HIGHLY COOPERATIVE FEEDBACK-CONTROL OF RETINAL ROD GUANYLATE-CYCLASE BY CALCIUM-IONS
HIGHLY COOPERATIVE FEEDBACK-CONTROL OF RETINAL ROD GUANYLATE-CYCLASE BY CALCIUM-IONS
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DOI：
10.1038/334064a0
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发表时间：
1988-07-07
期刊：
影响因子：
64.8
通讯作者：
STRYER, L
中科院分区：
文献类型：
--
作者：
KOCH, KW;STRYER, L
视网膜视杆细胞中的视觉兴奋是由级联介导的，该级联导致环GMP（cGMP）的放大水解和随后质膜中cGMP激活的阳离子特异性通道的关闭1 -3。黑暗状态的恢复需要cGMP的再合成，这是由鸟苷酸环化酶催化的，鸟苷酸环化酶是一种轴突相关酶4 -6。光照7 - 10后细胞溶质钙浓度（Cai）的降低被认为在刺激环化酶活性方面很重要11，12。这一假设得到以下发现的支持：当Cais降低至小于10 nM时，视杆外节的cGMP含量增加数倍13 -16。很明显，cGMP和Cailevel受负反馈的间接控制1，2。当钙水平从10 μM降低到10 nM时，蟾蜍ROS的鸟苷酸环化酶受到强烈刺激，但仅当它们被光激发时17。我们在这里表明，鸟苷酸环化酶活性的未照明的牛杆外节显着增加（5至20倍）时，钙水平从200 nM降低到50 nM。鸟苷酸环化酶活性在生理范围内对钙水平的这种陡峭依赖性具有3.9的希尔系数。在低钙水平下的刺激由蛋白质介导，该蛋白质可以通过用低盐缓冲液洗涤而从外节膜释放。通过将可溶性提取物加回洗涤过的膜，钙敏感性部分恢复。通过光诱导的Cai降低而高度协同激活鸟苷酸环化酶可能是激发后恢复暗电流的关键事件。
Visual excitation in retinal rod cells is mediated by a cascade that leads to the amplified hydrolysis of cyclic GMP (cGMP) and the consequent closure of cGMP-activated cation-specific channels in the plasma membrane1–3. Recovery of the dark state requires the resynthesis of cGMP, which is catalysed by guanylate cyclase, an axoneme-associated enzyme4–6. The lowering of the cytosolic calcium concentration (Cai) following illumination7–10is thought to be important in stimulating cyclase activity11,12. This hypothesis is supported by the finding that the cGMP content of rod outer segments increases several-fold when Caiis lowered to less than 10 nM13–16. It is evident that cGMP and Cailevels are reciprocally controlled by negative feedback1,2. Guanylate cyclase from toad ROS is strongly stimulated when the calcium level is lowered from 10 μM to 10 nM, but only if they are excited by light17. We show here that the guanylate cyclase activity of unilluminated bovine rod outer segments increases markedly (5 to 20-fold) when the calcium level is lowered from 200 nM to 50 nM. This steep dependence of guanylate cyclase activity on the calcium level in the physiological range has a Hill coefficient of 3.9. Stimulation at low calcium levels is mediated by a protein that can be released from the outer segment membranes by washing with a low salt buffer. Calcium sensitivity is partially restored by adding the soluble extract back to the washed membranes. The highly cooperative activation of guanylate cyclase by the light-induced lowering of Caiis likely to be a key event in restoring the dark current after excitation.