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-activated⁴⁵Ca accumulation. This calcium uptake requires magnesium and is specific for ATP; Other nucleotides, ITP, GTP, UTP and the non-hydrolysable analogue of ATP --methylene ATP did not substitute for ATP.⁴⁵Ca accumulation was inhibited by mersalyl, ethylmaleimide, ruthenium red, oligomycin and dicyclohexylcarbodiimide and was unaffected by ouabain. Addition of taurine to the incubation medium enhanced⁴⁵Ca uptake in a concentration-dependent manner; increases of more than 100% being produced by 25 mM taurine. The taurine-induced stimulation of⁴⁵Ca uptake was also sensitive to the tested inhibitors. The effect of taurine was only exerted on the bicarbonate-dependent, ATP-activated⁴⁵Ca uptake. Calcium accumulation observed in the absence of ATP or in a tris-buffered medium was unaffected by taurine. Other amino acids, glycine, GABA, -alanine, glutamic acid and the taurine analogue guanidinoethyl-sulfonate did not stimulate⁴⁵Ca uptake. These results suggest that taurine is affecting a Mg-ATPase activity responsible for calcium accumulation in frog ROS.