Abstract
Saponin, a cell-skinning reagent which perforates the cell membrane via its specific interaction with plasmalemmal cholesterol, was used to identify the subcellular origin of ATP-dependent Ca 2+ accumulation in the presence and absence of inorganic phosphate and oxalate by microsomal fractions isolated from rat vas deferens and dog aorta. The purified plasma membranes from rat gastric fundus muscle, which elicit the stimulation of ATP-dependent Ca 2+ accumulation by inorganic phosphate but not by oxalate, were used as a control reference. Saponin at concentrations effective for skinning smooth muscle fibres (10–50 μg/ml) inhibited Ca 2+ binding in the absence of ATP to a similar extent in all fractions, but the inhibition of ATP-dependent Ca 2+ accumulation was more pronounced in dog aorta microsomes and rat gastric fundus muscle plasma membranes than in rat vas deferens microsomes. The resistance of phosphate- and oxalate-stimulated ATP-dependent Ca 2+ accumulation to inhibition by saponin was much greater in rat vas deferens than in dog aorta microsomes. Our results suggest that phosphate- and oxalate-stimulated ATP-dependent Ca 2+ accumulation also occurs in plasma membrane vesicles isolated from smooth muscle and is by no means an unique property of endoplasmic reticulum.
Published Version
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