Obaid A L, Flores R, Salzberg B M
David Mahoney Institute of Neurological Science, Department of Physiology, School of Medicine, University of Pennsylvania, Philadelphia 19104-6085.
J Gen Physiol. 1989 Apr;93(4):715-29. doi: 10.1085/jgp.93.4.715.
Extrinsic absorption changes exhibited by potentiometric dyes have established the ionic basis of the action potential in synchronously activated populations of nerve terminals in the intact neurohypophyses of amphibia and mammals (Salzberg et al., 1983; Obaid et al., 1983, 1985b). Also, large and rapid changes in light scattering, measured as transparency, have been shown to follow membrane depolarization and to be intimately associated with the release of neuropeptides from the nerve terminals of the mouse neurohypophysis (Salzberg et al., 1985; Gainer et al., 1986). We report some experiments that help to define the pharmacological profile of the calcium channels present in intact neurosecretory terminals of vertebrates. For these, we used the peptide toxin omega-conotoxin GVIA (1-5 microM) and the dihydropyridine compounds Bay-K 8644 and nifedipine (2-5 microM), together with the after-hyperpolarization of the nerve terminal action potential. This undershoot depends upon the activation of a calcium-mediated potassium channel, as suggested by its sensitivity to [Ca++]o and charybdotoxin. omega-conotoxin GVIA substantially reduced the after-hyperpolarization in neurosecretory terminals of Xenopus, while neither of the dihydropyridine compounds had any effect under conditions that mimic natural stimulation. The effects of these calcium channel modifiers on the action potential recorded optically from the terminals of the Xenopus neurohypophysis were faithfully reflected in the behavior of the light-scattering changes observed in the neurohypophysis of the CD-1 mouse. omega-conotoxin GVIA (5 microM) reduced the size of the intrinsic optical signal associated with secretion by 50%, while the dihydropyridines had little effect. These observations suggest that the type of calcium channel that dominates the secretory behavior of intact vertebrate nerve terminals is at least partially blocked by omega-conotoxin GVIA and is insensitive, under normal conditions, to dihydropyridines.
电位染料所显示的外在吸收变化，已确定了两栖动物和哺乳动物完整神经垂体中同步激活的神经末梢群动作电位的离子基础（萨尔茨贝格等人，1983年；奥贝德等人，1983年、1985年b）。此外，以透明度衡量的光散射的大幅快速变化，已被证明跟随膜去极化，并与小鼠神经垂体神经末梢释放神经肽密切相关（萨尔茨贝格等人，1985年；盖纳等人，1986年）。我们报告了一些实验，这些实验有助于确定脊椎动物完整神经分泌末梢中存在的钙通道的药理学特征。为此，我们使用了肽毒素ω-芋螺毒素GVIA（1-5微摩尔）和二氢吡啶化合物Bay-K 8644及硝苯地平（2-5微摩尔），同时结合神经末梢动作电位的超极化后电位。这种负后电位取决于钙介导的钾通道的激活，这一点从其对细胞外[Ca++]和蝎毒素的敏感性可以看出。ω-芋螺毒素GVIA显著降低了非洲爪蟾神经分泌末梢的超极化后电位，而在模拟自然刺激的条件下，这两种二氢吡啶化合物均无任何作用。这些钙通道调节剂对从非洲爪蟾神经垂体末梢光学记录的动作电位的影响，在CD-1小鼠神经垂体中观察到的光散射变化行为中得到了如实反映。ω-芋螺毒素GVIA（5微摩尔）使与分泌相关的内在光信号大小降低了50%，而二氢吡啶则几乎没有影响。这些观察结果表明，在完整脊椎动物神经末梢分泌行为中起主导作用的钙通道类型，至少部分被ω-芋螺毒素GVIA阻断，并且在正常条件下对二氢吡啶不敏感。
