title: Calmodulin-sensitive ATP-dependent Ca2+ transport across adipocyte plasma membranes.
abstract: An ATP-dependent transport system which is active at concentrations of free Ca2+ in the submicromolar range has been identified in adipocyte plasma membranes. The system appears to represent the functional component of the high affinity insulin-sensitive calcium-stimulated, magnesium-dependent adenosine triphosphatase preveiously described in the same preparation (Pershadsingh, H. A., and McDonald, J. M. (1979) Nature 281, 495-497). This ATP-dependent Ca2+ transport pump was stimulated approximately 3-fold by the Ca2+-dependent regulatory protein, calmodulin. This effect was confined to the plasma membrane since a similar effect was undetectable in the fraction enriched in endoplasmic reticulum. Calmodulin stimulation was dose-dependent but saturable with half-maximal activation occurring at 0.72 microgram/ml (43 nM). Calmodulin appeared to stimulate the system primarily by decreasing the apparent half-maximal  saturation constant for free Ca2+ from 0.20 +/- 0.04 microM to 0.07 +/- 0.01 microM (n = 3). The Hill coefficient increased from 1.6 +/- 0.2 to 3.2 +/- 0.6 (n = 3), thus showing an increased positive cooperativity which allows the pump to be activated by an exceedingly narrow Ca2+ threshold in the presence of calmodulin. The calmodulin stimulation of the plasma membrane Ca2+ extrusion pump in adipocytes, working in opposition to metabolic signals which increase cytoplasmic Ca2+, could constitute a self-regulating negative feedback device for maintaining a low steady state level of intracellular Ca2+. This feedback system may be of critical importance in regulation of cellular metabolism by insulin.
mesh_terms: D000273:Adipose Tissue / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D017301:Ca(2+) Mg(2+)-ATPase; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008861:Microsomes / Q000378:metabolism; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D000001:Calcimycin; D011189:Potassium Chloride; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium