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2 Department of Physiology and 1 First Department of Operative Dentistry, Kyushu Dental College, Kokurakitaku, Kitakyushu 803-8580, and 3 Department of Physiology, School of Medicine, University of Occupational and Environmental Health, Yahatanishiku, Kitakyushu 807-8555, Japan
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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To find mechanisms of an extreme polydipsia in an inbred strain of mice, STR/N, this study was undertaken using Institute of Cancer Research (ICR) mice as a control. During food deprivation, daily water intake of both strains decreased. The decrement in the STR/N mice was larger than that in the ICR mice. During dehydration, daily food intake of the STR/N mice was smaller than that of the ICR mice. These data indicate that prandial drinking was more severely affected for the STR/N mice. Under anesthesia, the stimulated salivary secretion by pilocarpine of the STR/N mice was significantly smaller than that of the ICR mice. The submandibular gland of the STR/N mice was lighter and harder than that of the ICR mice. After desalivation from the major three salivary glands, the ICR mice drank as much as the STR/N mice. Young STR/N mice with undeveloped polydipsia did not show different salivary secretion stimulated by pilocarpine from the young ICR mice. These findings indicate a dysfunction with age in the salivary glands of the STR/N mice, and they suggest that the decreased saliva induces thirst and triggers extraordinary drinking in the polydipsic mice.
drinking; thirst; salivary gland
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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THE INBRED STRAIN OF MICE (STR/N), originally discovered in the late 1950s, shows extreme polydipsia and polyuria. These mice have a normal or higher amount of arginine vasopressin in their plasma, and they respond well to the exogenous vasopressin with antidiuretic action in the kidney (14). Within 6 mo after birth, most STR/N mice drink several times more water than the nonpolydipsic mice. Until the last decade, no extensive study has been done to find the causes for polydipsia in the STR/N mice despite the interesting characteristics on body fluid balance.
It is known that various regions of the brain, particularly the circumventricular organs and the hypothalamus, are involved in the integrative function of drinking behavior. The recent studies on STR/N mice reveal that the anteroventricular region of the third ventricle (AV3V), the region related to body fluid balance and drinking, has different affinities to some dipsogens such as ANG II (5) and opioids (4). Moreover, it is suggested that neurons in such regions are strongly activated by water deprivation in the STR/N mice (15).
Dipsogenic stimuli induce a feeling of thirst or dryness in the oral cavity. Some studies show that the central (10) and peripheral (16) dipsogenic stimuli decrease salivary secretion. Salivary dysfunction or salivary resection makes a mouth dry and increases drinking. Many years ago, Cannon (2) proposed a dry mouth theory of thirst. Although this is not enough to explain a whole process of drinking behavior induced by dipsogenic stimulation, it is clear that saliva is an important factor in controlling thirst sensation and drinking. Thus the purpose of this study is to investigate whether or not the extraordinary large amount of drinking in STR/N mice is based on a dysfunction of salivary glands.
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals
Adult male polydipsic inbred (STR/N strain) and control Institute of Cancer Research (ICR) strain mice at 6-11 mo of age were used in most of the present experiment. Mice less than 6 mo old were used when necessary. Both strains of mice were individually housed under 12:12-h light-dark cycles (lights on at 0800) and had free access, except where noted, to tap water and dry food (standard laboratory chow). Room temperature was maintained between 21 and 24°C. Relative humidity was controlled between 40 and 60%.The present study was carried out after receiving permission from the Animal Experiment Committee, Kyushu Dental College.
Measurement of Water and Food Intake
In this series of the experiment, daily water and food intake, daily water intake during food deprivation, and daily food intake during dehydration were measured between 1000 and 1130. Water and food intake was measured to the nearest 0.1 g by weighing the water bottle, the food basket, and the amount of spillage (A & D FY-3000). Body weights were also measured.Water and food intake during the 3 days before food deprivation and dehydration were measured and averaged, and the averaged values were used as a control. The STR/N mice, which showed >20 ml (per 40 g of body wt) of daily water intake, were selected for further experiments.
Salivary Gland
Mice were deeply anesthetized with urethan (1.65 g/kg ip). Body temperature was maintained with an animal blanket controller (Nihon Kohden, ATB-1100), if necessary. Measurements of the volume and osmolality of the saliva stimulated by pilocarpine and the change of blood flow in the submandibular gland also stimulated by pilocarpine were done from 1100 to 1500.Wet and dry weights of the submandibular and sublingual glands. The submandibular and sublingual glands on both sides were removed under a binocular microscope. Immediately after the removal, the glands were measured. The glands were kept in an oven at 70°C for 2 days to measure their dry weight.
Hardness of the submandibular glands. The hardness of the
extracted submandibular glands was measured with a homemade rheometer. The rheometer was made of a ballpoint pen (Zebra KC-80), a manipulator (Narishige, MP-1), and an electric balance (A & D, HF 400) (Fig. 1A). The main body of the ballpoint
pen was cut to 80-mm long. A spring originally present inside the pen
was replaced with a long, soft one (70-mm long, 4 mm in diameter, 0.36 mm/g spring constant). Held by a manipulator, the ballpoint pen was
used to press the submandibular glands down. After the head of the
ballpoint pen was attached to a surface of the glands, the pen was
quickly dropped 1 mm during 8 s. The number in the weight display of
the electric balance was then read aloud every 15 s for 2 min. As shown
in Fig. 1B, the values were time dependent so that a
y-intercept was taken as a value indicating the hardness of the
submandibular glands after a single exponential curve fitting was
accomplished.
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The hardness of the submandibular glands was compared with that of gelatin blocks. For this purpose, gelatin was dissolved in hot distilled water to make a 5, 10, 15, 20, 25, and 30% concentration, and it was stored in petri dishes (Falcon) and kept for a day at 4°C. The thickness of the gelatin block was 8 mm. Figure 1C shows a relationship between the hardness and the concentration of gelatin.
Volume and osmolality of secreted saliva stimulated by pilocarpine. The tracheas of the urethan-anesthetized mice were cannulated so as not to be stifled with secreted saliva. To collect saliva, which was secreted from the submandibular and sublingual glands, parotid ducts were dissected bilaterally. Mice were then placed in a homemade holder in a prone position. The upper incisors were hooked on an iron bar, which was fixed with the homemade holder, and the lower incisors were pulled with a string to keep the mouth open. By lifting the tongue up to the palatum durum and holding it with the aid of a filter paper, the sublingual caruncle could be directly seen.
Saliva secretion was induced by intraperitoneal administration of pilocarpine (20 µmol/kg). The dose of pilocarpine was chosen to induce the maximum response of salivary secretion in mice (13). A small cotton wick was placed on the sublingual caruncle to collect the saliva. The secreted saliva was sampled every 2 min and measured to the nearest 0.1 mg by weighing the cotton wicks. Saliva, which was secreted during the 55 min after the pilocarpine stimulation, was evaluated as total saliva. For measurement of the saliva osmolality, 15 µl of saliva standing in the mouth was sampled with a micropipette and measured with an osmometer (Fiske, One-Ten). The mice did not have a cotton wick in the mouth for this experiment.
Measurement of blood flow in the submandibular glands. Blood flow changes in the submandibular gland of the mice were monitored using a laser-Doppler flowmeter (BRC, BRL-100). The probe was placed against the left side of the submandibular gland without exerting any pressure on the tissue. The flow parameter indicates a blood flow of superficial vessels in the gland. Output from the apparatus was continuously monitored and stored by using a Mac-Lab system. The relative change of the blood flow during the induced salivary secretion was evaluated.
Removal of Salivary Glands
Mice were anesthetized with pentobarbital sodium (60 mg/kg ip). The submandibular and sublingual glands were approached ventrally, and after bilateral ligation of the ducts, they were removed. The parotid ducts were bilaterally ligated and cut at the peripheral end of ligation.Plasma Osmolality
Immediately after the decapitation of the animals, blood was collected with heparin sodium salt (5 units for 1-ml blood sample). The blood sample was centrifuged at 4,000 rpm for 10 min. Osmolality of the 15-µl plasma sample was measured in the same way as the measurement of saliva osmolality.Statistical Analysis
Data on water and food intake was converted to milliliters and grams consumed per 40 g body wt, respectively. The 40-g unit was chosen for easy comparison because it is a body weight roughly averaged between the two strains of mice. For data analysis, Mann-Whitney U test was used. Changes in daily water intake after the removal of the salivary glands were analyzed by Friedman test. The data are given as means ± SE. The data were compared between the two strains, ICR and STR/N, of mice.| |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Water and Food Intakes
Daily water intake of the ICR and STR/N mice used in this study was 5.4 ± 0.5 (n = 10) and 37.0 ± 4.0 ml (n = 15), respectively. Daily food intake of the ICR and STR/N mice was 4.6 ± 0.2 (n = 10) and 3.9 ± 0.3 g (n = 15), respectively.Figure 2 shows daily water intake during
the control period and during the 24-h food deprivation. During
24-h food deprivation, the daily water intake of the ICR mice
decreased from 5.4 ± 0.5 to 3.1 ± 0.4 ml (n = 10, 43%
decrease), whereas the daily water intake of the STR/N mice decreased
from 43.1 ± 7.0 to 15.0 ± 2.8 ml (n = 7, 75%
decrease). Although both strains of mice decreased daily
water intake, the amount of a decrease in the STR/N mice was larger
than that in the ICR (P < 0.01) mice. Figure
3 shows daily food intake during the
control period and during the 24-h dehydration. During the 24-h
dehydration, the food intake of the ICR mice decreased from 4.2 ± 0.2 to 2.5 ± 0.2 g (n = 10, 40% decrease), whereas the food
intake of the STR/N mice decreased from 4.4 ± 0.3 to 0.8 ± 0.2 g
(n = 7, 82% decrease). Two of seven STR/N mice did not eat food during dehydration. The difference in the
food intake during the 24-h dehydration was significant (P < 0.01) between both strains. This indicates that the STR/N mice show little appetite for food without water intake.
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Saliva Secretion Stimulated by Pilocarpine Administration
In this experiment, saliva secretion stimulated by an intraperitoneal injection of pilocarpine (20 µmol/kg) was measured in the ICR and STR/N mice under urethan anesthesia. When unstimulated, no obvious salivary secretion was detected under our experimental condition. Figure 4 shows changes in salivary secretion after an intraperitoneal injection of pilocarpine. In the ICR mice, the stimulated salivary secretion started to increase after 4 min and reached its maximum 7 min after the pilocarpine injection, and thereafter it gradually decreased. In the STR/N mice, the stimulated salivary secretion reached its maximum level faster than that in the ICR mice 5 min after the pilocarpine injection and thereafter quickly returned to the control level. The total amount of saliva secreted by the STR/N mice stimulated by pilocarpine was significantly smaller than that in the ICR (Fig. 4B) mice.
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The osmolality of the saliva stimulated by an intraperitoneal injection of pilocarpine was measured by collecting saliva around the period of maximal secretion. The osmolality of saliva was 192.6 ± 4.6 mosmol/kgH2O in the ICR mice (n = 7) and 268.7 ± 19.6 mosmol/kgH2O in the STR/N mice (n = 8), respectively. The saliva osmolality of the STR/N mice was significantly higher than that of the ICR (P < 0.01) mice.
Changes in Blood Flow in the Submandibular Gland by Intraperitoneal Pilocarpine Injection
Salivary secretion is influenced by the blood supply to the glands. For rapid secretion, a large amount of blood supply is required (17). With a laser-Doppler flowmeter, we measured changes in blood flow in the submandibular gland. Figure 5 (ICR mice, A; STR/N mice, B) shows changes in blood flow induced by an intraperitoneal injection of pilocarpine (20 µmol/kg). In the ICR mice, blood flow increased and recovered to the control level in the same manner as the salivary secretion, as shown in Fig. 4. In the STR/N mice, however, no obvious change of blood flow was observed. Figure 5C shows the percent changes in blood flow at their maximum levels after the pilocarpine stimulation in the two strains. Compared with the STR/N mice, the blood flow in the ICR mice significantly increased with an administration of pilocarpine.
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Wet and Dry Weights of the Submandibular Glands
The wet weight of the submandibular gland of the ICR and STR/N mice was measured. Figure 6 shows a relationship between the wet weight of the submandibular gland and the body weight. Because the body weight of the STR/N mice was lighter than that of the ICR mice, data from 10 ICR mice <6 mo old were added to cover for the range of light body weight (<37 g). Correlation coefficients of the ICR and STR/N mice were 0.876 and 0.737, respectively. These correlation coefficients showed significant difference (P < 0.05). As shown in Fig. 6, it is likely that the wet weights of the submandibular gland of the STR/N mice are lighter than those of the ICR mice with a range of >30 g in body wt. The dry weights of the submandibular and sublingual glands were also measured and evaluated as percentages against the wet weights. The percentages of the ICR and STR/N mice were 25.2 ± 0.4 (n = 14) and 26.3 ± 0.5% (n = 11), respectively. There was no significant difference.
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Hardness of Submandibular Glands
Figure 1 shows the hardness of the submandibular glands of the ICR and STR/N mice measured in arbitrary units. The submandibular glands of the STR/N mice (n = 13) were 2.1 times harder than those of the ICR mice (n = 14), and the difference was significant (P < 0.01). Figure 1 also shows that the hardness of the submandibular glands of the ICR and STR/N mice was roughly equal to that of the gelatin at 4 and 10% at 4°C, respectively.Plasma Osmolality
The plasma osmolality was measured in the ICR and STR/N mice. The plasma osmolality was 340.4 ± 2.8 mosmol/kgH2O in the ICR mice (n = 8) and 359.6 ± 4.3 mosmol/kgH2O (n = 5) in the STR/N mice, respectively. The plasma osmolality of the STR/N mice was significantly higher than that of the ICR (P < 0.01) mice.Removal of Salivary Glands
To understand how salivary secretion influences water intake in both strains of mice, the volume of daily water intake before and after desalivation was measured. Figure 7 shows that after this procedure, the ICR mice significantly increased their daily water intake (Friedman test, P < 0.01) almost to the same level as the STR/N mice, whereas the STR/N mice showed no change in water intake.
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Salivary Secretion of Young Mice Stimulated by Pilocarpine
Polydipsic characteristics in the STR/N mice develop with age. To understand the relationship between polydipsia and salivary functions of the STR/N mice, salivary secretion of young mice with the same stimulation (20 µmol/kg pilocarpine) was measured. The young mice used in this experiment were 8-10 wk old in both the ICR and STR/N strains. The young STR/N mice did not show extraordinary drinking (14). Figure 8 shows a time course of salivary secretion after an intraperitoneal injection of pilocarpine (20 µmol/kg) and the total amounts of saliva from both strains during 55 min. The total amounts of saliva in the ICR and STR/N mice were 237.3 ± 36.3 (n = 7) and 256.6 ± 20.5 µl (n = 7), respectively, and the difference was not significant (Fig. 8).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Under normal conditions, the inbred strain of mice, STR/N, showed a water intake 8.0 times higher than the ICR mice, which were used as control animals. During the 24-h food deprivation, daily water intake of both strains decreased. The decrement in the STR/N mice was significantly larger than that in the ICR mice, although the STR/N mice still showed a daily water intake 4.8 times higher than the ICR mice. During dehydration, the daily food intake of the STR/N mice was smaller than that of the ICR mice. Thus the STR/N mice drink much more than the ICR mice during prandial and nonprandial periods, although the polydipsic tendency is much greater during the prandial period.
Xerostomia, which is the subjective feeling of oral dryness, is accompanied by salivary gland dysfunction. A similar situation is found in the STR/N mice. We showed that salivary secretion from the submandibular and sublingual glands of the STR/N mice stimulated by pilocarpine was approximately one-half of that of the ICR mice at their maximal levels, and the osmolality of the saliva from the STR/N mice was significantly higher than that of the ICR mice. In addition, the salivary glands of the polydipsic mice were smaller and harder, and no increase in blood flow to the gland occurred during the stimulation by pilocarpine in the STR/N mice, whereas in the control mice, it increased to over 170%. Moreover, the STR/N mice showed no change in daily water intake after the removal of three major salivary glands, whereas the ICR mice increased their daily water intake by five to six times. These findings suggest a dysfunction of the salivary glands in the STR/N mice.
Because saliva has antimicrobial action, its decreased secretion may decrease antimicrobial action and help to develop periodontitis. It is interesting to note that the STR/N mice have been studied as model animals with periodontitis, which, with age, has a progressive development to gingivitis, inducing a periodontal pocket formation (8, 18). Alveolar bone loss becomes apparent beyond 8 mo (18). Cultivable microbial changes associated with advancing periodontitis and alveolar bone loss were also correlated with the age of the STR/N mice (18). We found that the young STR/N mice secreted the same volume of saliva after pilocarpine stimulation as the ICR mice, whereas the aged STR/N mice secreted less than the aged ICR mice. This indicates that the dysfunction of salivary secretion develops with age, and its development may be related to the development of periodontitis.
The causes of this extreme polydipsia in the STR/N mice are not yet completely clear. The previous studies have shown that the STR/N mice have various abnormalities in the central nervous system, particularly in the hypothalamus and the circumventricular organs (6, 7, 9). In this regard, it is worth noting that a lesion in the AV3V in normal rats induces a morphological change in the submandibular glands (12) and an impairment in salivary secretion induced by peripheral administration of pilocarpine (11). In diabetes mellitus, increased thirst and polyuria are prominent symptoms, but the blood glucose levels in the STR/N mice are the same as those in the ICR mice (8). Another condition having extreme polydipsia and polyuria, diabetes insipidus, also has to be ruled out, because the STR/N mice have particularly active vasopressinergic neurons in the hypothalamus and their plasma vasopressin levels are generally higher than those in the ICR mice (15). Our results, on the other hand, strongly suggest that the salivary dysfunction in the polydipsic mice is definitely one factor for this phenomenon. We found that after three major salivary glands were removed from the polydipsic and control mice, the former showed no increased water intake, whereas the water intake increased nearly six times in the latter, almost to the same level as that in the polydipsic mice. It is, however, still unclear whether salivary gland dysfunction is the cause or the result of the polydipsia.
Interestingly, it has been reported that in the streptozotocin-diabetes in rats, the structure and function of salivary glands are affected and salivary secretion is decreased (1). These phenomena may be comparable to those in the STR/N mice, although our findings on the submandibular glands suggested no atrophy because the ratio of the dry weight over the wet weight of the gland is the same in both the STR/N and ICR mice.
Our results on the plasma osmolality of the STR/N mice do not agree with the earlier finding (14). The value is ~20 mosm/kgH2O higher than that of the ICR mice. The plasma osmolality of the ICR mice in this study was identical to that of Swiss/Webster in the previous report. ICR, Swiss/Webster, and STR/N mice originate from the same strain, although Swiss/Webster mice are not presently available in Japan. The difference in the plasma osmolality of polydipsic mice is very important because hyperosmosis activates central and peripheral osmoreceptors and can induce drinking (3). Therefore, we collected the blood samples carefully and repeated the osmolality measurement. The osmometer used in this experiment needed only a small sample volume (15 µl), and it still had high reproducibility.
In conclusion, this study demonstrates a dysfunction of salivary glands in the STR/N mice, and it suggests that this dysfunction is partly responsible for the extraordinary drinking by the STR/N mice. However, we cannot exclude the possibility that the overhydration induces this dysfunction of salivary glands in the STR/N mice.
Perspectives
The present study provides evidence that the polydipsia of the inbred strain of mice, STR/N, is caused, at least partly, by their impaired salivary function. It is known (14) that the polydipsia is not due to polyuria; under restricted water intake, the STR/N mice secrete urine with similar or higher osmolality as that of the control mice. Even when water intake is restricted for as long as 16 mo after weaning, the STR/N mice survive well. Moreover, when water is given ad libitum for the first time, they drink copiously. Thus the polydipsia in the STR/N mice develops even under the water-restricted condition. Although recent studies on the STR/N mice have found many abnormalities in the brain functions, particularly in the circumventricular organs and the hypothalamus (see introduction and DISCUSSION), our findings point to a new direction of unraveling the mechanism of polydipsia in this strain of mice. However, a question still remains if salivary dysfunction is a result of overdrinking or the primary cause for polydipsia. Although we found that the young STR/N mice that had not yet developed polydipsia had normal salivary function, this finding could not give us the definite answer to the question. For this purpose, it may be useful to test whether or not the STR/N mice kept under water restriction for a long period develop dysfunction of salivary glands. In addition, we wish to investigate dose-response relationships of pilocarpine's effect on salivary secretion from the STR/N and control mice.| |
ACKNOWLEDGEMENTS |
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We express our appreciation to Rieko Nishi and Masumi Tsujisawa for technical assistance.
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FOOTNOTES |
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This work was supported in part by Grants-in-Aid for Scientific Research 09670076 and 11671848 to K. Inenaga and 07507004 to H. Yamashita from the Ministry of Education, Japan.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: K. Inenaga, Dept. of Physiology, Kyushu Dental College, 2-6-1, Manazuru, Kokurakitaku, Kitakyushu 803-8580, Japan (E-mail: ine{at}kyu-dent.ac.jp).
Received 30 June 1999; accepted in final form 28 September 1999.
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ABSTRACT
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RESULTS
DISCUSSION
REFERENCES
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34.5, linear regression coefficient = 0.88. For STR/N, y = 1.73x + 31.4, linear regression coefficient = 0.77. Significant
difference between two correlation coefficients (P < 0.05).
