The content and release of endogenous GABA in isolated horizontal cells of the goldfish retina.

The endogenous amino acid content of intact retinas and isolated horizontal cells was measured by high performance liquid chromatography (HPLC). In both of these preparations, the concentration of gamma-aminobutyric acid (GABA) was approximately 2 mM. The endogenous GABA of the isolated horizontal cells was specifically released upon depolarization of the cells by elevated external K+ concentrations. This K+ induced release of endogenous GABA has two components. There is a saturable, Ca2+-dependent portion which predominates with small depolarizations, and a Ca2+-independent fraction which is linearly dependent upon the membrane potential of the horizontal cells. The data presented suggest the possibility of a dual scheme for the release of GABA from goldfish horizontal cells. The first is a Ca2+-dependent, conventional synaptic release; and the second a Na+-driven, carrier mediated release, which is linearly dependent upon the cellular membrane potential and functions in the absence of extracellular Ca2+.