Impaired Mitochondria Fail to Ensure Sustained Soce: Possible Mechanism for Decreased Salivary Secretion Under Diabetes

Abstract

Xerostomia is a troublesome complication of diabetes mellitus associated with decreased salivation. Previously we showed the diabetes-induced alterations of ACh-mediated [Ca2+]cyt signaling in submandibular salivary gland which provides a major secretion of fluid and electrolytes. Since salivation is initiated by an InsP3-mediated Ca2+ release from the ER and subsequently depends on the elevated [Ca2+]cyt maintained by a store-operated Ca2+ entry (SOCE), we hypothesized that both processes could be altered under the diabetes contributing to gland dysfunctions. Diabetes was induced by a single i.p. injection of streptozotocin; [Ca2+]i was measured using fura-2/AM. We found a decrease of the amplitude and deceleration of ACh-induced [Ca2+]i signals under the diabetes. The increased contribution of mitochondria to the cytosolic calcium clearance in acinar cells was also found under diabetes revealed as: i) an increase in the amount of Ca2+accumulated in mitochondria under basal conditions (by 46%); ii) significantly smaller effect of mitochondrial Ca2+ uptake inhibition on the ACh-induced [Ca2+]i transients in Ca2+-containing extracellular medium (by 69% vs. 29%). Since both SOCE and ER Ca2+ refilling are precisely regulated by mitochondria (Kopach et al., 2009), we studied the effectiveness of these processes under diabetes. SOCE induced by short ACh stimulation was increased in diabetic cells (by 70%). Inhibition of mitochondrial Ca2+ accumulation equalized SOCE magnitude in control and diabetic cells indicating an increased role of mitochondria to provide positive feedback on SOCE under diabetes. In contrast, during the sustained cells stimulation SOCE was decreased and decelerated under diabetes (∼ by 40%) suggesting inability of acinar cells to maintain SOCE under potent agonist stimulation. Concluding, diabetes induces the impairment of intracellular mechanisms responsible for the activation and maintenance of SOCE suppressing mitochondrial Ca2+ handling.