Cobbett P, Ingram C D, Mason W T
Department of Neuroendocrinology, AFRC Institute of Animal Physiology and Genetics Research, Babraham, Cambridge, U.K.
Neuroscience. 1987 Nov;23(2):661-77. doi: 10.1016/0306-4522(87)90084-4.
The properties of whole cell Ba2+ currents were studied in immunocytochemically identified, normal bovine lactotrophs using the patch clamp technique. In the current clamp mode, current-induced and spontaneous Ba2+ action potentials were recorded. These were of longer duration and showed less inactivation with stimulation frequency when compared with Na+ action potentials. Under voltage clamp, isolated Ba2+ currents had an activation threshold of about -35 mV and peak value at -15 mV to +20 mV. Inactivation of the current to a potential-dependent, non-zero steady-state level indicated the presence of one rapidly and one slowly inactivating component to the current. These two components were also distinguished by: (1) the voltage dependence of the inactivation time constant of the current, (2) the differential frequency-dependent inactivation of the peak and steady-state currents, and (3) the presence of two half-inactivation potentials for the current. Analysis of the ensemble current variance of the non-inactivating component gave a single-channel amplitude of 0.19 pA at 0 mV and a slope conductance of 3 pS. Fluctuation analysis of the voltage-activated Ba2+ current noise revealed two time constants, one which was voltage dependent and the other was independent of potential. The contribution of these two currents to Ca2+-dependent hormone secretion remains to be clarified.
运用膜片钳技术，在经免疫细胞化学鉴定的正常牛催乳细胞中研究了全细胞Ba2+电流的特性。在电流钳模式下，记录了电流诱导的和自发的Ba2+动作电位。与Na+动作电位相比，这些动作电位持续时间更长，并且在刺激频率增加时失活较少。在电压钳条件下，分离出的Ba2+电流的激活阈值约为 -35 mV，峰值出现在 -15 mV至 +20 mV之间。电流失活至一个依赖电位的非零稳态水平，表明电流存在一个快速失活成分和一个缓慢失活成分。这两个成分还可通过以下方式区分：(1) 电流失活时间常数的电压依赖性；(2) 峰值电流和稳态电流的频率依赖性失活差异；(3) 电流存在两个半失活电位。对非失活成分的总体电流方差分析得出，在0 mV时单通道幅度为0.19 pA，斜率电导为3 pS。对电压激活的Ba2+电流噪声的波动分析揭示了两个时间常数，一个与电压有关，另一个与电位无关。这两种电流对Ca2+依赖性激素分泌的贡献仍有待阐明。
