Taurine activation of a bicarbonate-dependent, ATP-supported calcium uptake in frog rod outer segments

Isolated frog rod outer segments (ROS) with a leaky plasma membrane showed a bicarbonate-dependent, ATP-activated 45Ca accumulation. This calcium uptake requires magnesium and is specific for ATP; other nucleotides, ITP, GTP, UTP and the non-hydrolysable analogue of ATP beta-gamma-methylene ATP did not substitute for ATP. 45Ca accumulation was inhibited by mersalyl, ethylmaleimide, ruthenium red, oligomycin and dicyclohexylcarbodiimide and was unaffected by ouabain. Addiction of taurine to the incubation medium enhanced 45Ca uptake in a concentration-dependent manner; increases of more than 100% being produced by 25 mM taurine. The taurine-induced stimulation of 45Ca uptake was also sensitive to the tested inhibitors. The effect of taurine was only exerted on the bicarbonate-dependent, ATP-activated 45Ca uptake. Calcium accumulation observed in the absence of ATP or in a tris-buffered medium was unaffected by taurine. Other amino acids, glycine, GABA, beta-alanine, glutamic acid and the taurine analogue guanidinoethyl-sulfonate did not stimulate 45Ca uptake. These results suggest that taurine is affecting a Mg-ATPase activity responsible for calcium accumulation in frog ROS.