Freshly dissociated neuronal somata of the crab (Cardisoma carnifex) X-organ were studied in the whole cell patch-clamp configuration. To characterize the Ca2+ currents in these somata, recordings were made under conditions designed to suppress K+ and Na+ currents. 2. In 52 mM external Ca2+ the threshold for activation of Ca2+ currents was above -40 mV, with peak amplitudes occurring around +10 to +20 mV. The full component of the current was available for activation at -50 mV because no current increase was observed when the holding potential was increased to -90 mV. These characteristics of the current characterize it as a high-voltage activated (HVA) current. 3. The Ca2+ current was almost completely (60-90%) inactivated within 200 ms at maximal current potentials (+10 to +20 mV). The decay was best described by a double-exponential function with a fast and slow component of inactivation (tau f = 12 ms and tau s = 64 ms). Both Sr2+ and Ba2+ substitutions reduced the rates of inactivation. 4. In double-pulse experiments, plots of variable prepulse potential versus test pulse current produced a U-shaped curve with test pulse currents showing maximal inactivation at potentials that produced maximal Ca2+ influx during the prepulse. Tail currents also displayed a U-shaped inactivation curve. The extent of current-dependent inactivation was sequentially reduced by Sr2+ and Ba2+ substitutions. These data suggest that inactivation in crab somata is predominantly Ca2+ dependent. The remaining inactivation of Ba2+ currents suggests that there is also a component of voltage-dependent inactivation in the somata. 5. Part of the inactivated Ca2+ current could be recovered during short (4-10 ms) hyperpolarizing pulses to -130 mV. The absolute extent of recovery from inactivation was greatest for currents carried by Ca2+ rather than Sr2+ or Ba2+. When voltage-dependent inactivation was dominant (Ba2+ currents), the relative amount of current recovered was greater. The data suggest that hyperpolarizing pulses are more effective in removing voltage-dependent inactivation, but also allow some recovery from Ca(2+)-dependent inactivation. 6. In the crab saline, which contained 24 mM Mg2+, the amplitudes of currents carried by 52 mM Ca2+, Sr2+ and Ba2+ were similar. Removing the Mg2+ from the saline augmented both the Ba2+ and Sr2+ currents relative to the Ca2+ current.(ABSTRACT TRUNCATED AT 400 WORDS)
摘要
采用全细胞贴片钳记录配置，对蟹（食蟹硬壳蟹）X器官新鲜解离的神经元胞体进行了研究。为了表征这些胞体中的钙电流，在旨在抑制钾电流和钠电流的条件下进行记录。2. 在外部钙浓度为52 mM时，钙电流激活阈值高于 -40 mV，峰值幅度出现在 +10至 +20 mV左右。当钳制电位增加到 -90 mV时未观察到电流增加，因此在 -50 mV时电流的完整成分可用于激活。电流的这些特性将其表征为高电压激活（HVA）电流。3. 在最大电流电位（+10至 +20 mV）下，钙电流在200 ms内几乎完全（60 - 90%）失活。失活的衰减最好用具有快速和慢速失活成分的双指数函数来描述（快速失活时间常数tau f = 12 ms，慢速失活时间常数tau s = 64 ms）。用Sr2+和Ba2+替代都降低了失活速率。4. 在双脉冲实验中，可变预脉冲电位与测试脉冲电流的关系图产生了一个U形曲线，测试脉冲电流在预脉冲期间产生最大钙内流的电位下显示出最大失活。尾电流也显示出U形失活曲线。用Sr2+和Ba2+替代依次降低了电流依赖性失活的程度。这些数据表明，蟹胞体中的失活主要是钙依赖性的。Ba2+电流剩余的失活表明胞体中也存在电压依赖性失活成分。5. 部分失活的钙电流在短时间（4 - 10 ms）超极化脉冲至 -130 mV期间可以恢复。从失活中恢复的绝对程度对于由Ca2+携带的电流最大，而不是Sr2+或Ba2+。当电压依赖性失活占主导（Ba2+电流）时，恢复的电流相对量更大。数据表明，超极化脉冲在消除电压依赖性失活方面更有效，但也允许从钙依赖性失活中部分恢复。6. 在含有24 mM Mg2+的蟹盐溶液中，52 mM Ca2+、Sr2+和Ba2+携带的电流幅度相似。从盐溶液中去除Mg2+会使Ba2+和Sr2+电流相对于Ca2+电流都增加。（摘要截于400字）
