title: Voltage-dependence of Ca2+ uptake and ATP hydrolysis of reconstituted Ca2+-ATPase vesicles.
abstract: Ca2+-ATPase from sarcoplasmic reticulum was reconstituted into phospholipid/cholesterol (9:1) vesicles (RO). Sucrose density gradient centrifugation of the RO vesicles separated a light layer (RL) with a high lipid/protein ratio and a heavy layer (RH). RH vesicles exhibited a high rate of Ca2+-dependent ATP hydrolysis but did not accumulate Ca2+. RL vesicles, on the other hand, showed an initial molar ratio of Ca2+ uptake to ATP hydrolysis of approximately 1.0. Internal trapping of transported Ca2+ facilitated studies over periods of several minutes. Ca2+ transport and ATP hydrolysis declined concomitantly, reaching levels near 0 with external Ca2+ concentrations less than or equal to 2 microM. Ca2+ uptake was inhibited by the Ca2+ ionophore A23187, the detergent Triton X-100, and the metabolic inhibitor quercetin. Ca2+ transport generated a transient electrical potential difference, inside positive. This finding is consistent with the hypothesis that the Ca2+ pump is electrogenic. Steady state electrical potentials across the membrane were clamped by using potassium gradients and valinomycin, and monitored with voltage-sensitive dyes. Over a range of +50 to -100 mV, there was an inverse relationship between the initial rate of Ca2+ uptake and voltage, but the rate of ATP hydrolysis was nearly constant. In contrast, lowering the external Ca2+ concentration depressed both transport and ATP hydrolysis. These findings suggest that the membrane voltage influences the coupling between Ca2+ transport and ATP hydrolysis.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D006868:Hydrolysis; D007700:Kinetics; D008051:Lipid Bilayers; D008564:Membrane Potentials; D009132:Muscles / Q000201:enzymology; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology
chemical_list: D008051:Lipid Bilayers; D000001:Calcimycin; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium