INSULIN BINDING-SITES IN GILLS OF THE ESTUARINE CRAB CHASMAGNATHUS-GRANULATA

Bovine I-125-insulin was injected into the estuarine crab Chasmagnathu s granulata in order to study its distribution and specific uptake by tissues. The highest radioactivity uptake occurred in both anterior an d posterior gills, which reached maximum values at 30-60 min following labeled insulin administration. Heart and hepatopancreas concentrated a very low amount of radioactivity (only 9 and 3%, respectively, of t hat shown by gills). A significant reduction of the uptake was observe d in the gills when an excess of unlabeled insulin was injected togeth er with the labeled hormone. In vitro studies also showed specific upt ake of I-125-insulin by the gills incubated at 25 degrees C, which rea ched a plateau after 120-min incubation, suggesting a saturable proces s. The inhibition of I-125-insulin uptake was dose dependent on unlabe led insulin. Glucagon did not compete with radioactivity uptake by gil ls in vivo and in vitro. Further characterization of insulin-binding s ites was performed in gill membrane. The amount of unlabeled insulin t hat prevented 50% of the I-125-insulin uptake was 7.78 mu g/ml, and th e Scatchard plot analysis established the presence of binding site wit h Kd of 3.11 mu M and Bmax of 0.14 mu M (r = 0.99). Ovine prolactin wa s not able to prevent I-125-insulin binding to gill membrane. These fi ndings seem to indicate the presence of specific binding sites for ins ulin or insulin-like substance in crab gills, which deserves further s tudies. (C) 1997 Wiley-Liss, Inc.