Neurotransmitter release and its facilitation in crayfish. II. Duration of facilitation and removal processes of calcium from the terminal.

Excitatory postsynaptic currents (EPSCs) were recorded extracellularly from synaptic spots on crayfish opener muscles. Facilitation was measured in twin pulses with different intervals. When the extracellular calcium concentration, [Ca]0, was raised from 1.7 to 13.5 mM, the duration of facilitation was increased more steeply than can be explained by the higher entry of Ca. The discrepancy can be accounted for by assuming two saturable Ca removal processes, one being inhibited at high [Ca]0. If [Na]0 was reduced to 50 or 25%, facilitation at low [Ca]0 (e.g. 3.4 mM) was greatly prolonged, while at high [Ca]0 (13.5 mM) no effect was observed. It is concluded that one of the Ca removal processes, R1, depends on [Na]0 and is inhibited by [Ca]0; probably it is a Cai in equilibrium with Na0 exchange. R1 is predominant at low [Ca]0 and is largely responsible for the short duration of facilitation there. Other removal processes, R2, are not affected appreciably by [Ca]0 and [Na]0, and they predominate at high [Ca]0. Approximate values for the kinetic constants of R1 and R2 and the inhibition of R1 were estimated.