CA2+-DEPENDENT RELEASE OF [H-3] GABA IN CULTURED CHICK RETINA CELLS

Depolarization by K+ (50 mM) of cultured chick retina cells released 1.14+/-0.28% of the accumulated [H-3]gamma-aminobutyric acid (GABA) in the absence of Ca2+, but when 1.0 mM Ca2+ was present, the internal free calcium ion concentration [Ca2+]i rose by about 750 nM and the [H-3]GABA release about doubled to a value of 2.22+/-0.2% of the total [H-3]GABA. Nitrendipine (0.1 muM), a blocker of the L-type Ca2+ channels, blocked the [Ca2+]i response to K+ depolarization by about 65%, and the omega-Conotoxin GVIA (omega-CgTx) (0.5 muM), a blocker of the N-type of Ca2+ channels, inhibited by 27% the [Ca2+]i rise due to K+ depolarization. Parallel experiments showed that nitrendipine inhibits [H-3]GABA release to the level observed in the absence of Ca2+, whereas omega-CgTx did not inhibit significantly the release of [H-3]GABA. The results also show that the release of [H-3]GABA due to K+-depolarization in the absence of Ca2+ can be totally blocked by 1-(2-(((Diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (NNC-711), an inhibitor of the GABA carrier. However, in the presence of Ca2+, NNC-711 blocks the release only by about 66%, corresponding to the Ca2+-independent release. Thus, it is concluded that [H-3]GABA is released in chick retina cells by the exocytotic mechanism, which is Ca2+-dependent, and by reversal of the carrier, which is Ca2+-independent, in much the same way as has been found for other GABAergic neurons. Previous reports that cultured chick retina cells show no Ca2+-dependent release of GABA should be re-examined in light of our results obtained with a superfusion system.