Hypertonicity‐induced transmitter release at Drosophila neuromuscular junctions is partly mediated by integrins and cAMP/protein kinase A

The frequency of quantal transmitter release increases upon application of hypertonic solutions. This effect bypasses the Ca 2+ triggering step, but requires the presence of key molecules involved in vesicle fusion, and hence could be a useful tool for dissecting the molecular process of vesicle fusion. We have examined the hypertonicity response at neuromuscular junctions of Drosophila embryos in Ca 2+ ‐free saline. Relative to wild‐type, the response induced by puff application of hypertonic solution was enhanced in a mutant, dunce , in which the cAMP level is elevated, or in wild‐type embryos treated with forskolin, an activator of adenylyl cyclase, while protein kinase A (PKA) inhibitors decreased it. The response was also smaller in a mutant, DC0 , which lacks the major subunit of PKA. Thus the cAMP/PKA cascade is involved in the hypertonicity response. Peptides containing the sequence Arg‐Gly‐Asp (RGD), which inhibit binding of integrins to natural ligands, reduced the response, whereas a peptide containing the non‐binding sequence Arg‐Gly‐Glu (RGE) did not. A reduced response persisted in a mutant, myospheroid , which expresses no integrins, and the response in DC0 was unaffected by RGD peptides. These data indicate that there are at lease two components in the hypertonicity response: one that is integrin mediated and involves the cAMP/PKA cascade, and another that is not integrin mediated and does not involve the cAMP/PKA cascade.