High-affinity Ca2+-Mg2+-ATPase in kidney of euryhaline Gillichthys mirabilis: kinetics, subcellular distribution and effects of salinity

Abstract

1. 1. Two components of Ca2+-Mg2+-ATPase are observed in kidneys of G. mirabilis. The high-affinity component has a K0.5Ca of 0.23[mu]M; the low-affinity activity K0.5Ca is 90-110[mu]M. The high-affinity activity requires Mg2+, displays Michaelis-Menten kinetics, has peak activity at 1.2 [mu]M Ca2+, and is insensitive to ouabain and Na+ azide.2. 2. In subcellular fractions, the high-affinity component segregates with Na+-K+-ATPase and is localized predominantly in BLM. The low-affinity component is broadly distributed among membranous organelles, including brush border, and may be equivalent to alkaline phosphatase.3. 3. Specific activity of the high-affinity Ca2+-Mg2+-ATPase is modestly increased following adaptation of fish to FW, but total renal high-affinity activity is greatest in the hypertrophied kidneys of FW-adapted fish and is least in kidneys of fish adapted to 200% SW.4. 4. High-affinity Ca2+-Mg2+-ATPase may be associated with active Ca2+ transport or with regulation of intracellular Ca2+ concentration of tubular cells.