Nerve Activity but Not Intracellular Calcium Determines the Time Course of Endocytosis at the Frog Neuromuscular Junction

Abstract

We used FM1-43 imaging and intracellular recordings of synaptic potentials to measure the time course of endocytosis in frog motor nerve terminals following tetanic nerve stimulation, and we used fura-2 imaging of intraterminal Ca 2+ concentration to compare endocytic rate and [Ca 2+] i. Following a 30 Hz tetanus, endocytosis declined exponentially with a time constant that depended on the duration of stimulation. The level of [Ca 2+] i rose from a resting value of about 100 nM to more than 500 nM during 30 Hz stimulation, and rapidly declined to 200–250 nM after stimulation. [Ca 2+] i returned to resting level with a time course that, like endocytosis, depended on the duration of tetanic stimulation. However, the rate of [Ca 2+] i recovery was much slower than the rate of endocytosis, leading to the conclusion that endocytic rate is not determined solely by the instantaneous level of [Ca 2+] i.