Cyclic nucleotide-dependent phosphorylation of intestinal epithelium proteins

Abstract

THE function of cyclic AMP as an important regulator of ion and fluid transport in the small intestine -has been well documented¹. The non-dividing (maturating crypt cells and the differentiated villous cells are the main sources of cyclic AMP-mediated secretion of water and electrolytes during experimental cholera in rats and guinea pigs^2,3. It has been proposed that cyclic AMP, analogous to its possible mode of action in kidney⁴ and toad bladder⁵ epithelial cells, controls ion movements across the apical membrane of the intestinal epithelial cell (enterocyte) by changing the state of phosphorylation of some protein(s) in the intestinal brush border⁶. Studies have shown, however, that adenylate cyclase^2,7, the high affinity form of particulate cyclic AMP phosphodiesterase (unpublished observation) and receptor bound cyclic AMP (ref. 8) are localised predominantly in the basal–lateral membranes of the villous cell. In contrast, the luminal membrane is enriched in components of the cyclic GMP system, such as guanylate cyclase^9,10, cyclic GMP phosphodiesterase (unpublished observation) and receptor-bound cyclic GMP (ref. 8). The polarisation of both systems in the enterocyte would be more consistent with a model in which cyclic GMP, rather than cyclic AMP, is involved in the regulation of ion transport across the brush border, whereas cyclic AMP action might be confined to the contraluminal membrane. To test this hypothesis further, we have studied the subcellular distribution of specific substrate proteins for cyclic nucleotide-dependent protein kinases in isolated villous cells from rat small intestine.