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We simultaneously measured presynaptic free calcium ion concentration (Ca2+i) and synaptic strength at the crayfish claw opener neuromuscular junction (nmj) under a variety of experimental conditions. Our experiments were designed both to test the hypothesis that elevated Ca2+i is necessary and sufficient for the induction of a form of synaptic enhancement that persists for several seconds after tetanic stimulation--augmentation--and to determine the quantitative relationship between elevated Ca2+i and this enhancement. Action potential trains increased Ca2+i and enhanced transmission. During the decay phase of synaptic enhancement known as augmentation (time constant of decay approximately 7 sec at 20 degrees C with < 200 microM fura-2 in terminals), Ca2+i was elevated 700 nM or less above rest and an essentially linear relationship between Ca2+i and enhancement was observed. Introduction of exogenous Ca2+ buffers into the presynaptic terminal slowed the buildup and recovery kinetics of both Ca2+i and the component of synaptic enhancement corresponding to augmentation. The slope of the relationship relating delta Ca2+i to augmentation was not changed. The time course of augmentation and recovery of Ca2+i remained correlated as the temperature of the preparation was changed from about 10 degrees C to 20 degrees C, but the quantitative relationship of enhancement to Ca2+i was increased more than two- to threefold. During moderate frequency trains of action potentials, a slowly developing component of the total synaptic enhancement was approximately linearly related to residual Ca2+i measured with fura-2. The quantitative relationship between Ca2+i and this component of synaptic enhancement during trains was the same as that during synaptic augmentation after trains.(ABSTRACT TRUNCATED AT 250 WORDS)
Delaney et al. (Sat,) studied this question.