Juvenile hormone and hemolymph juvenile hormone binding protein titers and their interaction in the hemolymph of fourth stadium Manduca sexta

Abstract

A driving force in the initiation of an endocrine response is the modulation of hormone titers surrounding the responsive tissue. For nonpolar hormones, titer fluctuations are, in part, regulated by specific circulatory transport proteins that bind, protect and convey the hormones to the responsive tissues. In the present study, juvenile hormone (JH), hemolymph juvenile hormone binding protein (hJHBP), and total hemolymph protein in the fourth larval stadium of Manduca sexta were quantified from similarly staged insects to assess whether JH is bound or unbound when it interacts with the target. JH levels, as determined by radioimmunoassay, were highest, 12.87 nM, 4 h after ecdysis to the fourth stadium, and declined 6-fold to the lowest titer, 2.28 nM, at 44 h after ecydsis, i.e. 8 h after head critical period. Conversely, hJHBP titers peaked at 11.95 μM 44 h after ecdysis and dropped 5-fold by the time of ecdysis to the fifth stadium. Total hemolymph protein also reached its highest level, 19.83 mg/ml, at 44 h; however, hJHBP titers did not follow fluctuations in the total hemolymph protein. Calculations of bound and unbound JH indicated that >99% of the hormone was bound by hJHBP. In contrast, <1% of the binding protein was hormone-loaded at any time during the stadium. Due to the rapid metabolism of JH in the hemol the hemolymph is improbable at certain times during development. Thus, the rate limiting step in JH uptake at these times may be the interaction of hJHBP with the target cell.