Cyclic AMP in the fat body of Mamestra brassicae during the last instar and its possible involvement in the cellular autophagocytosis induced by 20-hydroxyecdysone
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Abstract
The amount of cAMP was assayed by a competitive protein binding method in fat body
cells of Mamestra brassicae, during the last larval stage and after administration
of 20-hydroxyecdysone. When expressed as picomoles of cAMP per milligram fresh weight
of tissue, two increases in its concentration were observed on the 3rd and on the
6th days. However, only the first peak appeared on the curve when cAMP concentration
was expressed as picomoles cAMP per milligram of protein of tissue homogenate. Electron
microscopical examination of the tissue showed that the first increase of cAMP level
coincided with the beginning of the formation of autophagic vacuoles and revealed
a heavy accumulation of protein storage granules in the cells, starting on the 4th
day. This process might mask the second rise of cAMP level when tissue protein content
is taken as the basis for calculation. 20-Hydroxyecdysone (5 micrograms/g body wt)
administered to 48-hr-old larvae induced premature autophagocytosis in the fat body
cells and a sharp rise in their cAMP content, reaching within 3 hr a level as high
as observed in the 3-day-old untreated larvae. Autophagy was also enhanced in the
cells exposed to dibutyryl cAMP or theophylline either in vivo or in vitro. Based
on these data we think that cAMP content of the fat body is controlled by ecdysone
and that cAMP plays a significant role in the regulation of autophagocytosis in this
tissue during metamorphosis.