Calcium dynamics and buffering in motoneurones of the mouse spinal cord.
1. A quantitative analysis of endogenous calcium homeostasis was performed on 65 motoneurones in slices of the lumbar spinal cord from 2- to 8-day-old mice by simultaneous patch-clamp and microfluorometric calcium measurements. 2. Somatic calcium concentrations were monitored with a temporal resolution in the millisecond time domain. Measurements were performed by using a monochromator for excitation and a photomultiplier detection system. 3. Somatic calcium signalling was investigated during defined voltage-clamp protocols. Calcium responses were observed for membrane depolarizations positive to -50 mV. A linear relation between depolarization time and free calcium concentrations ([Ca2+]i) indicated that voltage-dependent calcium influx dominated the response. 4. Endogenous calcium homeostasis was quantified by using the 'added buffer' approach. In the presence of fura-2 and mag-fura-5, calcium transients decayed according to a monoexponential function. Decay-time constants showed a linear dependence on dye concentration and the extrapolated constant in the absence of indicator dye was 371 +/- 120 ms (n = 13 cells, 21 C). 5. For moderate elevations (< 1 microM), recovery kinetics of depolarization-induced calcium transients were characterized by a calcium-independent, 'effective' extrusion rate gamma = 140 +/- 47 s-1 (n = 13 cells, 21 C). 6. The endogenous calcium binding ratio for fixed buffers in spinal motoneurones was kappaB' = 50 +/- 17 (n = 13 cells), indicating that less than 2 % of cytosolic calcium ions contributed to [Ca2+]i. 7. Endogenous binding ratios in spinal motoneurones were small compared to those found in hippocampal or cerebellar Purkinje neurones. From a functional perspective, they provided motoneurones with rapid dynamics of cytosolic [Ca2+]i for a given set of influx, extrusion and uptake mechanisms. 8. With respect to pathophysiological conditions, our measurements are in agreement with a model where the selective vulnerability of spinal motoneurones during excitotoxic conditions and motoneurone disease partially results from low endogenous calcium buffering.
Palecek
J
J
Zentrum Physiologie und Pathophysiologie, Universitat Gottingen, Humboldtallee 23, 37073 Gottingen, Germany.
Lips
M B
MB
Keller
B U
BU
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D005470:Fluorometry; D016257:Fura-2 / Q000031:analogs & derivatives; D006706:Homeostasis; D051379:Mice; D016472:Motor Neuron Disease / Q000378:metabolism; D009046:Motor Neurons / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011485:Protein Binding; D011689:Purkinje Cells / Q000378:metabolism; D013116:Spinal Cord / Q000378:metabolism*
D015220:Calcium Channels; C094537:mag-Fura 5; D002118:Calcium; D016257:Fura-2