Two modes of stimulation by ammonia of taurine release from cultured rabbit Muller cells

A previous study revealed that a 10-min ('acute') treatment of cultured Muller glia with ammonium ions (further referred to as 'ammonia') at 0.5-5 mM concentration stimulated the release of newly loaded taurine (Tau) by a cAMP-dependent, osmoresistant mechanism. Here we showed that a 24 h treatment of the cells with 1 mM ammonia increased both Tau release and intracellular cAMP content in a degree similar to acute treatment with 5 mM ammonia, and the effects were similarly resistant to an increase of medium tonicity by addition of 50 mM sucrose. A 65 min superfusion of the cells with a guanylate cyclase inhibitor [methylene blue (MB)], a protein kinase inhibitor (H7) and a calcium-free buffer containing 10 mM Mg2+ (OCa-10Mg) also increased Tau release and cAMP level in the cells. Acute treatment with 5 mM ammonia of cells pretreated for 24 h with 1 mM ammonia or for 65 min with MB, H7 or OCa-10Mg produced additional significant stimulation of Tau release, without further increasing the cAMP level in the cells. By contrast, a 10-min treatment with 65 mM KCl, which is a potent, cAMP independent stimulus of Tau release in untreated Muller glia, produced no further enhancement of Tau release in ammonia-, MB-, H7 or OCa-10Mg-pretreated cells. The results indicate that acute treatment with ammonia, on top of treatments that evoke Tau release associated with an increase of cAMP, produces an extra Tau release that is cAMP-independent. Tau released by this extra ammonia treatment possibly originates from a different pool than Tau liberated by the pretreatments or 65 mM KCl. (C) 1997 Elsevier Science Ltd.