Ecdysteroid regulation of salivary gland degeneration in the ixodid tick, Amblyomma hebraeum: a reconciliation of in vivo and in vitro observations.

Salivary gland degeneration in the female tick Amblyomma hebraeum Koch is triggered by an ecdysteroid (ES) hormone. Under both in vivo and in vitro conditions, degeneration requires 4 days for completion. In partially fed females that have fed beyond a "critical weight," the commitment period for salivary gland degeneration occurs between 24 and 48 h after removal from the host. Although tissue degeneration begins within 24 h postengorgement, ES titer as measured by radioimmunoassay (RIA) does not rise to threshold levels until 48 h postengorgement. To explain this anomaly we examined two hypotheses: (1) there is an early hormonal signal (e.g., 3-dehydroecdysone; 3DE) that is an ES not detectable by the antibody used in our RIA; and (2) the low hemolymph titer during the first 2 days postengorgement is not an accurate reflection of the ES concentration within the tissue itself. 3-Oxoecdysteroid 3 beta-reductase (ketoreductase) was present in salivary glands, but neither ketoreductase nor 3DE was detected in hemolymph. The ES concentration of salivary gland homogenates was similar to that of hemolymph, while that of saliva was undetectable. Together, these results support our second hypothesis that the metabolically active tissue of the salivary gland experiences a suprathreshold concentration of hormone even though the concentration in hemolymph is below threshold levels.