Neurotransmitter release and its facilitation in crayfish. I. Saturation kinetics of release, and of entry and removal of calcium.

Release and facilitated release of transmitter at neuromuscular junctions of the crayfish Astacus were measured as a function of [Ca]0 at single junctions using a patch clamp technique. Tests were made of a quantitative model that relates release of transmitter to [Ca]i. The model assumes three processes, entry of Ca during the action potential, release of transmitter as a function of [Ca]i, and removal of Ca after the action potential. Each process is described alternatively by linear kinetics or saturation kinetics, and predictions for different combinations of the equations are given. The main findings were in agreement with those predicted by the "saturation" model. The amplitude of synaptic current varies non-linearly with [Ca]0, log-log plot yielding a slope of about 1.6. The degree of facilitation at long intervals is an increasing function of [Ca]0. In addition, the duration of facilitation is prolonged as [Ca]0 is increased, to saturate at [Ca]0 of 9 mM.