Modulation of gastric mucosal calcium channels activity by platelet-derived growth factor

A dihydropyridine-sensitive gastric mucosal calcium channels were isolated from the solubilized epithelial cell membranes by affinity chromatography on wheat germ agglutinin. The channels following labeling the calcium antagonist receptor site with [³H]PN200-100 were reconstituted into phospholipid vesicles which exhibited active ⁴⁵Ca²⁺ uptake as evidenced by La³⁺ displacement assays. The uptake of calcium was independent of sodium and potassium gradients indicating the electroneutral nature of the process. The channels responded in a dose dependent manner to dihydropyridine calcium antagonist, PN200-110, which at 0.5/zm exerted maximal inhibitory affect of 66% on ⁴⁵Ca²⁺ uptake, while a 52% enhacement in ⁴⁵Ca²⁺ uptake occurred with a specific calcium channel activator, BAY K8644. On platelet-derived growth factor (PDGF) binding in the presence of ATP, channel protein showed an increase in tyrosine phosphorylation of 55 and 170 kDa calcium channel proteins. Such phosphorylated channels following reconstitution into vesicles displayed a 78% greater ⁴⁵Ca²⁺ uptake. The results demonstrate the importance of PDGF in the regulation of gastric mucosal calcium uptake.