Indirect coupling of calcium transport in chromaffin granule ghosts to the proton pump.
In chromaffin granule ghosts, the Na+/Ca2+ exchanger in the granule membrane can provide a high affinity (Km 1-3 microM) and high capacity (Vmax 50-100 nmol mg min-2) mechanism for Ca2+ accumulation. The activity of the Na+/H+ antiporter can be used to couple Ca2+ uptake via Na+/Ca2+ exchange to ATP-dependent proton translocation via the granule membrane H(+)-ATPase. Therefore, Ca2+ uptake can be indirectly linked to the proton pump. However, under conditions designed to mimic the environment of a granule in the cytosol of a chromaffin cell, measured rates of Ca2+ uptake are low, a free Ca2+ concentration of about 5 microM in the ghost matrix being attained. Under such circumstances, the granules seem unlikely to play a major role in calcium homeostasis in the intact cell.
Department of Biological Chemistry, UCLA School of Medicine 90024.
J R
JR
Haigh
J H
JH
Phillips
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism; D002837:Chromaffin Granules / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007474:Ion Exchange; D017494:Proton Pumps / Q000502:physiology*; D006180:Proton-Translocating ATPases / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D028021:Sodium-Potassium-Chloride Symporters; D013816:Thermodynamics
D002352:Carrier Proteins; D017494:Proton Pumps; D028021:Sodium-Potassium-Chloride Symporters; D000255:Adenosine Triphosphate; D012964:Sodium; D006180:Proton-Translocating ATPases; D002118:Calcium