Protein free diet feeding: Effects on sympathetic activity and salivary evoked secretion in the submandibular gland of the rat

Salivary gland function, development, and regeneration

Salivary glands produce and secrete saliva, which is essential for maintaining oral health and overall health. Understanding both the unique structure and physiological function of salivary glands, as well as how they are affected by disease and injury, will direct the development of therapy to repair and regenerate them. Significant recent advances, particularly in the OMICS field, increase our understanding of how salivary glands develop at the cellular, molecular, and genetic levels: the signaling pathways involved, the dynamics of progenitor cell lineages in development, homeostasis, and regeneration, and the role of the extracellular matrix microenvironment. These provide a template for cell and gene therapies as well as bioengineering approaches to repair or regenerate salivary function.

Switching to normal diet reverses kwashiorkor-induced salivary impairments via increased nitric oxide level and expression of aquaporin 5 in the submandibular glands of male Wistar rats

Kwashiorkor, a form of malnutrition, has been shown to cause impaired salivary secretion. However, there is dearth of information on the mechanism that underlies this complication. Also, whether returning to normal diet after kwashiorkor will reverse these complications or not is yet to be discerned. Thus, this study aimed at assessing the mechanisms that underlie kwashiorkor-induced salivary impairments and to evaluate the effects of switching back to normal-diet on kwashiorkor-induced salivary impairments. Weaning rats were randomly divided into 3 groups (control group, kwashiorkor group (KG), re-fed kwashiorkor group (RKG)) of 7 rats each. The control group had standard rat chow while the KG and RKG were fed 2% protein diet for 6 weeks to induce kwashiorkor. The RKG had their diet changed to standard rat-chow for another 6 weeks. Blood and stimulated saliva samples were collected for the analysis of total protein, electrolytes, amylase, immunoglobulin A (IgA) secretion rate, leptin, and ghrelin. Tissue total protein, nitric oxide level, expressions of Na⁺/K⁺-ATPase, muscarinic (M3) receptor, and aquaporin 5 in the submandibular glands were also determined. Data were presented as means ± SEM and compared using ANOVA with Tukey's post hoc test. RKG showed improved salivary function evidenced by reduced salivary lag-time and potassium and increased flow rate, sodium, amylase, IgA secretion rate, leptin, submandibular nitric oxide level, and aquaporin 5 expression compared with KG. This study for the first time demonstrated that kwashiorkor caused significant reduction in salivary secretion through reduction of nitric oxide level and aquaporin 5 expression in submandibular salivary glands. Normal-diet re-feeding after kwashiorkor returned salivary secretion to normal.

Chronic high-protein diet induces oxidative stress and alters the salivary gland function in rats

OBJECTIVE

Chronic high protein intake leads to an increase in reactive oxygen species (ROS) generation. However, there is no data on the impact of high-protein diet on the antioxidant barrier, oxidative stress and secretory function in the salivary glands of healthy individuals.

DESIGN

16 male Wistar rats were randomly divided into 2 groups (n=8): normal protein (C) and high-protein diet (HP) for 8 weeks. Salivary antioxidants: peroxidase (Px), catalase (CAT), superoxide dismutase 1 (SOD 1), uric acid (UA), total antioxidant status (TAS), total oxidant status (TOS) and the oxidative stress index (OSI), as well as protein carbonyls (PC), 4-hydroxynonenal protein adduct (4-HNE protein adduct), 8-isoprostanes (8-isoP), 8-hydroxy-2'-deoxyguanosine (8-OHdG) and protein content were determined in the salivary glands and plasma. Salivary unstimulated and stimulated flow rates were examined.

RESULTS

Parotid Px, TAS, UA, TOS, OSI, PC were significantly higher, the total protein content was statistically lower in the HP group as compared to the control. Submandibular UA, TOS, OSI, 8-isoP, 4-HNE-protein adduct, 8-OHdG were statistically elevated, SOD 1 and Px were significantly lower in the HP group as compared to the control rats. The unstimulated salivary flow rate was significantly depressed in the HP group as compared to the controls.

CONCLUSIONS

Higher antioxidant capacity in the parotid glands of HP rats vs. control rats seems to be a response to a higher ROS formation. In the submandibular glands severe oxidative modification of almost all cellular components was observed. Administration of HP resulted in the weakening of the salivary gland function.

Reduced methacholine-induced submandibular salivary secretion in rats with experimental periodontitis

OBJECTIVE

Saliva is the first barrier to the entry of bacteria and viruses into the body and is considered a necessary instrument in oral health. Intraperitoneal injection of lipopolysaccharide endotoxins results in submandibular gland (SMG) hyposalivation. The objective of present studies was to assess if periodontitis, a chronic inflammatory disease caused by oral bacteria, alters cholinergic-induced SMG salivary secretion.

DESIGN

An experimental periodontitis model (EP) (cotton thread ligature around the neck of the first lower molars) was used. Male Wistar rats (300-380g) were randomly divided into 3 groups: control, 7 days-bilateral EP and 7 days-unilateral EP (to study if there were different effects at the ipsilateral and contralateral side). The following determinations were performed in SMG: (1) dose-response curves to the cholinergic agonist methacholine, (2) prostaglandin E (PGE) content, (3) inducible nitric oxide synthase (iNOS) activity and (4) histology of gland sections.

RESULTS

The molars with EP, no matter the group, exhibited significant and similar bone loss (p<0.001). Bilateral EP reduced methacholine-induced salivary secretion (p<0.05, dose 1μg/kg; p<0.001, dose 3-30μg/kg), increased PGE content (p<0.01), stimulated iNOS activity (p<0.05). Ipsilateral glands of unilateral EP animals presented lower methacholine-induced salivary secretion (p<0.05, dose 3μg/kg; p<0.001, dose 10-30μg/kg), and higher PGE content than contralaterals (p<0.001). In turn, at 3 and 10μg/kg of methacholine, contralateral glands showed significantly lower secretion than control animals (p<0.001). Histological studies of glands revealed partial loss of secretor granular material and periductal oedema in the bilateral and unilateral EP groups as compared to controls.

CONCLUSIONS

As far as we know, the present results demonstrate for the first time that EP reduces methacholine-induced SMG salivary secretion.