Nasi E, Gomez M P
Department of Physiology, Boston University School of Medicine, Massachusetts 02118.
J Gen Physiol. 1992 May;99(5):747-69. doi: 10.1085/jgp.99.5.747.
Retinas from the scallop Pecten irradians were enzymatically dispersed, yielding a large number of isolated photoreceptors suitable for tight-seal recording. Whole-cell voltage clamp measurements demonstrated that the phototransducing machinery remained intact: quantum bumps could be elicited by dim illumination, while brighter flashes produced larger, smooth photocurrents. Single-channel currents specifically activated by light were recorded in cell-attached patches, and were almost exclusively confined to the rhabdomeric region. Their density is sufficiently high to account for the macroscopic photoresponse. Channel activation is graded with stimulus intensity in a range comparable to that of the whole-cell response, and can be recorded with illumination sufficiently dim to evoke only quantum bumps. Light-dependent channel openings are very brief, on average 1 ms or less at 20-22 degrees C, apparently not because of blockage by extracellular divalent cations. The mean open time does not change substantially with stimulus intensity. In particular, since dwell times are in the millisecond range even with the dimmest lights, the channel closing rate does not appear to be the rate-limiting step for the decay kinetics of discrete waves. The latency of the first opening after light onset is inversely related to light intensity, and the envelope of channel activity resembles the time course of the whole-cell photocurrent. Unitary currents are inward at resting potential, and have a reversal voltage similar to that of the macroscopic light response. Voltage modulates the activity of light-sensitive channels by increasing the opening rate and also by lengthening the mean open times as the patch is depolarized. The unitary conductance of the predominant class of events is approximately 48 pS, but at least one additional category of smaller-amplitude openings was observed. The relative incidence of large and small events does not appear to be related in a simple way to the state of adaptation of the cell.
将海湾扇贝的视网膜进行酶解分散，得到大量适合进行紧密封接记录的分离光感受器。全细胞电压钳测量表明，光转导机制保持完整：弱光照射可引发量子 bumps，而较强闪光则产生更大的平滑光电流。在细胞贴附式膜片中记录到了由光特异性激活的单通道电流，且几乎完全局限于视杆区域。其密度足够高，足以解释宏观光反应。通道激活在与全细胞反应相当的强度范围内随刺激强度分级，并且可以在足够暗的光照下记录，仅引发量子 bumps。光依赖性通道开放非常短暂，在20 - 22摄氏度时平均为1毫秒或更短，显然不是因为细胞外二价阳离子的阻断。平均开放时间不会随刺激强度而大幅变化。特别是，由于即使在最暗的光线下停留时间也在毫秒范围内，通道关闭速率似乎不是离散波衰减动力学的限速步骤。光开始后第一个开放的潜伏期与光强度呈反比，通道活动的包络类似于全细胞光电流的时间进程。在静息电位下，单位电流是内向的，并且具有与宏观光反应相似的反转电压。电压通过增加开放速率以及在膜片去极化时延长平均开放时间来调节光敏感通道的活性。主要事件类别的单位电导约为48 pS，但至少观察到了另一类较小幅度开放。大小事件的相对发生率似乎与细胞的适应状态没有简单的关系。
