Cytoplasmic free Ca in isolated snail neurons as revealed by fluorescent probe fura-2: Mechanisms of Ca recovery after Ca load and Ca release from intracellular stores

Summary Using the fluorescent probe fura-2, we measured the cytoplasmic concentration of free Ca²⁺ (Ca] _i ) and its changes in isolated, nonidentified neurons of the snailHelix pomatia. Ca] _i increased during membrane depolarization due to opening of Ca channels in the surface membrane. When the membrane potential returned to the resting level, Ca] _i recovered monoexponentially, with the time constant ranging from 10 to 30 sec. The rate of recovery remained unchanged after treatments that interferred with the normal functioning of both Ca/Na exchange and Ca-ATPase in the surface membrane or mitochondria. Ca] _i recovery slowed down upon cooling according to Q₁₀=2.3 and after intracellular injection of vanadate. The data obtained suggest that the rate of Ca] _i recovery after membrane depolarization is mainly determined by Ca pump of intracellular stores (presumably by the endoplasmic reticulum). Ca release from these stores could be induced in the presence of millimolar caffeine or theophylline in the external medium when Ca] _i increased up to a certain threshold level. This depolarization-induced Ca load triggered further transient increase in Ca] _i , which was accompanied by membrane hyperpolarization due to the development of Ca-activated potassium conductance. 1mm procaine or tetracaine, but not lidocaine, inhibited this Ca-induced Ca release. In some cases stable oscillations of Ca] _i were observed. They could be induced by producing a steady Ca influx by membrane depolarization.