title: Calmodulin stimulation of plasmalemmal Ca2+-pump of canine aortic smooth muscle.
abstract: Plasma-membrane-enriched fractions of canine aortic smooth muscle possess an ATP-supported Ca2+ accumulation which has an absolute requirement for Mg2+ and a high affinity for Ca2+ (Km approximately 0.5 microM). The rate of ATP-supported Ca2+ transport is not affected by several calmodulin antagonists, but is stimulated by exogenously added calmodulin. The maximal effect of calmodulin on the rate of ATP-dependent Ca2+ transport (at 5.0 microM Ca2+) occurs at 10 micrograms/ml calmodulin and represents an approximately 3-fold stimulation. This calmodulin stimulation of Ca2+ transport does not require pretreatment of the membranes by EGTA and is an intrinsic property of the plasma membranes. A high-affinity Ca2+-ATPase (Km for Ca2+ approximately 0.5 microM) is also present in the aortic smooth muscle plasma membrane. This high-affinity Ca2+-ATPase does not require Mg2+ for catalytic activity, but is in fact inhibited by increasing Mg2+ concentrations. Calmodulin at concentrations effective for the stimulation of the ATP-dependent Ca2+ transport has no effect on the high-affinity Ca2+-ATPase activity or on the basal ATPase activity stimulated by 5 mM Mg2+ or Ca2+. Our results indicate that isolated plasma membranes of canine aortic smooth muscle contain no endogenous calmodulin. The ability of exogenously added calmodulin to stimulate the rate of ATP-dependent Ca2+ transport by vascular smooth muscle plasma membranes suggests that calmodulin may play a role in lowering the cytoplasmic concentration of ionized calcium during vasodilatation. An Mg2+-independent, but not an Mg2+-dependent high-affinity Ca2+-ATPase, was identified in the plasma membranes. This may be separate from the plasmalemmal Ca2+-pump.
mesh_terms: D015720:5'-Nucleotidase; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002147:Calmodulin / Q000494:pharmacology*; D004285:Dogs; D008274:Magnesium / Q000494:pharmacology; D008861:Microsomes / Q000378:metabolism; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000378:metabolism; D009708:Nucleotidases / Q000378:metabolism; D014018:Tissue Distribution
chemical_list: D002147:Calmodulin; D000255:Adenosine Triphosphate; D009708:Nucleotidases; D015720:5'-Nucleotidase; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium