Rapid kinetic measurements of second messenger formation in olfactory cilia from channel catfish

The effect of stimulating olfactory cilia from the channel catfish (Ictalurus punctatus) with odorant amino acids on the formation of adenosine 3',5'-cyclic monophosphate, guanosine 3',5'-cyclic monophosphate, and inositol 1,4,5-trisphosphate (IP₃) was studied in the subsecond time scale using a quenched flow technique. L-Alanine (L-Ala) and L-cysteine (L-Cys) (100 μM) elicited a transient elevation in IP₃ levels that peaked at 25 ms. In contrast, even at high concentration a mixture of odorant amino acids (1 mM L-Ala, L-Cys, L-norleucine, L-glutamate, L-proline, and L-arginine) did not elicit a change in cAMP levels in this time scale and caused only relatively slow and minor increases in cGMP. The dose-response relationship for the IP₃ response of L-Cys and L-Ala in the range from 10 nM to 1 mM is consistent with previous electrophysiological and ligand binding experiments. Odorant amino acid-stimulated IP₃ formation was GTP dependent and was inhibited by guanosine 5'-O-(2-thiodiphosphate), suggesting that the response was G protein mediated. These results are consistent with a mediatory role for IP₃ in amino acid olfactory signal transduction in catfish.