The Y-organs of Cancer antennarius secrete the hormones ecdysone (E) and much greater amounts of 3-dehydroecdysone (3DE), but the dominant derivative in the circulation is 20hydroxyecdysone (20E). The uptake and clearance of E, 3DE, and metabolites were compared in selected tissues in vivo, and the capacity of each to generate 20E was assessed in vitro.
[ 3 H]-labeled E or 3DE, injected into intermolt crabs, rapidly cleared from hemolymph, while being taken up and then rapidly cleared from tissues (muscle, heart, epidermis, ovary) within 1 h. Labeled 20E from E or 3DE appeared in all tissues in the first hour, cleared to varying degrees from tissues, and then reaccumulated over 24 h. Among these tissues, ovary accumulated the most labeled 20E. Generally by 24 h more labeled 20E accumulated from 3DE than from E. Excising the eyestalks (inducing premolt) did not affect the outcomes except that deeyestalking caused much greater concentrations of 3DE than E to appear in all tissues initially. Tissue extracellular volumes ([ 14 C]inulin spaces) were not altered by deeyestalking, except transiently in epidermis, and otherwise did not account for the observed ecdysteroid dynamics.
A parallel study with eyestalk neural tissue from crabs receiving [ 3 H]3DE showed extraordinary accumulation of 3DE and labeled metabolites. The normalized tissue/hemolymph ratio was 72 at 8 h, compared with 0.6-2.6 for other tissues. The apparent extracellular volume of eyes was high (80%) but did not account for the high ecdysteroid accumulation.
Tissues were incubated 18 h with [ 3 H]3DE. Activities representing 3ฮฒ-reductase and 20-hydroxylase generally were present, evidenced by finding labeled E and 20E. Tissue blanks, hemolymph, and muscle were devoid of activity, hepatopancreas and epidermis were intermediate, ovary, testis, and hindgut were high, and eyestalk tissue was highest in activities. Low amounts of labeled high-polar material were usually present, and a labeled derivative tentatively identified as 3dehydro-20-hydroxyecdysone (3D20E) also appeared in ovary, hepatopancreas, hindgut, and eyes, indicating some direct conversion of 3DE to 3D20E.
In composite, these data indicate that 3DE secreted by Y-organs is converted to E, thence to 20E by several tissues, accounting for the preponderance of 20E in crustacean hemolymph. The findings support the hypotheses that 20E is the definitive molting hormone (receptor ligand) and that an ecdysteroid feedback mechanism controls release of the molt-inhibiting hormone from the eyestalks.