Callaway J C, Lasser-Ross N, Stuart A E, Ross W N
Department of Physiology, University of North Carolina, Chapel Hill 27599.
J Neurosci. 1993 Mar;13(3):1157-66. doi: 10.1523/JNEUROSCI.13-03-01157.1993.
At photoreceptor synapses, transmitter release is continuous and graded. At this type of synapse, the control of presynaptic [Ca2+]i and calcium's role in releasing transmitter might be different than at terminals invaded by all-or-none action potentials. To examine this possibility, we measured the spatial and temporal changes of [Ca2+]i in response to depolarization of individual photoreceptor terminals of the barnacle Balanus nubilus, which had been injected with the Ca2+ indicator Fura-2. Depolarizing pulses produced voltage-dependent Ca2+ entry that was confined to the tips of the arbor where the release sites are located. At increasing distances from the tips, the rate of [Ca2+]i increase was slower and the peak [Ca2+]i occurred later, suggesting that Ca2+ entered the tips and diffused back into the larger processes of the arbor. Consistent with this result, a stable gradient of [Ca2+]i was observed at maintained depolarizations, with the highest values at the tips of the arbor. Removal of external Na+ did not affect the time course of Ca2+ decline in the terminal, indicating that Na+/Ca2+ exchange was not the primary mechanism for restoring [Ca2+]i to basal levels. Computer simulations, assuming only Ca2+ entry at the arbor's tips and diffusion of Ca2+ away from the entry site, qualitatively reproduced these observations. The threshold for Ca2+ entry was near -60 mV, and entry was maintained during prolonged depolarizations, in agreement with previous experiments showing that Ca2+ channels in the terminal region do not inactivate. The time course of the measured [Ca2+]i change in the terminal paralleled voltage changes due to a Ca(2+)-activated K+ conductance, which senses [Ca2+]i just under the membrane. This parallelism is expected since the release sites are located on processes of small-enough diameter to permit radial equilibration of [Ca2+]i within the time course of physiological voltage changes. Therefore, the optical measurements reflect the mean level of [Ca2+]i under the membrane. Whether this mean concentration is also the value at the sites that trigger exocytosis will depend on how close the Ca2+ channels are to these sites.
在光感受器突触中，递质释放是连续且分级的。在这类突触中，突触前[Ca2+]i的调控以及钙在递质释放中的作用可能与全或无动作电位侵入的终末不同。为了检验这种可能性，我们测量了注射了Ca2+指示剂Fura-2的藤壶Balanus nubilus单个光感受器终末去极化时[Ca2+]i的时空变化。去极化脉冲产生了电压依赖性Ca2+内流，其局限于释放位点所在的树突尖端。离尖端距离增加时，[Ca2+]i增加的速率变慢且[Ca2+]i峰值出现得更晚，这表明Ca2+进入尖端并扩散回树突的较大分支。与该结果一致，在持续去极化时观察到[Ca2+]i的稳定梯度，树突尖端的值最高。去除细胞外Na+并不影响终末中Ca2+下降的时间进程，表明Na+/Ca2+交换不是将[Ca2+]i恢复到基础水平的主要机制。计算机模拟仅假设Ca2+在树突尖端进入并从进入位点扩散，定性地重现了这些观察结果。Ca2+进入的阈值接近 -60 mV，并且在长时间去极化期间保持进入，这与先前显示终末区域的Ca2+通道不会失活的实验一致。终末中测量到的[Ca2+]i变化的时间进程与由Ca(2+)-激活的K+电导引起的电压变化平行，该电导在膜下感知[Ca2+]i。这种平行性是预期的，因为释放位点位于直径足够小的分支上，以允许在生理电压变化的时间进程内[Ca2+]i进行径向平衡。因此，光学测量反映了膜下[Ca2+]i的平均水平。这个平均浓度是否也是触发胞吐作用位点的值将取决于Ca2+通道与这些位点的接近程度。
