Inhibition Of Cholera Toxin Research Articles

Progesterone-induced meiosis in xenopus laevis oocytes: a role for cAMP at the “maturation-promoting factor” level

Cholera toxin inhibition of progesterone-induced meiosis of Xenopus laevis oocytes in vitro has been correlated with increased cAMP levels. Inhibition of germinal vesicle breakdown (Gvbd) and cAMP increase occurred after a lag period of 2 hr, when cholera toxin was injected, or 4–5 hr, when applied externally. The ability of the maturation-promoting factor (Mpf) to provoke Gvbd when injected into recipient oocytes was found to be dependent upon whether the oocytes had been exposed to cholera toxin alone or to toxin and progesterone. With the former, cAMP levels were elevated and Mpf activity was abolished, whereas with the latter, the increase in cAMP was less pronounced and Mpf activity was observed. Injection of cAMP or its 8-thio derivatives shortly before the appearance of progesterone-induced Mpf abolished Gvbd. If injected earlier or later, no inhibition was observed. In contrast, cholera toxin inhibited maturation even when added several hours before progesterone, suggesting a sustained accumulation of cAMP. No Gvbd occurred when 8-thio-methyl-cAMP was injected together with Mpf. These data suggest that cAMP is involved in the control of the formation/amplification and/or activity of Mpf—a result which may be of general significance in cell division mechanisms.

Selective inhibition of cholera toxin- and catecholamine-stimulated lipolysis by blocking agents.

Vibrio cholerae enterotoxin stimulates lipolysis in rat epididymal fat cell suspensions. Like hormones this toxin increases adenylate cyclase activity, raising levels of cyclic adenosine 3',5'-monophosphate (cAMP), which activates a cellular lipase. Using specific blocking agents, we studied the responses to the adrenergic lipolytic hormones epinephrine, norepinephrine, and isoproterenol, and to cholera toxin. All stimulators were used at 100 x threshold dose. Propranolol (34 muM), a beta blocking agent, inhibited epinephrine stimulation (P less than 0.001) but not that of toxin (P greater than 0.2). Choleragenoid (25 mug/ml), a natural toxoid of cholera toxin, blocked stimulation by toxin (P less than 0.001) but not that of the adrenergic agents (P greater than 0.2). A beta blocker, practolol (3 mM), inhibited stimulation by the catecholamines tested (P less than 0.005) but not that of toxin (P greater than 0.05). Higher concentrations of propranolol (340 muM) and the alpha blocking agents phenoxybenzamine (3 mM) and phentolamine (1.6 mM) inhibited all agonists (P less than 0.001). The response to theophylline was inhibited by all blockers (P less than 0.05) except propranolol at the lower concentration (34 muM). A combined beta and alpha blockade using propranolol and epinephrine together did not inhibit toxin-mediated lipolysis. It appears that stimulation by cholera toxin is independent of beta adrenergic receptors. A major inhibition of theophylline-mediated lipolysis by alpha blocking drugs indicated a nonspecific effect of these agents at the concentrations used. The uninhibited response to toxin in the presence of propranolol and epinephrine suggests a lack of relationship of the toxin receptor to either alpha or beta receptors.