Dual regulation of ACTH secretion by guanine nucleotides in permeabilized AtT-20 cells
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We have examined the effects of guanine nucleotides on ACTH secretion from digitonin-permeabilized AtT-20 cells, with the aim of analyzing the involvement of GTP-binding proteins (G proteins) in the secretory process.
AtT-20 cells permeabilized with 20µM digitonin displayed calciumdependent secretion. The EC50 of calcium was ~2µM and the maximal stimulation was 350% of basal release.
Nonhydrolyzable guanine nucleotides also stimulated ACTH release, in a virtually Ca2+-free medium. The EC50 of guanosine 5′-(3-O-thio)triphosphate (GTP γ S) was ~15µM and the maximal stimulation was ~230% of basal release. The effects of calcium and guanine nucleotides were not additive.
In the presence of the inhibitory hormone, somatostatin guanine nucleotides inhibited the calcium-stimulated secretion.
Both the stimulatory and the inhibitory effects on secretion of guanine nucleotides were independent of changes in cyclic AMP (cAMP) and calcium. It is suggested that G proteins influence an unknown step in the secretion process, which would be near or at the exocytotic site.
The results can be explained by assuming the existence of two types of G proteins, one with stimulatory effects on exocytotic release (GeS) and another with inhibitory effects (GeI).
Key wordsGTP-binding proteins calcium exocytosis guanine nucleotides adrenocorticotropic hormone (ACTH) AtT-20 cells
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