Interactions of saponin with microsomal membranes isolated from vascular smooth muscle

The effects of the plant glycosides extract, saponin, on various biochemical properties of microsomal fractions isolated from dog aortae and mesenteric arteries were investigated. These properties include binding and transport of Ca2+, ouabain-sensitive K+ activated p-nitrophenyl phosphatase (hereafter termed K+-pNPPase) and binding of radiolabelled antagonists to adrenoceptors. We found that saponin inhibited both Ca2+ binding and transport by the vascular muscle microsomes in a dose-dependent manner, but it caused marked enhancement of the K+-pNPPase activity. Saponin only slightly reduced the Bmax, but not the Kd of [125I]-iodocyanopindolol binding to the beta-adrenoceptors of the dog mesenteric artery microsomes. A similar finding on the [3H]-prazosin binding to the alpha 1-adrenoceptor in dog aortic microsomes was also observed. Our findings are consistent with the "skinning" action of saponin on the smooth muscle and provide direct evidence that saponin does not attenuate the recognition properties of the adrenoceptors in the cell membranes.