Heat-stable toxin from Escherichia coli activates chloride current via cGMP-dependent protein kinase

Heat-stable toxin (STa) increases cyclic GMP (cGMP) in iso- lated intestinal cells and in T84 cells, a colonic secretory cell line. Whole-cell current recordings from patch clamp experiments show identical properties for currents activated by either STa or the cystic fibrosis transmembrane conductance regulator (CFTR) channel. STa-activated currents display a linear current-voltage relationship and a relative permeability sequence of Br > Cl > I. STa or 8-Br-cGMP-activated currents remain when 20μM Walsh inhibitor, a blocker of protein kinase A (PKA), is added in the pipette, suggesting that cGMP-dependent protein kinase (PKG) activates the currents. Intracellular addition of Rp-8-Br-cGMP, an agent that activates PKGII and inhibits PKGI and PKA, causes induction of a chloride conductance identical to that stimulated by STa. We conclude that STa activates CFTR by phosphorylation with cGMP-dependent protein kinase.