Na(+)-Ca2+ exchangers from rod outer segments and cardiac sarcolemma: comparison of properties

The properties of the Na(+)-Ca2+ exchangers from cardiac sarcolemma (SL) and rod outer segments (ROS) were studied in parallel by measuring the Na+ gradient-dependent Ca2+ uptake into SL or ROS vesicles. The ROS exchanger, but not the SL exchanger, has a striking specific dependence on K+. The ROS exchanger is stimulated by K+ with an apparent concentration at half-maximum of 1 mM. The addition of valinomycin, to produce an inside-positive membrane potential, stimulates the SL exchanger 1.8-fold and the ROS exchanger 1.2-fold. The Michaelis constant for half-maximal transport rate for Ca2+ and the maximal transport rate for the exchangers, in the absence of valinomycin, are estimated to be 20 microM and 8 nmol.mg-1.2.2 s-1 in SL and 5 microM and 1 nmol.mg-1.2.2 s-1 in ROS. Both exchangers are modulated by the same regulatory influences. For example, both are stimulated by proteases, phospholipase D, and intravesicular Ca2+.