CAPACITY OF INSECT (MANDUCA-SEXTA) PROTHORACIC GLANDS TO SECRETE ECDYSTEROIDS - RELATION TO GLANDULAR GROWTH

The capacity of prothoracic glands to secrete ecdysteroids changes dur ing the last larval stadium of the tobacco hornworm, Manduca sexta. In the present study, the protein content of prothoracic glands was obse rved to change significantly during this time period. Peaks in glandul ar protein content occurred on Day 4 (15.4 mu g/gland) and Day 7 (14.6 mu g/gland). These correspond to times of maximal ecdysteroid secreti on in vitro. Ecdysteroid secretion in vitro was determined as a functi on of glandular protein content for Day 1, 3, and 7 glands. For unstim ulated glands, secretion increased from 0.05 ng/mu g on Day 1, to 0.54 ng/mu g on Day 3, to 1.37 ng/mu g on Day 7. For glands incubated with big PTTH, secretion increased from 0.27 ng/mu g on Day 1, to 2.05 ng/ mu g on Day 3, to 2.60 ng/mu g on Day 7. The results suggested that de velopmental changes in secretory capacity are influenced by both the a mount and type of glandular proteins. Glandular protein metabolism was assessed by monitoring the incorporation of [S-35]methionine. A time course study revealed the rate of incorporation for Day 3 and Day 5 gl ands was significantly greater than the rate for Day 1 and Day 7 gland s. Electrophoretic separation of radiolabeled glandular proteins revea led developmental changes in the pattern of protein synthesis. However , a band whose intensity changed in parallel with developmental change s in glandular secretory capacity was nor detected. Finally, incorpora tion of BrdU by cells of the prothoracic glands was assessed using imm unohistochemistry: Incorporation of BrdU was not observed on Days 1 or 7, occurred in only a few cells on Day 5, and was most pronounced on Day 3 (12.3% of the cells were labeled). The combined results indicate that changes in ecdysteroidogenic capacity are associated with (a) a change in glandular protein content, (b) a change in the types of prot eins synthesized by prothoracic glands, and (c) a temporally restricte d pulse of DNA synthesis, the latter being a possible indicant of cell proliferation. (C) 1995 Academic Press, Inc.