Cooperative Ca 2+ Removal from Presynaptic Terminals of the Spiny Lobster Neuromuscular Junction

Abstract

Stimulation-induced changes in presynaptic free calcium concentration ([Ca 2+] i) were examined by fluorescent imaging at the spiny lobster excitor motor nerve terminals. The Ca 2+ removal process in the terminal was analyzed based on a single compartment model, under the assumption that the Ca 2+ removal rate from the terminal cytoplasm is proportional to nth power of [Ca 2+] i. During 100 nerve stimuli at 10–100 Hz, [Ca 2+] i reached a plateau that increased in a less-than-linear way with stimulation frequency, and the power index, n, was about 2. In the decay time course after stimulation, n changed with the number of stimuli from about 1.4 after 10 stimuli to about 2 after 100 stimuli. With the change of n from 1.4 to 2, the rate became larger at high [Ca 2+] i (>1.5 μM), but was smaller at low [Ca 2+] i (<1 μM). These results suggest that a cooperative Ca 2+ removal mechanism of n = 2, such as mitochondria, may play an important role in the terminal. This view is supported by the gradual increase in the [Ca 2+] i plateau during long-term stimulation at 20–50 Hz for 60 s and by the existence of a very slow [Ca 2+] i recovery process after this stimulation, both of which may be due to accumulation of Ca 2+ in the organelle.