+ influx through voltage-gated Ca2+ channels in the salivary cells of Haementeria ghilianii.</jats:p>