Adenosine Triphosphate-dependent Inhibition of Insulin-stimulated Glucose Transport in Fat Cells

The role of extracellular Ca2+ and cyclic nucleotides in the mechanism of enzyme secretion from the cat pancreas

The role of Ca2+ in protein secretion from the isolated perfused cat's pancreas, the effect of the dibutyryl analogues of cAMP and cGMP, and the interrelation of Ca2+ and the nucleotides were studied. The following results were obtained: 1. Pancreatic enzyme secretion can be elicited by CaCl2 injections into the pancreatic arteries and is linearily related to the peak Ca2+ concentration in the effluent perfusate. Different background Ca2+ concentrations in the perfusate (3 mM or 0.125 mM) do not disturb this relation, indicating that no adaptation occurs. The effect of Ca2+ injections is of the same magnitude as that evoked by the hormones pancreozymin or acetylcholine. 2. Injections of Ca2+ potentiate the effects of dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP), dibutyryl guanosine 3',5'-cyclic monophosphate (dbcGMP) or theophylline. 3. Infusion of low doses of pancreozymin increases the Ca2+ effect. The findings indicate that extracellular Ca2+ is involved in the mechanism of enzyme secretion and that Ca2+ and cyclic nucleotides have a synergistic action on the target.

Mechanism of activation of adenylate cyclase by Vibrio cholerae enterotoxin

The kinetics and properties of the activation of adenylate cyclase by cholera enterotoxin have been examined primarily in toad erythrocytes, but also in avian erythrocytes, rat fat cells and cultured melanoma cells. When cholera toxin is incubated with intact cells it stimulates adenylate cyclase activity, as measured in the subsequently isolated plasma membranes, according to a triphasic time course. This consists of a true lag period of about 30 min, followed by a stage of exponentially increasing adenylate cyclase activity which continues for 110 to 130 min, and finally a period of slow activation which may extend as long as 30 hr in cultured melanoma cells. The progressive activation of adenylate cyclase activity by cholera toxin is interrupted by cell lysis; continued incubation of the isolated membranes under nearly identical conditions does not lead to further activation of the enzyme. The delay in the action of the toxin is not grossly dependent of the number of toxin-receptor (GM1 ganglioside) complexes, and is still seen upon adding a second dose of toxin to partially stimulated cells. Direct measurements indicate negligible intracellular levels of biologically active radioiodinated toxin in either a soluble or a nuclear-bound form. The effects are not prevented by Actinomycin D (20 mug/ml), uromycin (30 mug/ml), cycloheximide (30 mug/ml), sodium fluoride (10 mM) or sodium azide (1 mM); KCN, however, almost completely prevents the action of cholera toxin. The action of the toxin is temperature dependent, occurring at very slow or negligible rates below certain critical temperatures, the values of which depend on the specific animal species. Thetransition for toad erythrocytes occurs at 15 to 17 degrees C, while rat adipocytes and turkey erythrocytes demonstrate a discontinuity at 26 to 30 degrees C. The temperature effects are evident during the lag period as well as during the exponential phase of activation. The rate of decay of the stimulated adenylate cyclase activity of cultured melanoma cells indicates a half-time of about 36 hr. The rate of exponentially increasing activity and extent of enzyme activation are related to the number of bound toxin molecules according to a Langmuir adsorption isotherm and are half-maximal when about 2000 molecules of toxin are bound per cell. It is proposed that initially cholera toxin binds ineffectively, but that it is converted to an active form during the lag phase. This process may involve lateral motion of toxin-GM1 ganglioside complex within the plane of the membrane. The kinetics of adenylate cyclase activation are consistent with the possibility that during the exponential phase a bimolecular association is proceeding between the active form of the cholera toxin and some other membrane component. The possibility is considered that the cholera toxin molecule may bind directly to adenylate cyclase. These considerations may prove useful in understanding the possible interactions of active hormone-receptor complexes with adenylate cyclase in cell membranes.