title: Mechanisms of Ca2+ transport in plasma membrane vesicles prepared from cultured pituitary cells. II. (Ca2+ + Mg2+)-ATPase-dependent Ca2+ transport activity.
abstract: Purified plasma membrane vesicles from GH3 rat anterior pituitary cells exhibit a Mg2+-ATP-dependent Ca2+ transport activity. Concentrative uptake of Ca2+ is abolished by exclusion of either Mg2+ or ATP or by inclusion of the Ca2+ ionophore A23187. Furthermore, addition of A23187 to vesicles which have reached a steady state of ATP-supported Ca2+ accumulation rapidly and completely discharges accumulated cation. Ca2+ uptake is unaffected by treatment of vesicles with oligomycin, the uncoupler CCCP, or valinomycin and is greatly reduced in non-plasma membrane fractions. Likewise, Ca2+ accumulation is not stimulated by oxalate, consistent with the plasma membrane origin of this transport system. (Na+, K+)-ATPase participation in the Ca2+ transport process (i.e. via coupled Na+/Ca2+ exchange) was eliminated by omitting Na+ and including ouabain in the reaction medium. Ca2+ transport activity in GH3 vesicles has a similar pH dependence as that seen in a number of other plasma membrane systems and is inhibited by orthovanadate in the micromolar range. Inhibition is enhanced if the membranes are preincubated with vanadate for a short time. A kinetic analysis of transport indicates that the apparent Km for free Ca2+ and ATP are 0.7 and 125 microM, respectively. The average Vmax is 3.6 nmol of Ca2+/min/mg of protein at 37 degrees C. Addition of exogenous calmodulin or calmodulin antagonists had no significant effect on these kinetic properties. GH3 plasma membranes also contain a Na+/Ca2+ exchange system. The apparent Km for Ca2+ is almost 10-fold higher in this system than that for ATP-driven Ca2+ uptake. When both processes are compared under similar conditions, the Vmax of the exchanger is approximately 2-3 times that of ATP-dependent Ca2+ accumulation. Similar results are obtained when purified plasma membranes from bovine anterior pituitary glands were investigated. It is suggested that both Na+/Ca2+ exchange and the (Ca2+ + Mg2+)-ATPase are important in controlling intracellular levels of Ca2+ in anterior pituitary cells.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013997:Time Factors; D014634:Valinomycin / Q000494:pharmacology
chemical_list: D014634:Valinomycin; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D000255:Adenosine Triphosphate; D012964:Sodium; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium