Chemosignal Transduction in the Vomeronasal Organ of Garter Snakes: Ca2+-Dependent Regulation of Adenylate Cyclase

Earthworm shock secretion contains a 20-kDa vomeronasally mediated chemoattractive protein for garter snakes. Both the ligand–receptor binding and the chemoattractivity of ES20 are Ca2+-dependent. When ES20 binds to its G-protein-coupled receptors in the vomeronasal epithelium, the inositol 1,4,5-trisphosphate (IP3) level is increased, but the level of cAMP is reduced. Furthermore, forskolin-stimulated levels of cAMP are completely blocked by ES20-receptor binding or by Ca2+alone and the effect of calcium ions can be nullified by EGTA. Previously, we hypothesized that the decrease in cAMP was due to activation of a Ca2+-dependent phosphodiesterase. In the present study, we provide evidence that the decrease in cAMP is due mainly to the regulation of adenylate cyclase (AC) activity by Ca2+or is indirectly mediated by ES20. Results obtained with intact vomeronasal sensory epithelium suggest that the binding of ES20 to its receptors facilitates generation of IP3which mobilizes intracellularly sequestered Ca2+, resulting in an increase of cystosolic Ca2+. A further increase in cytosolic Ca2+occurs through Ca2+influx from extracellular sources. Garter snake vomeronasal AC does not require calmodulin for its activity and shows a biphasic response to increasing concentrations of Ca2+; its activity is modulated both positively and negatively by this bivalent cation.