Printed in Great Britain A MORPHOMETRIC AND BIOCHEMICAL INVESTIGATION OF THE VESICULATION OF

The vesiculation of kidney proximal tubule microvilli has been examined in tissue slices, isolated brush borders and isolated microvilli. Vesiculation could be induced in tissue slices by 2,4-dinitrophenol and anoxia. Cycloheximide and fluoride had no effect. In brush borders and microvilli, the vesiculation was found to be essentially temperature-dependent. Whilst an osmotic swelling could be produced by hypo-osmolar media, the temperature-dependent vesiculation could not be prevented in hyper-osmolar media. Of a wide variety of reagents tested, only glutaraldehyde, mercuric chloride and mersalyl were effective in arresting the vesiculation. Electron micrographs show that vesiculation involves a collapse of the internal structure of the microvillus. However, the collapse was not associated with depolymerization of the microvillus actin filaments. Rather it appeared to be due to the parting of cross-bridges between the membrane and the actin filaments. The nature of these cross-bridges is discussed: it is suggested that a-actinin possesses the characteristics for the cross-bridging protein and that vesiculation might be explained by the displacement of a-actinin by tropomyosin