Ischemia preconditioning protects rat submandibular glands from ischemia/reperfusion injuries

Effect of ionizing radiation on the secretion of the paracellular pathway in rat submandibular glands

OBJECTIVES

This study aims to investigate the effects of ionizing radiation on the secretion of the paracellular pathway in rat submandibular glands (SMGs) and reveal the changes in the tight junction (TJ) protein claudin-4.

METHODS

A total of 24 Wistar rats were randomly divided into control and irradiation groups. The irradiation groups were further divided into 1, 4, and 12 weeks groups after irradiation. One-time 20 Gy irradiation was given to the SMG area on the experimental side of the irradiation group. At 1, 4, and 12 weeks after irradiation, the secretion of SMGs was measured using the Schirmer's test. The pathological changes in the gland tissues were observed under light microscopy after hematoxylin⁃eosin (HE) staining. The changes in the TJ ultrastructure were observed under transmission electron microscopy. The immunofluorescence staining and Western blot were used to detect the expression levels of muscarinic acetylcholine M3 receptor, aquaporin 5 (AQP5), and claudin-4 protein.

RESULTS

At 1, 4, and 12 weeks after irradiation, the secretion of SMGs in the irradiation group was significantly decreased and lower than that in the control group (P<0.01). At 1 week, the interstitial edema was observed in SMG tissues. Nuclear pyknosis, decreased number of acinar cells, and small focal necrosis with inflammatory infiltration were also observed over time. However, these changes were most evident at 12 weeks after irradiation. In the irradiation group, the TJ ultrastructure of glands at different times appeared to be fuzzy, collapsed, and had decreased electron density. Moreover, the width of TJs was remarkably decreased (P<0.01). The expression levels of M3 and AQP5 were decreased in a time-dependent manner, and the fluorescence intensity was significantly reduced after irradiation. However, the expression levels and fluorescence intensity of claudin-4 were enhanced in different degrees.

CONCLUSIONS

The changes in the TJ structure, the upregulation of the claudin-4 expression, and the damage in the paracellular pathway were involved in the hyposecretion of SMGs after irradiation.

Pilocarpine improves submandibular gland dysfunction in irradiated rats by downregulating the tight junction protein claudin-4

OBJECTIVES

To investigate the effects of radiation on paracellular pathway of rat submandibular glands (SMGs) and the mechanism of increasing secretion following treatment with pilocarpine.

MATERIALS AND METHODS

In situ irradiation models of SMGs in Wistar rats were conducted, and the glands were exposed to X-radiation at a single dose of 20 Gy. Pilocarpine was intraperitoneally injected 60 min prior to radiation and continuous 6 days postirradiation for a total of 7 days. Salivary secretion, histological changes, pro-inflammatory cytokines, alterations in tight junctions (TJs), and functional membrane proteins aquaporin-5 (AQP5) and claudin-4 mediated by the muscarinic acetylcholine M3 subtype receptor were determined at 1 and 12 weeks after irradiation.

RESULTS

Salivary secretion of the irradiated glands was reduced at 1 and 12 weeks. As well, acinar cell numbers, TJ width, and the levels of M3 receptor and AQP5 were decreased. In contrast, tumor necrosis factor-α, interleukin 6, interleukin 1α, and the expression of the TJ protein claudin-4 were significantly increased in irradiated SMGs. Notably, all the alterations were attenuated by pilocarpine treatment.

CONCLUSIONS

Pilocarpine could improve the secretory function of irradiated rat SMGs via reducing inflammation, ameliorating the structural injury of TJs, and attenuating the up-regulation of claudin-4 expression.

Effect of ischemic preconditioning on radiation damage to the submandibular gland in rats

To investigate the effects of radiation on rat submandibular glands and the possible protective effects of ischemic preconditioning, the submandibular glands of Wistar rats were subjected to in situ radiation after ischemic preconditioning. The glands were exposed to X-radiation at a single dose of 20 Gy. Ischemic preconditioning was achieved by three min of ischemia and three min of reperfusion, repeated three times before irradiation. Salivary secretion, histological changes, alterations in tight junctions, and the levels of oxidative stress, pro-inflammatory cytokines, and water secretion proteins mediated by the muscarinic acetylcholine M3 subtype receptor were determined at 1 and 12 weeks post-irradiation. In glands subjected to irradiation only, the secretion, superoxide dismutase activity, tight junction width, acinar cell number, and M3 receptor and aquaporin-5 levels were lower at 1 and 12 weeks than seen in the ischemically preconditioned irradiated glands. In contrast, tumor necrosis factor-α, malondialdehyde, myeloperoxidase activity, and the expression of the tight junction protein claudin-4 were significantly higher in the irradiated only glands. Our study revealed that radiation caused a series of injury-stress responses, especially damage to the water secretion pathway mediated by the M3 receptor that ultimately led to hyposecretion, which might play an important role in the dysfunction of the irradiated only glands. Ischemic preconditioning reduced the radiation-induced injury to submandibular glands and ameliorated salivary hyposecretion.

Expression patterns of tight junction proteins in porcine major salivary glands: a comparison study with human and murine glands

Tight junction (TJ) proteins play a dynamic role in paracellular fluid transport in salivary gland epithelia. Most TJ studies are carried out in mice and rats. However, the morphology of rodent salivary glands differs from that of human glands. This study aimed to compare the histological features and the expression pattern of TJ proteins in porcine salivary glands with those of human and mouse. The results showed that porcine parotid glands were pure serous glands. Submandibular glands (SMGs) were serous acinar cell-predominated mixed glands, whereas sublingual glands were mucous acinar cell-predominated. Human SMGs were mixed glands containing fewer mucous cells than porcine SMGs, whereas the acinar cells of murine SMGs are seromucous. The histological features of the duct system in the porcine and human SMGs were similar and included intercalated, striated and excretory ducts, but the murine SMG contained a specific structure, the granular convoluted tubule. TJ proteins, including claudin-1 to claudin-12, occludin and zonula occludin-1 (ZO-1), were detected in the porcine major salivary glands and human SMGs by RT-PCR; however, claudin-6, claudin-9 and claudin-11 were not detected in the murine SMG. As shown by immunofluorescence, claudin-1, claudin-3, claudin-4, occludin and ZO-1 were distributed in both acinar and ductal cells in the porcine and human SMGs, whereas claudin-1 and claudin-3 were mainly present in acinar cells, and claudin-4 was mainly distributed in ductal cells in the murine SMG. In addition, 3D images showed that the TJ proteins arranged in a honeycomb-like structure on the luminal surface of the ducts, whereas their arrangements in acini were irregular in porcine SMGs. In summary, the expression pattern of TJ proteins in salivary glands is similar between human and miniature pig, which may be a candidate animal for studies on salivary gland TJ function.