title: Functional characterization of neuroendocrine regulation of branchial carbonic anhydrase induction in the euryhaline crab Callinectes sapidus.
abstract: Carbonic anhydrase (CA) plays an essential role as a provider of counterions for Na(+)/H(+) and Cl(-)/HCO3 (-) exchange in branchial ionic uptake processes in euryhaline crustaceans. CA activity and gene expression are low in crabs acclimated to full-strength seawater, with transfer to low salinity resulting in large-scale inductions of mRNA and subsequent enzyme activity in the posterior ion-regulating gills (e.g., G7). In the green crab Carcinus maenas, CA has been shown to be under inhibitory neuroendocrine control by a putative hormone in the x-organ-sinus gland complex (XOSG), located in the eyestalk. This study characterizes the neuroendocrine regulation of CA induction in the blue crab Callinectes sapidus, a commonly used experimental organism for crustacean osmoregulation. In crabs acclimated to full-strength seawater, eyestalk ligation (ESL) triggered a 1.8- and 100-fold increase in CA activity and mRNA, respectively. Re-injection with eyestalk homogenates abolished increases in CA activity and fractionally reduced CA gene expression. ESL also enhanced CA induction by 33% after 96 h in crabs transferred to 15 ppt salinity. Injection of eyestalk homogenates into intact crabs transferred from 35 to 15 ppt diminished by 43% the CA induction stimulated by low salinity. These results point to the presence of a repressor hormone in the eyestalk. Separate injections of medullary tissue (MT) and sinus gland (SG), two components of the eyestalk, reduced salinity-stimulated CA activity by 22% and 49%, suggesting that the putative repressor is localized to the SG. Crabs injected with SG extract harvested from crabs acclimated to 5 ppt showed no decrease in CA activity, demonstrating that the hormone is down-regulated at low salinity. Our results show the presence in the XOSG of an inhibitory compound that regulates salinity-stimulated CA induction.
authors.affiliation: Auburn University, Department of Biological Sciences, Auburn, Alabama 36840 rzm0015@auburn.edu.
authors.forename: Reed T
authors.initials: RT
authors.lastname: Mitchell
authors.affiliation: Auburn University, Department of Biological Sciences, Auburn, Alabama 36840.
authors.forename: Raymond P
authors.initials: RP
authors.lastname: Henry
mesh_terms: D000818:Animals; D003386:Brachyura / Q000201:enzymology*; D002256:Carbonic Anhydrases / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation / Q000502:physiology; D015971:Gene Expression Regulation, Enzymologic; D005880:Gills / Q000201:enzymology; D009490:Neurosecretory Systems / Q000502:physiology; D054712:Salinity
chemical_list: D002256:Carbonic Anhydrases