Brush-border tyrosine phosphorylation stimulates ileal neutral NaCl absorption and brush-border Na⁺-H⁺ exchange

The drug genistein, a tyrosine (Tyr) kinase inhibitor, was used to define a role for Tyr phosphorylation in regulation of basal and stimulated neutral NaCl absorption in rabbit ileum. Brush-border vesicles contain Tyr- phosphorylated peptides. Genistein freeze-thawed into the vesicles caused a concentration-dependent inhibition of at least three peptides with M(r) 111,000, 83,000, and 80,000. Studied with the Ussing chamber-voltage clamp technique, genistein added to the ileal mucosal surface inhibited neutral NaCl absorption. Direct addition of genistein to brush-border vesicles made from ileal villus cells inhibited brush-border Na⁺-H⁺ exchange but not D- glucose-stimulated Na⁺ uptake. These effects were not duplicated by genistin, a drug with similar structure to genistein but lacking Tyr kinase inhibiting properties. Serosal but not mucosal epidermal growth factor (EGF) stimulated NaCl absorption. Mucosal genistein but not genistin also altered second-messenger regulation of neutral NaCl absorption, inhibiting the effect of Ca²⁺ ionophore A-23187 and of serosal EGF but not affecting the transport changes caused by 8-bromoadenosine 3',5'-cyclic monophosphate (8- BrcAMP). In contrast, the Cl secretory effects indicated by the increase in short-circuit current for all three agents, A-23187, EGF, and 8-BrcAMP, were inhibited by mucosal genistein. These results suggest that 1) a Tyr kinase is involved in basally stimulating ileal neutral NaCl absorption and brush- border Na⁺-H⁺ exchange; 2) EGF stimulates NaCl absorption by an effect exerted from the serosal surface, but the effect also involves a brush- border Tyr kinase; 3) brush-border Tyr kinase is involved in the ability of Ca²⁺ ionophore A-23187 to inhibit neutral NaCl absorption but is not involved in the transport effects of cAMP. This study suggests that Tyr kinase(s) acting over short time periods is involved in stimulation of neutral NaCl absorption and brush-border Na⁺-H⁺ exchange and also in Ca²⁺-induced inhibition of NaCl absorption. These studies represent the first example of a brush-border Tyr kinase being involved in short-term signal transduction in intestinal epithelial cells.