REGULATION OF ECDYSTEROID BIOSYNTHESIS IN CRAYFISH Y-ORGANS .2. ROLE OF CYCLIC NUCLEOTIDE-DEPENDENT PROTEIN-KINASES

The ecdysteroid production of the crustacean molting gland, the Y-organ, is known to be suppressed by the so-called molt inhibiting hormone (MIH), a neuropeptide from the X-organ-sinus gland complex. Cyclic nucleotides appear to be involved in intracellular effects of MIH in Y-organs. In the crab Cancer antennarius cAMP synthesis seems to be an initial event in MIH action, while in crayfish results indicate a more important role for cGMP in intracellular effects of MIH. To gain a better understanding of the intracellular mechanisms we examined the cAMP- and cGMP-dependent protein kinase (A- and G-kinase) activity in Y-organs of the crayfish Orconectes limosus at different molting stages induced by eyestalk ablation. The activity ratio of cGMP-dependent protein kinase was lower for premolt animals (0.51) than for intermolt animals (0.81), indicating a more pronounced activation of G-kinase in Y-organs of intermolt animals by endogenous cGMP. Moreover, preincubation of Y-organs from destalked animals with sinus gland extract reestablished a G-kinase activity ratio characteristic for intact animals. On the contrary, the activity ratio of A-kinase did not vary depending on molting stage and was not altered by sinus gland extract. These results strongly suggest that MIH action in Orconectes limosus Y-organ is mediated by cGMP rather than by cAMP.