Ion channels in human neutrophils activated by a rise in free cytosolic calcium concentration

Abstract

A rapid, transient rise in the free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) is one of the earliest events in neutrophil activation and is assumed to be involved in many of the subsequent cellular reactions^1–3. Both Ca²⁺ release from intracellular stores and Ca²⁺ influx from the extracellular space contribute to the rise in [Ca²⁺]_i⁴. In an attempt to assess the relative importance of these pools and the sequences leading to the rise in [Ca²⁺]_i, we have studied the time course of changes in [Ca²⁺]_i after stimulation with N-formylmethionyl-leucyl-phenylalanine (fMLP) or platelet-activating factor (PAF) using the Ca²⁺ indicators quin-2 and fura-2. We observed a time lag of 1–3 s between stimulation and rise in [Ca²⁺]_i. This lag depends on the agonist concentration but is independent of extracellular Ca²⁺. Thus Ca²⁺ release from intracellular stores is rate limiting for the rise in [Ca²⁺]_i. After this, cation channels in the plasma membrane (measured with the patch clamp method⁵) are opened. These non-selective channels, which also pass Ca²⁺, are activated by the initial rise in [Ca²⁺]_i, but by neither fMLP nor inositol 1,4,5-trisphosphate (IP₃) directly.