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 excitor motor nerve terminals. The Ca2+ removal process in the terminal was analyzed based on a single compartment model, under the assumption that the Ca2+ removal 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 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 [Ca2+]i (>1.5 microM), but was smaller at low [Ca2+]i (<1 microM). These results suggest that a cooperative Ca2+ 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 [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 be due to accumulation of Ca2+ in the organelle.
Ohnuma
K
K
Department of Physics, School of Science, Nagoya University, Nagoya 464-8602, Japan.
Kazawa
T
T
Ogawa
S
S
Suzuki
N
N
Miwa
A
A
Kijima
H
H
D000818:Animals; D003400:Astacoidea / Q000378:metabolism; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism*; D004558:Electric Stimulation; D005456:Fluorescent Dyes; D066298:In Vitro Techniques; D017136:Ion Transport; D007700:Kinetics; D008959:Models, Neurological; D008121:Nephropidae / Q000378:metabolism*; D009469:Neuromuscular Junction / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
D005456:Fluorescent Dyes; D002118:Calcium