Abstract
Adenyl cyclase (AC) plays a pivotal role in cell signaling. The AC system of bivalves has received little attention so far, and our study has been addressed to the characterization of AC properties in the gills of T. philippinarum. The enzyme showed a K~m~ value of 0.77 mM for ATP in the presence of 5 mM Mg^2+^; in the absence of agonists, it was poorly affected by GTP, while it was stimulated by GTPγS and GppNHp up to 14‐fold and 4‐fold, respectively. Similarly to other invertebrates, the enzyme activity was scarcely stimulated by forskolin. The receptor agonist serotonin (5‐HT) significantly stimulated the AC activity, and the pharmacological profile of the 5‐HT receptor/s was as follows: (+)butaclamol > dihydroergocryptine > methysergide > prazosin > yohimbine. The AC activity was assessed in vitro in the presence of tributyltin chloride and HgCl~2~, which reduced the AC activity only at the highest dose tested (10–100 µM). Our data indicate the presence of a membrane‐bound AC in gill membranes of T. philippinarum, coupled to G~s~ proteins and to a specific class of 5‐HT receptors. Such receptors show a pharmacological profile slightly different from that reported for 5‐HT invertebrate receptors cloned so far. J. Exp. Zool. 301A:952–960, 2004. © 2004 Wiley‐Liss, Inc.