低钙对鲫鱼视网膜中视杆、视锥信号向水平细胞传递的不同作用

DIFFERENTIAL EFFECTS OF LOW CALCIUM ON SIGNAL TRANSMISSION FROM RODS AND CONES TO HORIZONTAL CELLS IN CARP RITINA

本工作应用细胞内记录技术，在灌流的鲫鱼离体视网膜的标本上，研究了低钙对外网状层中视锥、视杆信号向水平细胞传递的不同影响。当降低灌流溶液中Ｃａ（２＋）浓度至０．１ｍｍｏｌ／Ｌ时使视锥－和视杆－水平细胞（Ｃｏｎｅ－ＨＣ和Ｒｏｄ－ＨＣ）均去极化，但对两者的光反应性有不同的影响：使Ｌ型Ｃｏｎｅ－ＨＣ对６８０ｕｍ和５００ｕｍ闪光反应的幅度有不同程度的减小，但使Ｒｏｄ－ＨＣ对光反应的幅度显著增大。在０．１ｍｍｏｌ／ＬＣａ（２＋）Ｒｉｎｇｅｒ液灌流时，无论是明视ＰⅢ（反映机锥活动）或暗视ＰⅢ（反映视杆活动），其反应的幅度均增大，提示发生在低钙时Ｃｏｎｅ－ＨＣ和Ｒｏｄ－ＨＣ光反应性的不同变化产生于突触传递的过程中。进而，磷酸二酯酶抑制剂ＩＢＭＸ对ＰⅢ的影响与０．１ｍｍｏｌ／ＬＣａ（２＋）Ｒｉｎｇｅｒ液的作用相似，而对Ｌ型Ｃｏｎｅ－ＨＣ和Ｒｏｄ－ＨＣ的作用均是使光反应性增大。这提示，在０．１ｍｍｏｌ／ＬＣａ（２＋）的Ｒｉｎｇｅｒ液中，Ｃｏｎｅ－ＨＣ和Ｒｏｄ－ＨＣ光反应性的变化差异可能是由于低钙对视锥和视杆终末Ｃａ２＋内流的不同影响而产生。

Intracellular recordings were made from neurons in the superfused isolated carp retina and the effects of calcium on synaptic transmission from cones and rods to horizontal cells were studied by changing the ionic composition of the external medium.When extracellular Ca2+ is lowered from 1.2 mmol/L to 0.1 mmol/L, both cone-and rod-horizontal cells (Cone-HC and Rod-HC) are depolarized, but light responsiveness of these cells are differentially affected: light responses of Cone-HCs are reduced in size, whereas those of Rod-HCs are significantly enhanced. In 0. 1 mmol/L Ca2+, glutamate-isolated P Ⅲ components, both photopic and scotopic, are enhanced, suggesting that the differential effects of low calcium on the horizontal cells must occur during signal transferring from photoreceptors to horizontal cells. Similar to the effect of 0.1 mmol/L Ca2+, phosphodiesterase inhibitor IBMX (50 μmol/L) is also capable of enhancing light responsiveness of both L-type Cone-HC and Rod-HC. These results suggest that the differentail effects of low calcium on the two types of horizontal cells may be due to different changes in the calcium influx into the receptor terminals.