Calmodulin Stimulation of Plasmalemmal Ca<sup>2+</sup>-Pump of Canine Aortic Smooth Muscle Journal Articles uri icon

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abstract

  • Plasma-membrane-enriched fractions of canine aortic smooth muscle possess an ATP-supported Ca<sup>2+</sup> accumulation which has an absolute requirement for Mg<sup>2+</sup> and a high affinity for Ca<sup>2+</sup> (Km ∼ 0.5 µM). The rate of ATP-supported Ca<sup>2+</sup> 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 Ca<sup>2+</sup> transport (at 5.0 µM Ca<sup>2+</sup>) occurs at 10 µg/ml calmodulin and represents an approximately 3-fold stimulation. This calmodulin stimulation of Ca<sup>2+</sup> transport does not require pretreatment of the membranes by EGTA and is an intrinsic property of the plasma membranes. A high-affinity Ca<sup>2+</sup>-ATPase (K<sub>m</sub> for Ca<sup>2+</sup> ∼0.5 µM) is also present in the aortic smooth muscle plasma membrane. This high-affinity Ca<sup>2+</sup>-ATPase does not require Mg<sup>2+</sup> for catalytic activity, but is in fact inhibited by increasing Mg<sup>2+</sup> concentrations. Calmodulin at concentrations effective for the stimulation of the ATP-dependent Ca<sup>2+</sup> transport has no effect on the high-affinity Ca<sup>2+</sup>-ATPase activity or on the basal ATPase activity stimulated by 5 mM Mg<sup>2+</sup> or Ca<sup>2+</sup>. 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 Ca<sup>2+</sup> 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 Mg<sup>2+</sup>-independent, but not an Mg<sup>2+</sup>-dependent high-affinity Ca<sup>2+</sup>-ATPase, was identified in the plasma membranes. This may be separate from the plasmalemmal Ca<sup>2+</sup>-pump.

publication date

  • 1986