Involvement of calpain and synaptotagmin Ca2+ sensors in hormone secretion from excitable endocrine cells

The requirement for Ca(2+) to regulate hormone secretion from endocrine cells is long established, but the precise function of Ca(2+) sensors in stimulus-secretion coupling remains unclear. In the current study, we examined the expression of calpain and synaptotagmin in INS-1 pancreatic and GH3 and AtT20 pituitary cells, and investigated the sensitivity of hormone secretion from these cells to inhibition of the calpain family of cysteine proteases. Little difference in expression of mu-calpain was observed between the different endocrine cells. However, AtT20 cells did exhibit an extremely low abundance of both m-calpain and the 54 kDa isoform of calpain-10 relative to their expression in INS-1 and GH3 cells. Interestingly, secretagog-stimulated secretion from both INS-1 and GH3 cells was completely abolished following pre-incubation with the cysteine protease inhibitor E64, whereas stimulated secretion from AtT20 cells was modest and completely insensitive to E64 inhibition. These results are in stark contrast to synaptotagmin data. Synaptotagmin expression in AtT20 cells is abundant, whereas INS-1 cells express extremely low levels of this Ca(2+) sensor, relative to the pituitary cells. We hypothesize that the expression pattern of calpain and synaptotagmin isoforms may reflect alternative mechanisms of stimulus-secretion coupling in excitable endocrine cells.