Cooperative Ca2+ removal from presynaptic terminals of the spiny lobster neuromuscular junction

Stimulation-induced changes in presynaptic free calcium concentration ([Ca2 +](i)) were examined by fluorescent imaging at the spiny lobster exciter mo tor nerve terminals. The Ca2+ removal process in the terminal was analyzed based on a single compartment model, under the assumption that the Ca2+ rem oval rate from the terminal cytoplasm is proportional to nth power of [Ca2](i). During 100 nerve stimuli at 10-100 Hz, [Ca2+](i) reached a plateau th at 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 l arger at high [Ca2+](i) (>1.5 mu M), but was smaller at low [Ca2+](i) (< 1 mu M). These results suggest that a cooperative Ca2+ removal mechanism of n = 2, such as mitochondria, may play an important role in the terminal. Thi s view is supported by the gradual increase in the [Ca2+](i) plateau during long-term stimulation at 20-50 Hz for 60 s and by the existence of a very slow [Ca2+](i) recovery process after this stimulation, both of which may b e due to accumulation of Ca2+ in the organelle.