In the membrane fraction of mouse parotid gland (PG), the protein level of aquaporin 5 (AQP5), a member of the water channel family, was increased by injection (i.p.) of isoproterenol (IPR), a β-adrenergic agonist, at 1 h, and stayed at high level until 6 h; this change occurred simultaneously as amylase secretion. The AQP5 level then decreased and returned toward the original level at 12-48 h. After IPR, the AQP5 mRNA gradually increased and reached a maximal at 24 h. The facts suggest a rapid appearance of AQP5 at plasma membrane by IPR followed by its degradation/metabolism by activation of proteolytic systems. Pre-treatment of animals with 2 calpain inhibitors, N-Ac-Leu-Leu-methininal (ALLM) and calpeptin, as well as a protein synthesis inhibitor, cycloheximide (CHX), significantly suppressed the IPR-induced AQP5 degradation in the PG membrane fraction; such suppression was not observed by 2 proteasome inhibitors, MG132 and lactacystin, and a lysosome denaturant, chloroquine, although most of these inhibitors increased AQP5 protein levels in unstimulated mice. The AQP5 protein was degraded also by μ-calpain in vitro. Furthermore, we demonstrated that μ-calpain was co-localized with AQP5 in the acinar cells by immunohistochemistry, and its activity in the PG was increased at 6 h after IPR injection. These results suggest that the calpain system was responsible for IPR-induced AQP5 degradation in the parotid gland and that such a system was coupled to the secretory-restoration cycle of amylase in the PG.