Native Xenopus oocytes were voltage clamped and exposed to Ringer solutions containing low concentrations of divalent cations. Oocytes, held at -60 mV, developed a reversible non-inactivating smooth inward current (Ic) associated with an increase in membrane conductance. 2. Ic was selectively carried by cations (Na+, K+), indicating that the current was not the result of a non-specific membrane breakdown, but was due instead to removal of a blocking effect of divalent cations on a specific population of endogenous ionic channels located in the oocyte membrane. 3. The blocking effects of Ca2+ and Mg2+ were voltage dependent, implying action at a binding site within the pore of the cationic channel. For example, the half-maximal inhibition (IC50) of Ic by Ca2+ was 61 microM in oocytes held at -60 mV and 212 microM in oocytes held at 0 mV. 4. The Ic channels could be unblocked by depolarization of the membrane even in the presence of physiological concentrations of Ca2+ or Mg2+. The unblocking of the channels was observed as a slowly developing outward current. 5. The novel cationic current was substantially reduced following in vitro maturation of oocytes by treatment with progesterone (10 microM, 4-5 h). 6. The physiological role of Ic channels remains to be elucidated. Nonetheless, their characteristics explain the ionic basis of the sensitivity of oocytes to reductions in extracellular divalent cations and raise the possibility that the channels play a role in calcium homeostasis.
摘要
对非洲爪蟾的卵母细胞进行电压钳制，并将其暴露于含有低浓度二价阳离子的林格氏液中。将卵母细胞钳制在 -60 mV，会产生一种可逆的、非失活的平滑内向电流（Ic），这与膜电导的增加有关。2. Ic 选择性地由阳离子（Na⁺、K⁺）携带，这表明该电流不是非特异性膜破裂的结果，而是由于二价阳离子对位于卵母细胞膜上的特定内源性离子通道群体的阻断作用被消除。3. Ca²⁺ 和 Mg²⁺ 的阻断作用是电压依赖性的，这意味着在阳离子通道孔内的结合位点起作用。例如，在钳制于 -60 mV 的卵母细胞中，Ca²⁺ 对 Ic 的半数最大抑制（IC50）为 61 μM，而在钳制于 0 mV 的卵母细胞中为 212 μM。4. 即使在存在生理浓度的 Ca²⁺ 或 Mg²⁺ 的情况下，通过膜去极化也可使 Ic 通道解除阻断。通道的解除阻断表现为缓慢发展的外向电流。5. 在用孕酮（10 μM，4 - 5 小时）处理使卵母细胞体外成熟后，这种新型阳离子电流大幅降低。6. Ic 通道的生理作用仍有待阐明。尽管如此，它们的特性解释了卵母细胞对细胞外二价阳离子减少敏感性的离子基础，并增加了这些通道在钙稳态中起作用的可能性。
