Hackos D H, Korenbrot J I
Department of Physiology and Graduate Program in Biophysics, School of Medicine, University of California at San Francisco, San Francisco, California 94143, USA.
J Gen Physiol. 1999 Jun;113(6):799-818. doi: 10.1085/jgp.113.6.799.
The selectivity of Ca2+ over Na+ is approximately 3.3-fold larger in cGMP-gated channels of cone photoreceptors than in those of rods when measured under saturating cGMP concentrations, where the probability of channel opening is 85-90%. Under physiological conditions, however, the probability of opening of the cGMP-gated channels ranges from its largest value in darkness of 1-5% to essentially zero under continuous, bright illumination. We investigated the ion selectivity of cGMP-gated channels as a function of cyclic nucleotide concentration in membrane patches detached from the outer segments of rod and cone photoreceptors and have found that ion selectivity is linked to gating. We determined ion selectivity relative to Na+ (PX/PNa) from the value of reversal potentials measured under ion concentration gradients. The selectivity for Ca2+ over Na+ increases continuously as the probability of channel opening rises. The dependence of PCa/PNa on cGMP concentration, in both rods and cones, is well described by the same Hill function that describes the cGMP dependence of current amplitude. At the cytoplasmic cGMP concentrations expected in dark-adapted intact photoreceptors, PCa/PNa in cone channels is approximately 7.4-fold greater than that in rods. The linkage between selectivity and gating is specific for divalent cations. The selectivity of Ca2+ and Sr2+ changes with cGMP concentration, but the selectivity of inorganic monovalent cations, Cs+ and NH4+, and organic cations, methylammonium+ and dimethylammonium+, is invariant with cGMP. Cyclic nucleotide-gated channels in rod photoreceptors are heteromeric assemblies of alpha and beta subunits. The maximal PCa/PNa of channels formed from alpha subunits of bovine rod channels is less than that of heteromeric channels formed from alpha and beta subunits. In addition, Ca2+ is a more effective blocker of channels formed by alpha subunits than of channels formed by alpha and beta subunits. The cGMP-dependent shift in divalent cation selectivity is a property of alphabeta channels and not of channels formed from alpha subunits alone.
在饱和cGMP浓度下进行测量时，视锥光感受器的cGMP门控通道对Ca2+的选择性比视杆光感受器的cGMP门控通道对Na+的选择性大约大3.3倍，此时通道开放的概率为85 - 90%。然而，在生理条件下，cGMP门控通道的开放概率范围从黑暗中最大值的1 - 5%到持续强光照射下基本为零。我们研究了从视杆和视锥光感受器外段分离的膜片中cGMP门控通道的离子选择性与环核苷酸浓度的关系，发现离子选择性与门控相关。我们根据在离子浓度梯度下测量的反转电位值确定相对于Na+的离子选择性（PX/PNa）。随着通道开放概率的增加，对Ca2+相对于Na+的选择性持续增加。视杆和视锥中PCa/PNa对cGMP浓度的依赖性，都可以用描述电流幅度对cGMP依赖性的相同希尔函数很好地描述。在暗适应的完整光感受器中预期的细胞质cGMP浓度下，视锥通道中的PCa/PNa比视杆通道中的大约大7.4倍。选择性与门控之间的联系对二价阳离子具有特异性。Ca2+和Sr2+的选择性随cGMP浓度变化，但无机单价阳离子Cs+和NH4+以及有机阳离子甲基铵+和二甲基铵+的选择性不随cGMP变化。视杆光感受器中的环核苷酸门控通道是α和β亚基的异源寡聚体。由牛视杆通道α亚基形成的通道的最大PCa/PNa小于由α和β亚基形成的异源通道的最大PCa/PNa。此外，Ca2+对由α亚基形成的通道的阻断作用比对由α和β亚基形成的通道更有效。二价阳离子选择性的cGMP依赖性变化是αβ通道的特性，而不是仅由α亚基形成的通道的特性。
