Failure of pertussis toxin to inhibit activation of guanylate cyclase by the heat-stable enterotoxin of Escherichia coli (STa) in the T84 cell line.

The heat-stable enterotoxin (STa) of Escherichia coli causes intestinal secretion by stimulating guanylate cyclase, an enzyme believed to be distinct from the STa receptor. Pertussis toxin (PT) has been reported to block the ability of STa to stimulate guanylate cyclase in rat intestinal mucosa (S. A. Epstein, R. A. Giannella, and H. J. Brandwein, FEBS Lett. 203:44-48, 1986). This suggested that a guanine nucleotide regulatory protein (G protein) coupled the STa receptor to guanylate cyclase, a function not previously recognized for G proteins. We sought to explore this phenomenon and, if possible, to identify this G protein. Initial experiments with the human colon carcinoma cell line T84 revealed that higher-than-expected concentrations (1 micrograms/ml) of PT were needed to intoxicate cells, as assessed by ADP-ribosylation of endogenous PT substrate, but that 99 to 100% intoxication could be achieved. Homogenates made from fully intoxicated cells did not differ from controls in basal or STa-stimulated guanylate cyclase activity, and cyclic GMP accumulation in intact T84 cells was not changed by PT treatment. We conclude that a PT-sensitive G protein is not involved in the stimulation of cyclic GMP production by the enterotoxin STa.