Lu J, Fishman H M
Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77555-0641.
Biophys J. 1994 Oct;67(4):1525-33. doi: 10.1016/S0006-3495(94)80626-5.
The exquisite sensitivity of elasmobranch fishes to electric fields is thought to reside in electroreceptive organs called ampullae of Lorenzini. We measured the stimulus-response behavior of ampullary organs excised from skates. Under open-circuit conditions, the ampullary organ showed three distinct response states: spontaneous repetitive spikes, evoked spikes, and small, damped oscillatory responses. Under short-circuit conditions, the amplitude range for a linear current response to a sinusoidal (0.5 Hz) voltage clamp of an organ (assessed by spectral analysis of the harmonics generated) was 7-200 microV rms. Changes in the spike firing rate of the afferent nerve innervating the organ were evident for voltage clamps of the ampullary epithelium of 3 microV and the spike rate saturated for clamp steps exceeding 100 microV. Thus, the linear response range of the ampullary epithelium exceeded the range in spike firing rate of the afferent nerve. The steady-state transorgan electrical properties under voltage clamp conditions were obtained by analysis of complex admittance determinations in the frequency range 0.05-20 Hz for perturbations (< 100 microV rms) in the linear range. Admittance functions were distinctly related to the preparation states observed under open-circuit conditions. A negative real part in the organ admittance (i.e., a steady-state negative conductance generated by the preparation) was a common characteristic of the two (open-circuit) excitable states. The negative conductance was also confirmed by the direction of current flow through the ampullary epithelium in response to step voltage clamps. We conclude that the steady state-negative conductance is an essential property of the ampullary epithelium,and we suggest that the interplay of negative and positive conductances generated by ion channels in apical and basal membranes of receptor cells results in signal amplification that may contribute significantly to the electric field sensitivity of ampullary organs.
人们认为，软骨鱼类对电场的极高敏感性存在于一种名为罗伦氏壶腹的电感受器官中。我们测量了从鳐鱼身上切除的壶腹器官的刺激-反应行为。在开路条件下，壶腹器官表现出三种不同的反应状态：自发重复尖峰、诱发尖峰和小的、衰减的振荡反应。在短路条件下，器官对正弦（0.5赫兹）电压钳的线性电流反应的幅度范围（通过对产生的谐波进行频谱分析来评估）为7至200微伏均方根值。对于壶腹上皮电压钳为3微伏时，支配该器官的传入神经的尖峰发放率有明显变化，而对于超过100微伏的钳位阶跃，尖峰率达到饱和。因此，壶腹上皮的线性反应范围超过了传入神经尖峰发放率的范围。通过分析在0.05至20赫兹频率范围内、线性范围内扰动（<100微伏均方根值）时的复导纳测定，获得了电压钳条件下的稳态跨器官电学特性。导纳函数与开路条件下观察到的标本状态明显相关。器官导纳中的负实部（即标本产生的稳态负电导）是两种（开路）可兴奋状态的共同特征。通过阶跃电压钳时通过壶腹上皮的电流方向也证实了负电导。我们得出结论，稳态负电导是壶腹上皮的一个基本特性，并且我们认为受体细胞顶端和基底膜中离子通道产生的正负电导之间的相互作用导致信号放大，这可能对壶腹器官的电场敏感性有显著贡献。
