ATP DEPENDENT CALCIUM UPTAKE ACTIVITY IN PLASMA MEMBRANE FRACTION OF INTESTINAL SMOOTH MUSCLE

Abstract

Ca²⁺ binding and ATP dependent Ca²⁺ uptake by plasma membrane fraction were studied and the effect of treating plasma membrane fraction with trypsin, phospholipase c and neuraminidase on both activities was observed.
Results of the above tests are as follows;
1. Plasma membrane fraction possessed the ability to bind Ca²⁺. The Scatchard plot of Ca²⁺ binding shwoed that plasma membrane had at least two types of Ca²⁺ binding sites of high and low affinity.
2. The amount of Ca²⁺ uptake by plasma membrane fraction was 18.51±1.31 nmoles Ca²⁺/mg protein/30min at 10^-5M Ca²⁺. The Ca²⁺ concentration for half maximal activation in Ca²⁺ uptake was 3.7×10^-7M.
3. The ATP dependent Ca²⁺ uptake by the microsome isolated from guinea pig intestine was clearly stimulated by 4mM K₂-oxalate whereas that of plasma membrane fraction was not.
4. Trypsin and phospholipase c treatment led to a 40-88% reduction in the ATP dependent Ca²⁺ uptake. The Ca²⁺ concentration for half maximal activation Ca²⁺ uptake shifted to a high concentration. On the other hand, neuraminidase treatment resulted in a 43-93% increase in ATP dependent Ca²⁺ uptake. But, the Ca²⁺ concentration for half maximal activation in Ca²⁺ uptake was not shifted by neuraminidase treatment.
The results show that plasma membrane plays an important role in regulating the cytoplasmic Ca²⁺ concentration. The results also suggest that a glycoprotein linked with sialic acid is involved in the flux of Ca²⁺ across the plasma membrane.