Os sensores de glicose T1R2 e T1R3 regulam a translocação do cotransportador NA+/GLICOSE/ÁGUA SGLT1 em glândula salivar: regulação da secreção salivar no diabetes Mellitus

Abstract

The Na+/glucose/water cotransporter SGLT1 in the luminal membrane of ductal cells (MLCD) in salivary glands can be activated by the sympathetic activity via β-adrenergic receptor-adenylate cyclase-PKA. It has been shown that diabetes promotes a decrease of sympathetic activity to the salivary glands and an increase in SGLT1 expression in MLCD, which correlates with reduced salivary flow. Our hypothesis is that occurs a parallel activation via of PKA by the presence of glucose sensors of the GPCR family 1 in the ductal cells. The present study sought to evaluate the subcellular expression of glucose sensors T1R2/T1R3 and glucose transporter GLUT2 in salivary glands and determine the role of these sensors in the translocation of SGLT1 in submandibular glands by microinjection of glucose in the submandibular artery. Furthermore, we evaluated the effect of inhibition of SGLTs in salivary ducts by intraductal microinjection of phlorizin seeking to evaluate their role in the flow and salivary glucose concentration. Were used rats Wistar nondiabetic (ND), diabetics treated with saline (DS) or insulin (DI). The diabetes was induced 28 days before the study (alloxan, 40 mg/kg, iv). From the 21st day, the animals were treated for 7 consecutive days with saline or insulin. On day 28, the rats were anesthetized (sodium pentobarbital 60 mg/kg, i.p.) to perform the intraductal microinjection of phlorizin or saline. The saliva was collected during 10 minutes under pilocarpine stimulation (4 mg/kg, ip). In another set of rats, submandibular glands were removed and processed for analysis of T1R2, T1R3 and GLUT2 protein by immunofluorescence. In a third group of rats, was administered a concentrated glucose solution or saline in the submandibular artery and evaluated the subcellular expression of SGLT1 by immunofluorescence. The results were expressed as mean ± SEM and compared with ANOVA- Newman-Keuls/ Test T student/ Pearson correlation (P < 0,05). The glycemia of DS was increased (P < 0,05) compared to ND and DI. The glucose sensors T1R2 and T1R3 and the glucose transporter GLUT2 were described for the first time, in the basolateral membrane of ductal cells in submandibular glands (GS). After the microinjection of glucose, the immunofluorescence analysis for SGLT1 showed an increasing in the marking of this protein in MLCD of GS. The microinjection of phlorizin increased (P < 0,05) the salivary flow in ND (47 ± 5 μl) and DS (28 ± 3 μl) compared with the group that received microinjection of saline (ND: 22 ± 3 μl, DS: 8 ± 2 μl). The phlorizin has not change salivary glucose concentration when compared to the saline groups. However, phlorizin promoted an increase (P < 0.05) in salivary glucose excretion from DS-phlorizin and DI phlorizin as compared with ND-saline and DI-saline, respectively. The results indicate that the increasing of the presence of glucose in the extracellular fluid of diabetic patients may increase the stimulation of the sensors, T1R2 and T1R3, which promotes the increase in the translocation of SGLT1 to MLCD in GS. We suggest that the reabsorption of glucose is mediated by SGLT1 on the luminal membrane and by the GLUT2 in the basolateral membrane of ductal cells. Moreover, water uptake by SGLT1 could be an important mechanism for the reduction of salivary secretion in diabetes.