Spontaneous and evoked release of [3H]taurine from a P2 subcellular fraction of the rat retina

The effects of spontaneous and evoked ³H]taurine release from a P₂ fraction prepared from rat retinas were studied. The P₂ fraction was preloaded with ³H]taurine under conditions of high-affinity uptake and then examined for ³H]taurine efflux utilizing superfusion techniques. Exposure of the P₂ fraction to high K⁺ (56 mM) evoked a Ca²⁺-independent release of ³H]taurine. Li⁺ (56 mM) and veratridine (100 M) had significantly less effect (8–15% and 15–30%, respectively) on releasing ³H]taurine compared to the K⁺-evoked release. 4-Aminopyridine (1 mM) had no effect on the release of ³H]taurine. The spontaneous release of ³H]taurine was also Ca²⁺-independent. When Na⁺ was omitted from the incubation medium K⁺-evoked ³H]taurine release was inhibited by approximately 40% at the first 5 minute depolarization period but was not affected at a second subsequent 5 minute depolarization period. The spontaneous release of ³H]taurine was inhibited by 60% in the absence of Na⁺. Substitution of Br^– for Cl^– had no effect on the release of either spontaneous or K⁺-evoked ³H]taurine release. However, substitution of the Cl^– with acetate, isethionate, or gluconate decreased K⁺-evoked ³H]taurine release. Addition of taurine to the superfusion medium (homoexchange) resulted in no significant increase in ³H]taurine efflux. The taurine-transport inhibitor guanidinoethanesulfonic acid increased the spontaneous release of ³H]taurine by approximately 40%. These results suggest that the taurine release of ³H]taurine is not simply a reversal of the carrier-mediated uptake system. It also appears that taurine is not released from vesicles within the synaptosomes but does not rule out the possibility that taurine is a neurotransmitter. The data involving chloride substitution with permeant and impermeant anions support the concept that the major portion of ³H]taurine release is due to an osmoregulatory action of taurine while depolarization accounts for only a small portion of ³H]taurine release.