In today’s world wrong nutritional habits together with a low level of physical activity have given rise to the development of obesity and its comorbidity, insulin resistance. More specifically, many researches indicate that lipids are vitally involved in the onset of a peripheral tissue (e.g., skeletal muscle, heart, and liver) insulin resistance. Moreover, it seems that diabetes can also induce changes in respect of lipid composition of both the salivary glands and saliva. However, judging by the number of research articles, the salivary glands lipid profile still has not been sufficiently explored. In the current study we aim to assess the changes in the main lipid fractions, namely, triacylglycerols, phospholipids, free fatty acids, and diacylglycerols, in the parotid and the submandibular salivary glands of rats exposed to a 5-week high fat diet regimen. We observed that the high caloric fat diet caused a significant change in the salivary glands lipid composition, especially with respect to PH and TG, but not DAG or FFAs, classes. The observed reduction in PH concentration is an interesting phenomenon frequently signifying the atrophy and malfunctions in the saliva secreting organs. On the other hand, the increased accumulation of TG in the glands may be an important clinical manifestation of metabolic syndrome and type 2 diabetes mellitus.
Carbohydrates and lipids are the two most important classes of molecules in respect of the body energy provisions. In today’s world, however, wrong nutritional habits together with a low level of physical activity have given rise to the development of obesity and its comorbidity, insulin resistance (IR). More specifically, a lot of research indicates that lipids are vitally involved in the onset of a peripheral tissue (e.g., skeletal muscle, heart, liver, and adipose tissue) insulin resistance [
The main function of the salivary glands is, unsurprisingly, the synthesis of saliva, a watery fluid containing electrolytes, mucus, and enzymes. Alongside its role in digestion saliva serves also some protective functions in respect of the oral mucosa and gingiva. Moreover, research of Tomita et al. indicates that the secretions of the parotid and the submandibular salivary glands contain also lipid component (in an amount of about 5–10 mg per 100 mL of the secretion) [
Previously conducted research indicates that diabetes frequently leads to the imbalance in the salivary glands lipid metabolism which subsequently results in the cytoplasm lipid droplets accumulation. However, judging by the limited number of the research articles, the salivary glands lipid profile still has not been sufficiently explored. In the current study we aim to assess, in detail, changes in the main lipid fractions, namely, triacylglycerols, phospholipids, free fatty acids, and diacylglycerols, concentration in the parotid (PSG) and the submandibular (SMSG) salivary glands of rats exposed to a 5-week high fat diet regimen.
The research was conducted on male Wistar rats randomly assigned to one of the experimental groups (8 specimens in each group). Prior to any experiments all procedures concerning animal treatment and maintenance were approved by the Local Ethical Committee for Animal Experiments of the Medical University of Bialystok. The rats were maintained in the appropriate conditions. Throughout the experiment a stable temperature (21-22°C), humidity, twenty-four-hour rhythm (12 h/12 h light-dark cycle), and free access to food and water were preserved.
At the beginning of the experiment the animals were randomly allocated into one of the two groups: control (C): with unrestricted access to a standard rodent diet; and high fat diet fed group (HFD): with unrestricted access to a high caloric research diet (60% of energy derived from fats, as described before [
In addition to the abovementioned procedures also blood from the abdominal aorta was collected. The blood was destined for the analysis of the fasting glucose (Accu-check glucometer, Byer, Germany), insulin (chemiluminescence, Abbot, USA), and free fatty acids levels (as described by Bligh and Dyer [
According to the protocol [
Between groups differences were detected using unpaired Student’s
The average daily food intake was similar in both studied groups. The high fat diet fed rats were characterized by a significantly increased body mass, as compared with the control group (
Effect of high fat diet feeding on body weight, fasting serum glucose level, fasting serum insulin level, the Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) index, and serum FFA level (measured at the beginning of the 6th week).
C | HFD | |
---|---|---|
Body weight (g) | 315.6 ± 17.0 | 375.4 ± 18.1 |
Glucose level (mg/dL) | 101.3 ± 6.4 | 164.5 ± 12.4 |
Insulin level ( |
4.6 ± 0.6 | 55.7 ± 5.7 |
HOMA-IR | 1.6 ± 1.1 | 20.0 ± 2.5 |
FFA level ( |
88.6 ± 10.4 | 152.4 ± 10.1 |
C: control group. HFD: group fed with high fat diet.
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
In the parotid salivary glands we did not observe any significant differences in total PH concentrations between the C and HFD group. However, we noticed an increment of oleic (18:1), linoleic (18:2), eicosapentaenoic (20:5), and docosahexaenoic (22:6) acids concentration in the HFD group in comparison with the C group (
Effects of high fat diet feeding on the salivary glands lipid profile ((a) parotid salivary glands; (b) submandibular salivary glands). Ctrl: control group (
Phospholipids composition in the parotid salivary glands (nmol/g).
Fatty acid | Control | High fat diet |
---|---|---|
Myristic (14:0) | 1067.79 ± 245.404 | 1064.25 ± 183.734 |
Palmitic (16:0) | 18308.42 ± 3549.057 | 20006.66 ± 3724.717 |
Palmitoleic (16:1) | 2262.11 ± 752.999 | 1716.34 ± 255.614 |
Stearic (18:0) | 1666.38 ± 290.83 | 1936.75 ± 250.423 |
Oleic (18:1) | 3515.13 ± 567.385 | 4726.36 ± 910.837 |
Linoleic (18:2) | 1920.61 ± 575.49 | 2656.63 ± 432.943 |
Arachidic (20:0) | 101.46 ± 21.18 | 118.36 ± 36.992 |
|
257.25 ± 90.915 | 212.67 ± 30.417 |
Behenic (22:0) | 167.36 ± 28.941 | 131.77 ± 32.863 |
Arachidonic (20:4) | 737.81 ± 101.194 | 731.71 ± 123.755 |
Lignoceric (24:0) | 98.77 ± 18.389 | 120.46 ± 37.501 |
Eicosapentaenoic (20:5) | 30.46 ± 8.774 | 55.63 ± 8.098 |
Nervonic (24:1) | 88.62 ± 13.577 | 80.72 ± 19.464 |
Docosahexaenoic (22:6) | 451.08 ± 69.753 | 614.43 ± 175.756 |
UFA | 9263.07 ± 971.175 | 10794.47 ± 1410.346 |
SFA | 21410.18 ± 3888.879 | 23378.25 ± 3956.379 |
Total | 30673.25 ± 4728.9 | 34172.72 ± 4828.508 |
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
Phospholipids composition in the submandibular salivary glands (nmol/g).
Fatty acid | Control | High fat diet |
---|---|---|
Myristic (14:0) | 1812.03 ± 410.021 | 220.06 ± 50.003 |
Palmitic (16:0) | 20700.35 ± 5080.428 | 4084.86 ± 826.012 |
Palmitoleic (16:1) | 4888.46 ± 1330.273 | 347.52 ± 90.284 |
Stearic (18:0) | 2551.09 ± 568.847 | 1235.59 ± 228.771 |
Oleic (18:1) | 15946.36 ± 3926.205 | 4915.29 ± 1193.102 |
Linoleic (18:2) | 10439.62 ± 3124.438 | 2841.45 ± 698.514 |
Arachidic (20:0) | 46.67 ± 14.163 | 31.41 ± 3.803 |
|
488.17 ± 261.042 | 157.67 ± 37.696 |
Behenic (22:0) | 51.8 ± 7.359 | 32.97 ± 2.694 |
Arachidonic (20:4) | 391.03 ± 70.611 | 196.61 ± 18.14 |
Lignoceric (24:0) | 14.08 ± 3.693 | 12.93 ± 2.181 |
Eicosapentaenoic (20:5) | 15.41 ± 3.307 | 18.88 ± 1.696 |
Nervonic (24:1) | 13.09 ± 2.719 | 5.57 ± 0.947 |
Docosahexaenoic (22:6) | 92.91 ± 26.462 | 38.75 ± 5.619 |
UFA | 32275.05 ± 6588.666 | 8521.74 ± 1983.94 |
SFA | 25176.03 ± 5704.184 | 5617.82 ± 1005.162 |
Total | 57451.08 ± 11482.449 | 14139.56 ± 2787.592 |
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
On the contrary, the submandibular salivary glands total PH content was significantly decreased (
In the parotid salivary glands total FFAs content was significantly decreased (
Free fatty acids composition in the parotid salivary glands (nmol/g).
Fatty acid | Control | High fat diet |
---|---|---|
Myristic (14:0) | 73.74 ± 11.539 | 38.11 ± 4.69 |
Palmitic (16:0) | 1225.19 ± 312.786 | 721.51 ± 90.679 |
Palmitoleic (16:1) | 475.43 ± 90.237 | 87.46 ± 21.989 |
Stearic (18:0) | 430.62 ± 120.932 | 230.14 ± 43.29 |
Oleic (18:1) | 369.43 ± 50.221 | 156.9 ± 39.858 |
Linoleic (18:2) | 337.67 ± 50.205 | 151.54 ± 35.83 |
Arachidic (20:0) | 6.67 ± 1.861 | 6.25 ± 0.806 |
|
31.86 ± 4.276 | 11.21 ± 3.25 |
Behenic (22:0) | 18.35 ± 4.609 | 4.2 ± 0.822 |
Arachidonic (20:4) | 558.74 ± 242.799 | 136.16 ± 41.227 |
Lignoceric (24:0) | 5.3 ± 1.381 | 3.97 ± 0.72 |
Eicosapentaenoic (20:5) | 28.94 ± 17.181 | 9.99 ± 3.425 |
Nervonic (24:1) | 3.88 ± 0.962 | 2.15 ± 0.29 |
Docosahexaenoic (22:6) | 70.78 ± 7.997 | 5.88 ± 1.923 |
UFA | 1876.72 ± 267.732 | 561.29 ± 134.462 |
SFA | 1759.88 ± 383.428 | 1004.18 ± 134.206 |
Total | 3636.6 ± 622.343 | 1565.47 ± 188.869 |
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
Free fatty acids composition in the submandibular salivary glands (nmol/g).
Fatty acid | Control | High fat diet |
---|---|---|
Myristic (14:0) | 50.36 ± 7.533 | 13.05 ± 2.304 |
Palmitic (16:0) | 1044.21 ± 293.655 | 136.95 ± 22.83 |
Palmitoleic (16:1) | 136.64 ± 33.283 | 3.2 ± 0.647 |
Stearic (18:0) | 673.43 ± 215.344 | 147.14 ± 21.13 |
Oleic (18:1) | 374.01 ± 68.926 | 21.52 ± 4.774 |
Linoleic (18:2) | 347.78 ± 77.772 | 11.45 ± 2.765 |
Arachidic (20:0) | 10.32 ± 2.892 | 5.01 ± 0.88 |
|
31.6 ± 11.089 | 1.97 ± 0.495 |
Behenic (22:0) | 11.71 ± 0.945 | 2.39 ± 0.336 |
Arachidonic (20:4) | 828.63 ± 332.348 | 5.27 ± 1.259 |
Lignoceric (24:0) | 3.67 ± 0.97 | 2.53 ± 0.444 |
Eicosapentaenoic (20:5) | 24.19 ± 6.655 | 1.55 ± 0.656 |
Nervonic (24:1) | 3.78 ± 0.859 | 1.32 ± 0.417 |
Docosahexaenoic (22:6) | 46.98 ± 7.058 | 1.1 ± 0.145 |
UFA | 1793.6 ± 488.952 | 47.38 ± 7.813 |
SFA | 1793.71 ± 515.048 | 307.07 ± 44.118 |
Total | 3587.31 ± 959.758 | 354.44 ± 46.163 |
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
Also in the submandibular salivary glands total FFAs content was significantly decreased (
In the parotid salivary glands total DAG content was significantly decreased (
Diacylglycerols composition in the parotid salivary glands (nmol/g).
Fatty acid | Control | High fat diet |
---|---|---|
Myristic (14:0) | 87.79 ± 22.188 | 56.03 ± 7.414 |
Palmitic (16:0) | 984.94 ± 210.904 | 575.82 ± 94.842 |
Palmitoleic (16:1) | 184.17 ± 24.947 | 49.8 ± 9.994 |
Stearic (18:0) | 431.31 ± 101.976 | 200.57 ± 29.904 |
Oleic (18:1) | 134.86 ± 21.235 | 93.98 ± 12.105 |
Linoleic (18:2) | 171.37 ± 35.577 | 130.41 ± 16.157 |
Arachidic (20:0) | 5.98 ± 1.84 | 5.05 ± 0.855 |
|
7.31 ± 1.464 | 5.26 ± 0.914 |
Behenic (22:0) | 12.08 ± 3.819 | 3.79 ± 0.715 |
Arachidonic (20:4) | 184.27 ± 76.864 | 94.84 ± 19.375 |
Lignoceric (24:0) | 3.77 ± 1.47 | 3.24 ± 0.514 |
Eicosapentaenoic (20:5) | 7.5 ± 3.514 | 5.38 ± 0.913 |
Nervonic (24:1) | 1.26 ± 0.382 | 1.31 ± 0.221 |
Docosahexaenoic (22:6) | 28.71 ± 5.259 | 5.89 ± 0.927 |
UFA | 719.47 ± 127.861 | 386.87 ± 51.931 |
SFA | 1525.87 ± 303.224 | 844.5 ± 99.547 |
Total | 2245.33 ± 426.18 | 1231.37 ± 128.942 |
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
Diacylglycerols composition in the submandibular salivary glands (nmol/g).
Fatty acid | Control | High fat diet |
---|---|---|
Myristic (14:0) | 48.38 ± 11.363 | 38.27 ± 7.62 |
Palmitic (16:0) | 599.62 ± 94.795 | 281.06 ± 47.48 |
Palmitoleic (16:1) | 58.54 ± 19.612 | 4.72 ± 0.704 |
Stearic (18:0) | 364.07 ± 54.254 | 261.05 ± 48.843 |
Oleic (18:1) | 142.47 ± 40.036 | 44.75 ± 5.25 |
Linoleic (18:2) | 186.84 ± 50.092 | 43.5 ± 5.515 |
Arachidic (20:0) | 9.04 ± 1.714 | 6.11 ± 0.856 |
|
8.62 ± 1.633 | 3.13 ± 0.448 |
Behenic (22:0) | 7.08 ± 0.724 | 2.94 ± 0.475 |
Arachidonic (20:4) | 184.5 ± 48.12 | 35.4 ± 3.616 |
Lignoceric (24:0) | 2.3 ± 0.579 | 2.14 ± 0.393 |
Eicosapentaenoic (20:5) | 4.65 ± 1.736 | 2.32 ± 0.334 |
Nervonic (24:1) | 0.75 ± 0.188 | 1.04 ± 0.211 |
Docosahexaenoic (22:6) | 13.61 ± 2.375 | 2.06 ± 0.324 |
UFA | 600 ± 134.423 | 136.92 ± 10.521 |
SFA | 1030.49 ± 155.56 | 591.57 ± 101.192 |
Total | 1630.49 ± 266.998 | 728.49 ± 107.393 |
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
Also in the submandibular salivary glands total DAG content was significantly decreased (
In the parotid salivary glands we did not observe any significant differences in total TG concentrations between the C and HFD groups. However, we noticed an increment of linoleic (18:2), eicosapentaenoic (20:5), and docosahexaenoic acid (22:6) concentration coexisting with a decrement of behenic acid content (22:0) in the HFD group in comparison with the C group (
Triacylglycerols composition in the parotid salivary glands (nmol/g).
Fatty acid | Control | High fat diet |
---|---|---|
Myristic (14:0) | 95.58 ± 16.557 | 93.13 ± 10.494 |
Palmitic (16:0) | 3753.26 ± 577.212 | 4106.75 ± 355.387 |
Palmitoleic (16:1) | 725.82 ± 311.454 | 457.56 ± 60.399 |
Stearic (18:0) | 1545.87 ± 203.585 | 1739.3 ± 179.592 |
Oleic (18:1) | 740.19 ± 141.33 | 758.58 ± 60.541 |
Linoleic (18:2) | 1745.84 ± 508.939 | 2123 ± 239.624 |
Arachidic (20:0) | 6.46 ± 0.973 | 6.98 ± 1.074 |
|
15.69 ± 5.458 | 14.51 ± 3.86 |
Behenic (22:0) | 57.18 ± 12.029 | 25.77 ± 5.108 |
Arachidonic (20:4) | 2336.43 ± 247.191 | 2462.9 ± 323.202 |
Lignoceric (24:0) | 6.2 ± 1.302 | 7.04 ± 1.189 |
Eicosapentaenoic (20:5) | 63.45 ± 20.07 | 114.81 ± 19.906 |
Nervonic (24:1) | 6 ± 3.685 | 4.11 ± 2.077 |
Docosahexaenoic (22:6) | 175.64 ± 37.506 | 250.05 ± 49.041 |
UFA | 5809.07 ± 895.885 | 6185.53 ± 508.847 |
SFA | 5464.55 ± 774.298 | 5978.97 ± 530.721 |
Total | 11273.62 ± 1662.308 | 12164.5 ± 1031.873 |
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
Triacylglycerols composition in the submandibular salivary glands (nmol/g).
Fatty acid | Control | High fat diet |
---|---|---|
Myristic (14:0) | 72.45 ± 10.702 | 47.77 ± 3.466 |
Palmitic (16:0) | 2978.17 ± 381.425 | 2979.54 ± 157.422 |
Palmitoleic (16:1) | 183.65 ± 28.609 | 67.47 ± 4.61 |
Stearic (18:0) | 1575.82 ± 255.862 | 2004.72 ± 79.831 |
Oleic (18:1) | 732.15 ± 62.753 | 953.72 ± 35.941 |
Linoleic (18:2) | 1416.05 ± 347.755 | 2068.17 ± 272.292 |
Arachidic (20:0) | 14.95 ± 4.395 | 19.21 ± 3.536 |
|
15.17 ± 5.569 | 14.62 ± 1.299 |
Behenic (22:0) | 31.32 ± 9.904 | 28.75 ± 5.13 |
Arachidonic (20:4) | 2542.74 ± 561.505 | 2542.69 ± 176.752 |
Lignoceric (24:0) | 15.01 ± 1.78 | 12.17 ± 1.746 |
Eicosapentaenoic (20:5) | 40.37 ± 13.144 | 95.49 ± 20.396 |
Nervonic (24:1) | 9.62 ± 4.199 | 6.56 ± 1.041 |
Docosahexaenoic (22:6) | 171.85 ± 52.848 | 239.27 ± 29.357 |
UFA | 5111.61 ± 933.047 | 5988 ± 227.362 |
SFA | 4687.72 ± 623.025 | 5092.15 ± 206.762 |
Total | 9799.33 ± 1548.921 | 11080.15 ± 422.14 |
Results are based on 8 independent preparations for each experimental treatment (means ± SD).
On the contrary, the submandibular salivary glands total TG content was significantly increased (
Obesity is currently a predominant medical condition because for several dozens of years an alarming increase in its prevalence has been observed. Unfortunately, raised body mass index (BMI) is an important risk factor for the development of several diseases, including insulin resistance and type 2 diabetes. Tissues which are responsible for the onset and development of the aforementioned conditions are mainly skeletal muscles, liver, and white adipose tissue. However, glucose homeostasis imbalance may affect any other tissue, including the salivary glands. Therefore, in the present study we examined the effect(s) of a diet induced insulin resistance and obesity on lipid profile in both the parotid and the submandibular salivary glands.
Some of the previously published reports indicate that chronic high fat diet feeding causes hyperglycemia which can contribute to the accumulation of lipid droplets in the cytoplasm of many nonadipose tissues, including skeletal muscles, liver, and even salivary glands [
Scientific investigation has led to the discovery of more than 30 different biological components present in the salivary glands, of which lipids seem to be a particularly important representative [
On opposition to the above-discussed phospholipid fraction free fatty acids are believed to be heavily implicated in the processes of intracellular signal transmission. Previously published observations performed on insulin resistant diabetic animals clearly demonstrated increased FFAs concentrations in many tissues, including skeletal muscles and liver [
Interestingly, in the present study we have noticed a decreased level of DAG in the parotid and the submandibular salivary glands of the diabetic rats. We demonstrated, likewise in the case of FFAs, that this decrement applies to both the saturated and the unsaturated fatty acid species. At first glance, this novel and intriguing finding seems to be quite unexpected. Many authors point on the causative relationship between DAG over accumulation and insulin resistance in peripheral tissues (e.g., skeletal muscles, adipose tissue, and liver) [
Another plausible explanation for the observed reductions in the salivary glands lipid content is a possible tissue atrophy frequently observed in some of the previous studies [
Some of the previously published studies indicate that the salivary glands intraorgan lipid accumulation occurs along with the time-course of insulin resistance and diabetes; moreover, its magnitude usually accompanies the changes in the serum glucose concentration [
In conclusion, due to the scarcity of the literature data the precise effects of insulin resistance and type 2 diabetes with respect to the salivary glands lipid profile were not, so far, extensively elucidated. In the present study we aimed to fill this gap. Firstly, we observed that the high fat diet regimen had caused significant changes in the salivary glands lipid composition, especially in regard to PH and TG, but not DAG or FFAs, classes. The observed reduction in PH concentration is an interesting phenomenon frequently signifying the atrophy and malfunctions in the saliva secreting organs. On the other hand, the increased accumulation of TG in the glands may be an important clinical manifestation of metabolic syndrome and type 2 diabetes mellitus.
The authors declare that there are no competing interests regarding this paper publication.
The study was supported by Medical University of Bialystok Grants nos. 153-18701L (N/ST/ZB/15/0012/1118) and 153-18700L (N/ST/ZB/15/0011/1118).