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Fluid secretion rates from mouse and rat parotid glands are markedly different following pilocarpine stimulation
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Comp. Eiochem. Physiol. Vol. 88A, No. 2, pp. 307-310, 1987 Printedin Great Britain
FLUID SECRETION RATES FROM MOUSE AND RAT PAROTID GLANDS ARE MARKEDLY DIFFERENT FOLLOWING PILOCARPINE STIMULATION YITZHAK MARMARY,* PHILIP C. Fox and BRUCE J. BAUM~ Clinical Investigations and Patient Care Branch, National Institute of Dental Research, National Institutes of Health, Bethesda, MD 20892, USA. Telephone: (301) 496-1363
(Received 23 January 1987)
Abstract-1. Parotid saliva production in two commonly employed laboratory animals, mouse and rat, was studied following pilocarpine stimulation. 2. When normalized to body wt, average parotid saliva output rates in mice were 34-fold greater than those observed in rats. When parotid salivary flow rates were normalized to gland weight, mice still displayed 2-3-fold higher values than rats. 3. The Na+ and K+ content of parotid saliva showed small differences between the two species, while saliva from rats contained 3-fold higher protein levels than observed with mice.
INTRODUCTION
Vertebrate salivary glands generally show increased development with higher position on the evolutionary scale (Young and Van Lennep, 1978). For example, salivary glands are virtually absent in fish, poorly developed in amphibians and well-developed in birds and terrestrial mammals. The principle function of salivary glands is the exocrine production of saliva. This fluid has two main roles. Saliva is critically important both to facilitate alimentation (mastication, degluttition) and to protect the oral tissues (Burgen and Emmelin, 1961; Young and Van Lennep, 1978; Mandel, 1987). The former role is served by a number of salivary constituents including specific lubricatory proteins (e.g. mucins), hydrolytic enzymes and by the fluid produced. The latter role is a result of the action of many anti-microbial proteins (e.g. lactoferrin, lysozyme, lactoperoxidase, secretory IgA) and the hydrating effects of the fluid. The relative size and degree of specialization of salivary glands can vary widely, particularly depending upon the feeding habits and environment of a given species. Examples of specialization include the toxins produced by reptile venom glands (e.g. Bonilla et al., 1971), a variety of hormones and growth factors produced by certain mammalian salivary glands (e.g. Levi-Montalcini and Angeletti, 1961; Cohen and Taylor, 1974; Lawrence et al., 1977), a role in thermoregulation for many mammals (e.g. Horowitz, 1976) and the exceptionally high fluid output of ruminants which provides a digestive milieu for the forestomach (Argenzio, 1984). Salivary glands are composed of two general regions, an acinar endpiece, which is involved in both primary fluid elaboration and most exocrine protein
*On sabbatical leave from Hebrew University-Hadassah School of Dental Medicine, Jerusalem, Israel. tTo whom all correspondence should be addressed.
production, and a ductal segment, which modifies the primary fluid through electrolyte fluxes (e.g. Na+, Cl- reabsorption) as well as releasing some additional proteins (Young and Van Lennep, 1978; Baum, 1987). Salivary gland acinar cells have been a frequently used model for studying cell surface receptor signal transduction mechanisms (e.g. Schramm and Selinger, 1975; Berridge, 1984). The two most common laboratory animals employed for such in oitro studies have been the rat (primarily) and the mouse. These closely related rodents are abundant omnivores. They display an early sexual maturity and have secretion tightly regulated by autonomic stimulation. Therefore they have proved to be very good tools for understanding exocrine gland function (Petersen, 1986). Recently we became interested in studying mouse salivary gland function in vivo because of the availability of genetically inbred mice which develop autoimmune disease similar to humans (e.g. Fernandes and Good, 1984). While many laboratories have studied rat salivary gland function in vivo (e.g. Yoshida et al., 1967; Mangos et al., 1966; Abe and Dawes, 1978; Bodner et al., 1983) relatively little study of mouse salivary secretion in vivo has occurred (Young and Van Lennep, 1979; Bobyock et al., 1985). It is the purpose of the present study to report the observation that mouse parotid glands, after pilocarpine stimulation, display a remarkably high fluid secretory capacity when compared to rat parotid glands. MATERIALS AND METHODS Wistar-strain rats were obtained from Harlan-SpragueDawley and CS7 Bl/6 mice were obtained from Jackson Laboratory. All animals were given access to laboratory chow and water ad libitum until the time of experiments. Data reported herein are derived from animals 34 months old. Studies were performed, however, with rats as old as 10 months and mice as old as 13 months, with essentially 307
YITZHAK MARMARY er
308
similar results. Saliva was collected following extra-oral cannulation of the parotid main excretory ducts of anesthetized (pentobarbital, 30 mg/kg, i.p.) animals (four to six per group) using the method described by Bobyock et al. (1985) with polyethylene tubing (PE 60, Clay Adams) attached to cannulae. Secretion was stimulated by the subcutaneous injection of pilocarpine HCI (5 mg/kg body wt; Sigma Chemical Co.). The rate of salivary secretion was followed by marking the advancing fluid front in the tubing at 5-min intervals (Bobyock et al., 1985). Animals were weighed prior to each experiment and after collections salivary glands were removed by careful dissection using a dissecting microscope (Zeiss, OPMI-I), and then weighed. Collected saliva samples were analysed for total protein content by absorbance at 215nm (Ameberg, 1971) and for Na+ and K+ by atomic absorption using a Perkin-Elmer Model 2380 Spectrometer. RESULTS AND
DlSCUSSION
Parotid saliva was collected from mice and rats, the flow rate in pl/min was normalized to 100 g body wt and the results, over a 30-min time course, are depicted in Fig. l(A). The pattern of parotid fluid secretion was generally similar in the two species; a high initial flow rate seen immediately after pilocarpine administration which declined steadily thereafter. On average, parotid saliva flow rates, calculated in this manner, were 34fold greater in mice than in rats. When flow rates were calculated in an alternative way, based on parotid gland wt (normalized to 100 mg parotid tissue), generally similar findings were observed [Fig. l(B)]. Mice exhibited 2-3-fold greater rates of fluid output than rats. The salivary flow rates observed here with mice are similar to previously reported findings by Bobyock et al. (1985), while those with rats are comparable to earlier results from our laboratory (Bodner and Baum, 1986). Saliva samples collected from animals were analysed for their Na+, KC and total protein content. These data are shown in Table 1. While small differences were detected in the average concen-
al.
Table 1. Na+, K+ and total protein concentration in mouse and rat oarotid saliva* Na+ (mM)
K+ (mM)
Protein (mg/ml)
Mouse
88.1 * 14.2 15.7 f 1.3 5.2 + 0.8 (6) (5) (6) Rat 72.2 f 14.6 27.0 f 6.5 17.6k4.1 (4) (5) (4) *Data are presented as the mean + SD of values assayed as described in Materials and Methods for the number of determinations given in parentheses.
trations of both major salivary cations, a dramatic difference in total protein content was observed. Rat parotid saliva contained about 3 times more protein than mouse parotid saliva (17.6 vs 5.2 mg protein/ml saliva). The absolute values for pilocarpinestimulated parotid salivary protein, Na+ and K+ found in this study also are similar to previously reported data (Bobyock et al., 1985; Bodner and Baum, 1986). Finally, we examined the relative mass of parotid glands in both species. Mouse parotid glands were, in absolute terms, only 20% by wt of rat parotid tissue. However, when expressed as a proportion of animal body wt, the mouse parotid gland was twice as large as that of the rat (0.37% vs 0.19% of total body wt, Table 2). Similarly, data from Abe and Dawes (1982)
Table 2. Body wt and parotid gland wt in mice and rats*
Mouse Rat
Body wt (g) 32.3 + 3.0 406 + 28
Glands as percent body wt W)
Parotid wt (mg) 61.1 it 14.9 341 * 47
0.37 f 0.10 0.19 f0.05
*Data are presented as the mean f SD. Parotid glands were dissected with the aid of a dissecting microscope as described in Materials and Methods. The mouse data are from four animals (body wt) and seven individual parotid glands. The rat data are from three animals (body wt) and six individual parotid glands.
B.
16
16 I
1 14
TIME IminI
Fig. 1. Time course of pilocarpine-stimulated parotid saliva production in mice (w-0) and rats (A---A). Data, mean f SD, are normalized to body wt (A) or parotid gland wt [(B), mean only].
Parotid secretion in mice and rats
show that the mouse submandibular gland is proportionally much larger than the rat submandibular gland. We have been unable to find any previous reports noting the relatively high stimulated flow rates of mouse parotid salivary glands. From the findings of Abe and Dawes (1982), a similar difference between mice and rats in fluid secretion from the submandibular gland, after pilocarpine stimulation, does not occur. As previously mentioned, there has been very little study of in uiuo mouse parotid salivary secretion. However, it has been reported that primary saliva elaborated by the mouse parotid is similar to that of the rat with respect to ionic composition (Mangos et al., 1966, 1973; Young and Van Lennep, 1979). These earlier studies also suggest that the acinar cells of both species are the site of all fluid transport in parotid glands (Mangos et al., 1966, 1973). It therefore follows from the present data that mouse parotid acinar cells, following a pilocarpine stimulus, are capable of elaborating primary fluid at a higher rate than acinar cells of the rat parotid. Such a conclusion is surprising because mice and rats, besides being phylogenetically closely related, are quite similar with respect to many physiological and behavioral considerations of which saliva production might be a reflection. For example, both species have similar nutritional requirements (Mosesson and Scher, 1968) and similar upper digestive tracts (Stephens and Patton, 1984). The morphology of both glands is also similar (Young and Van Lennep, 1978). Functionally, saliva in both species, besides protective and alimentary roles, also functions in thermoregulation, grooming and systemic hormone production (e.g. Horowitz, 1976; Young and Van Lennep, 1978; Ferguson, 1985). In a more general sense, higher rates of saliva production by mice might be an indicator of higher metabolic activity. Indeed, oxygen consumption when normalized to body mass is -2-fold higher in the mouse (Melby and Altman, 1976; Ferguson, 1985). However, both species have a similar length of life, heart rate and respiratory rate (Ruitenberg and Peters, 1986). Two explanations for these results seem possible, one behavioral and one pharmacologic. The behavioral relationship is related to food consumption. Current laboratory animal handling procedures recommend that both mice and rats be allowed food and water at all times (Mosesson and Scher, 1968; Stephen and Patton, 1984). Mice consume 4-6 g of food/day while rats consume 15-20 g of food/day. If one assumes an average mouse (in this study weighing 32 g) consumes 5 g of food/day, then the ratio of daily food intake to body wt = 0.16. For the average rat in this study (406 g body wt), if food consumption is 18 g/day then the above ratio = 0.044, i.e. about 25% that of the mouse. Perhaps to facilitate alimentation for meeting its daily food requirement, the mouse needs to produce a substantially larger amount of salivary fluid than the rat. Interestingly, the total output of exocrine proteins, when normalized to body or gland wt, is roughly the same in both species. The possible pharmacologic explanation is related to our choice of pilocarpine as a secretogogue. This
309
agent is commonly employed to elicit salivary secretion and is generally considered to have a parasympathomimetic mode of action. However, Schneyer (1965) reported that pilocarpine has /I -adrenergic agonist properties as evidenced by the finding that /I-adrenergic antagonists could modify pilocarpineelicited secretory responses. Similarly, Bobyock et al. (1985) showed that propranolol markedly reduced amylase secretion from mouse parotid glands after pilocarpine injection. Also, Abe and Dawes (1982) reported that the /I-adrenergic agonist isoproterenol was (relative to pilocarpine) a much more potent stimulus for submandibular fluid secretion in the mouse than in the rat. Thus the observations reported here may be a reflection of the mixed agonist properties of pilocarpine and a more important role for fl-adrenoreceptors in fluid secretion in the mouse parotid gland than in the rat. This latter possibility is amenable to direct testing and requires investigation. Our observations also may be interesting with respect to model choice for studies of mechanisms involved in the transepithelial transport of water. Our data suggest that the mouse would be a useful model to study the regulatory and ion transport events which drive water movement. From recent studies, these events are primarily related to Cl- fluxes in acinar cells (Young, 1982; Martinez and Cassity, 1985; Kawaguchi et al., 1986). Independent of the explanation for our findings, the data suggest that the mouse parotid gland may be significantly enriched in the relevant ion transporters needed for water secretion, compared to the rat, upon which most studies thus far have focused. REFERENCES
Abe K. and Dawes C. (1978) The effects of electrical and pharmacological stimulation of the types of proteins secreted by rat parotid and submandibular glands. Archs Oral Biol. 23, 367-372. Abe K. and Dawes C. (1982) Secretion of protein by the submandibular glands of the rat, mouse and hamster in response to various parasympatho- and sympathomimetic drugs. J. dent. Res. 61, 1454-1457. Argenzio R. A. (1984) Secretory functions of the gastrointestinal tract. In Dukes’ Physiology of Domestic Animals, 10th edn (Edited by Swenson M. J.)._. D. _ 292. Cornstock, Ithaca, New York. Ameberg P. (1971) Quantitative determinations of orotein in Sal&a. A comparison of analytical methods. &and. J. dent. Res. 19, 60-64. Baum B. J. (1987) Neurotransmitter control of secretion. J. dent. Res. 66, 628632. Berridge M. (1984) Inositol trisphosphate and diacylglycerol as second messengers. Biochem. J. 220, 345-360. Bobyock E., Chemick W. S. and DiGregorio G. J. (1985) The mouse parotid gland preparation: an in vivo pharmacological tool. J. dent. Res. 64, 1121-1125. Bodner L., Qwamstrom E., Omnell K-A., Hand A. R. and Baum B. J. (1983) Rat submandibular gland secretion: A bilateral and longitudinal comparative study. Comp. Biochem. Physiol. WA, 829-83 1. Bodner L. and Baum B. J. (1986) Characteristics of stimulated parotid gland secretion in the aging rat. Mech. Agng Devl. 31, 337-342. Bonilla C. A., Fiero M. K. and Seifert W. (1971) Comparative biochemistry and pharmacology of salivary gland secretions. J. Chromatogr. !%, 368-372.
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Burgen A. S. V. and Emmelin N. G. (1961) Physiology of the Salivary Clam&. Williams and Wilkins, Baltimore. Cohen S. and Taylor J. M. (1974) Epidermal growth factor: chemical and biological characterization. Rec. Prog. Horm. Res. 30, 533-550.
Ferguson J. H. (1985) Mammalian Physiology, pp. 243-246, 280. Merrill, Columbus, Ohio. Femandes G. and Good R. A. (1984) Inhibition by restricted-calorie diet of lymphoproliferative disease and renal damage in MRL/lpr mice. Proc. natn. Acad. Sci. U.S.A. 81, 6144-6148.
Horowitz M. (1976) Acclimation of rats to moderate heat: body water distribution and adaptability of the submaxillary gland. Ppiigers Arch. gen. Physiol. 366, 173-176. Kawaguchi M., Turner R. J. and Baum B. J. (1986) ‘%land *6Rb+ uptake in rat parotid acinar cells. Archs Oral Biol. 31, 679-683.
Lawrence A. M., Tan S., Hojvat S. and Kirsteins L. (1977) Salivary gland hyperglycemic factor: An extrapancreatic source of glucagon-like material. Science 195, 7&72. Levi-Montalcini R. and Angeletti, P. U. (1961) Growth control of the sympathetic system by a specific protein factor. Q. Rev. Biol. 36, 99-108. Mandel I. D. (1987) The functions of saliva. J. dent. Res. 66, 623-627.
Mangos J. A., Braun G. and Hamann K. F. (1966) Micropuncture study of sodium and potassium excretion in the rat parotid saliva Pftigers Archlgen. Physiol. 291,99-106. Mangos J. A., McSherry N. R., Nousia-Arvanitakis S. and Irwin K. (1973) Secretion and transductal fluxes of ions in exocrine glands of the mouse. Am. J. Physiol. 225, 18-24. Martinez J. R. and Cassity N. (1985) 36C1fluxes in dispersed rat submandibular acini: Effects of acetylcholine and transport inhibitors. PfIiigrs Arch. gen. Physiol. 403, 5(r54.
Melby E. C. Jr and Altman N. H. (1976) Handbook of Laboratory Animal Science, pp. 23-64. CRC Press, Cleveland, Ohio. Mosesson G. R. and Scher S. (1968) Breeding Laboratory Animals, pp. 57-79. Sterling, .New. York. Petersen 0. H. (1986) Calcium-activated ootassium channels and fluid secretion by exocrine glands. Am. J. Physiol. 251, Gl-Gl3. Ruitenberg E. J. and Peters P. W. J. (1986) Laboratory Animals, p. 16. Elsevier, Amsterdam. Schneyer C. A. (1965) Modification of action of pilocarpine by adrenergic blocking agents. Proc. Sot. exp.-Biol. &fed. 120. 23&232.
Schramm M. and Selinger Z. (1975) Enzyme secretion and K+ release mediated by beta and alpha adrenergic receptors in rat parotid slices. Mefh. Enzym. 39, 461466. Stephens U. K. and Patton N. M. (1984) Manual for Laboratory Animal Technicians, pp. 3243. American Association of Laboratory Animal Science, Joliet, Illinois. Yoshida T., Sprecher R. L., Schneyer C. A. and Schneyer L. H. (1967) Role of /f-receptors in sympathetic regulation of electrolytes in rat submaxillary saliva. Proc. Sot. exp. Biol. Med. 126, 912-916. Young J. A. (1982) Is Na/Cl cotransport the basis of transepithelial transport in absorptive and secretory epithelia? In Electrolyte and Water Transport Across Gastrointestinal Epithelia (Edited by Case R. M., Garner A., Turnberg L. A. and Young J. A.), pp. 181-198. Raven Press, New York. Young J. A. and Van Lennep E. W. (1978) The Morphology of Salivary Glands. Academic Press, London. Young J. A. and Van Lennep E. W. (1979) Transport in salivary and salt glands. In Membrane Transport in Biology Vol. 4B (Edited by Giebisch G., Tosteson D. C. and Ussing H. H.), pp. 563674. Springer, Berlin.
Comp. Eiochem. Physiol. Vol. 88A, No. 2, pp. 307-310, 1987 Printedin Great Britain
FLUID SECRETION RATES FROM MOUSE AND RAT PAROTID GLANDS ARE MARKEDLY DIFFERENT FOLLOWING PILOCARPINE STIMULATION YITZHAK MARMARY,* PHILIP C. Fox and BRUCE J. BAUM~ Clinical Investigations and Patient Care Branch, National Institute of Dental Research, National Institutes of Health, Bethesda, MD 20892, USA. Telephone: (301) 496-1363
(Received 23 January 1987)
Abstract-1. Parotid saliva production in two commonly employed laboratory animals, mouse and rat, was studied following pilocarpine stimulation. 2. When normalized to body wt, average parotid saliva output rates in mice were 34-fold greater than those observed in rats. When parotid salivary flow rates were normalized to gland weight, mice still displayed 2-3-fold higher values than rats. 3. The Na+ and K+ content of parotid saliva showed small differences between the two species, while saliva from rats contained 3-fold higher protein levels than observed with mice.
INTRODUCTION
Vertebrate salivary glands generally show increased development with higher position on the evolutionary scale (Young and Van Lennep, 1978). For example, salivary glands are virtually absent in fish, poorly developed in amphibians and well-developed in birds and terrestrial mammals. The principle function of salivary glands is the exocrine production of saliva. This fluid has two main roles. Saliva is critically important both to facilitate alimentation (mastication, degluttition) and to protect the oral tissues (Burgen and Emmelin, 1961; Young and Van Lennep, 1978; Mandel, 1987). The former role is served by a number of salivary constituents including specific lubricatory proteins (e.g. mucins), hydrolytic enzymes and by the fluid produced. The latter role is a result of the action of many anti-microbial proteins (e.g. lactoferrin, lysozyme, lactoperoxidase, secretory IgA) and the hydrating effects of the fluid. The relative size and degree of specialization of salivary glands can vary widely, particularly depending upon the feeding habits and environment of a given species. Examples of specialization include the toxins produced by reptile venom glands (e.g. Bonilla et al., 1971), a variety of hormones and growth factors produced by certain mammalian salivary glands (e.g. Levi-Montalcini and Angeletti, 1961; Cohen and Taylor, 1974; Lawrence et al., 1977), a role in thermoregulation for many mammals (e.g. Horowitz, 1976) and the exceptionally high fluid output of ruminants which provides a digestive milieu for the forestomach (Argenzio, 1984). Salivary glands are composed of two general regions, an acinar endpiece, which is involved in both primary fluid elaboration and most exocrine protein
*On sabbatical leave from Hebrew University-Hadassah School of Dental Medicine, Jerusalem, Israel. tTo whom all correspondence should be addressed.
production, and a ductal segment, which modifies the primary fluid through electrolyte fluxes (e.g. Na+, Cl- reabsorption) as well as releasing some additional proteins (Young and Van Lennep, 1978; Baum, 1987). Salivary gland acinar cells have been a frequently used model for studying cell surface receptor signal transduction mechanisms (e.g. Schramm and Selinger, 1975; Berridge, 1984). The two most common laboratory animals employed for such in oitro studies have been the rat (primarily) and the mouse. These closely related rodents are abundant omnivores. They display an early sexual maturity and have secretion tightly regulated by autonomic stimulation. Therefore they have proved to be very good tools for understanding exocrine gland function (Petersen, 1986). Recently we became interested in studying mouse salivary gland function in vivo because of the availability of genetically inbred mice which develop autoimmune disease similar to humans (e.g. Fernandes and Good, 1984). While many laboratories have studied rat salivary gland function in vivo (e.g. Yoshida et al., 1967; Mangos et al., 1966; Abe and Dawes, 1978; Bodner et al., 1983) relatively little study of mouse salivary secretion in vivo has occurred (Young and Van Lennep, 1979; Bobyock et al., 1985). It is the purpose of the present study to report the observation that mouse parotid glands, after pilocarpine stimulation, display a remarkably high fluid secretory capacity when compared to rat parotid glands. MATERIALS AND METHODS Wistar-strain rats were obtained from Harlan-SpragueDawley and CS7 Bl/6 mice were obtained from Jackson Laboratory. All animals were given access to laboratory chow and water ad libitum until the time of experiments. Data reported herein are derived from animals 34 months old. Studies were performed, however, with rats as old as 10 months and mice as old as 13 months, with essentially 307
YITZHAK MARMARY er
308
similar results. Saliva was collected following extra-oral cannulation of the parotid main excretory ducts of anesthetized (pentobarbital, 30 mg/kg, i.p.) animals (four to six per group) using the method described by Bobyock et al. (1985) with polyethylene tubing (PE 60, Clay Adams) attached to cannulae. Secretion was stimulated by the subcutaneous injection of pilocarpine HCI (5 mg/kg body wt; Sigma Chemical Co.). The rate of salivary secretion was followed by marking the advancing fluid front in the tubing at 5-min intervals (Bobyock et al., 1985). Animals were weighed prior to each experiment and after collections salivary glands were removed by careful dissection using a dissecting microscope (Zeiss, OPMI-I), and then weighed. Collected saliva samples were analysed for total protein content by absorbance at 215nm (Ameberg, 1971) and for Na+ and K+ by atomic absorption using a Perkin-Elmer Model 2380 Spectrometer. RESULTS AND
DlSCUSSION
Parotid saliva was collected from mice and rats, the flow rate in pl/min was normalized to 100 g body wt and the results, over a 30-min time course, are depicted in Fig. l(A). The pattern of parotid fluid secretion was generally similar in the two species; a high initial flow rate seen immediately after pilocarpine administration which declined steadily thereafter. On average, parotid saliva flow rates, calculated in this manner, were 34fold greater in mice than in rats. When flow rates were calculated in an alternative way, based on parotid gland wt (normalized to 100 mg parotid tissue), generally similar findings were observed [Fig. l(B)]. Mice exhibited 2-3-fold greater rates of fluid output than rats. The salivary flow rates observed here with mice are similar to previously reported findings by Bobyock et al. (1985), while those with rats are comparable to earlier results from our laboratory (Bodner and Baum, 1986). Saliva samples collected from animals were analysed for their Na+, KC and total protein content. These data are shown in Table 1. While small differences were detected in the average concen-
al.
Table 1. Na+, K+ and total protein concentration in mouse and rat oarotid saliva* Na+ (mM)
K+ (mM)
Protein (mg/ml)
Mouse
88.1 * 14.2 15.7 f 1.3 5.2 + 0.8 (6) (5) (6) Rat 72.2 f 14.6 27.0 f 6.5 17.6k4.1 (4) (5) (4) *Data are presented as the mean + SD of values assayed as described in Materials and Methods for the number of determinations given in parentheses.
trations of both major salivary cations, a dramatic difference in total protein content was observed. Rat parotid saliva contained about 3 times more protein than mouse parotid saliva (17.6 vs 5.2 mg protein/ml saliva). The absolute values for pilocarpinestimulated parotid salivary protein, Na+ and K+ found in this study also are similar to previously reported data (Bobyock et al., 1985; Bodner and Baum, 1986). Finally, we examined the relative mass of parotid glands in both species. Mouse parotid glands were, in absolute terms, only 20% by wt of rat parotid tissue. However, when expressed as a proportion of animal body wt, the mouse parotid gland was twice as large as that of the rat (0.37% vs 0.19% of total body wt, Table 2). Similarly, data from Abe and Dawes (1982)
Table 2. Body wt and parotid gland wt in mice and rats*
Mouse Rat
Body wt (g) 32.3 + 3.0 406 + 28
Glands as percent body wt W)
Parotid wt (mg) 61.1 it 14.9 341 * 47
0.37 f 0.10 0.19 f0.05
*Data are presented as the mean f SD. Parotid glands were dissected with the aid of a dissecting microscope as described in Materials and Methods. The mouse data are from four animals (body wt) and seven individual parotid glands. The rat data are from three animals (body wt) and six individual parotid glands.
B.
16
16 I
1 14
TIME IminI
Fig. 1. Time course of pilocarpine-stimulated parotid saliva production in mice (w-0) and rats (A---A). Data, mean f SD, are normalized to body wt (A) or parotid gland wt [(B), mean only].
Parotid secretion in mice and rats
show that the mouse submandibular gland is proportionally much larger than the rat submandibular gland. We have been unable to find any previous reports noting the relatively high stimulated flow rates of mouse parotid salivary glands. From the findings of Abe and Dawes (1982), a similar difference between mice and rats in fluid secretion from the submandibular gland, after pilocarpine stimulation, does not occur. As previously mentioned, there has been very little study of in uiuo mouse parotid salivary secretion. However, it has been reported that primary saliva elaborated by the mouse parotid is similar to that of the rat with respect to ionic composition (Mangos et al., 1966, 1973; Young and Van Lennep, 1979). These earlier studies also suggest that the acinar cells of both species are the site of all fluid transport in parotid glands (Mangos et al., 1966, 1973). It therefore follows from the present data that mouse parotid acinar cells, following a pilocarpine stimulus, are capable of elaborating primary fluid at a higher rate than acinar cells of the rat parotid. Such a conclusion is surprising because mice and rats, besides being phylogenetically closely related, are quite similar with respect to many physiological and behavioral considerations of which saliva production might be a reflection. For example, both species have similar nutritional requirements (Mosesson and Scher, 1968) and similar upper digestive tracts (Stephens and Patton, 1984). The morphology of both glands is also similar (Young and Van Lennep, 1978). Functionally, saliva in both species, besides protective and alimentary roles, also functions in thermoregulation, grooming and systemic hormone production (e.g. Horowitz, 1976; Young and Van Lennep, 1978; Ferguson, 1985). In a more general sense, higher rates of saliva production by mice might be an indicator of higher metabolic activity. Indeed, oxygen consumption when normalized to body mass is -2-fold higher in the mouse (Melby and Altman, 1976; Ferguson, 1985). However, both species have a similar length of life, heart rate and respiratory rate (Ruitenberg and Peters, 1986). Two explanations for these results seem possible, one behavioral and one pharmacologic. The behavioral relationship is related to food consumption. Current laboratory animal handling procedures recommend that both mice and rats be allowed food and water at all times (Mosesson and Scher, 1968; Stephen and Patton, 1984). Mice consume 4-6 g of food/day while rats consume 15-20 g of food/day. If one assumes an average mouse (in this study weighing 32 g) consumes 5 g of food/day, then the ratio of daily food intake to body wt = 0.16. For the average rat in this study (406 g body wt), if food consumption is 18 g/day then the above ratio = 0.044, i.e. about 25% that of the mouse. Perhaps to facilitate alimentation for meeting its daily food requirement, the mouse needs to produce a substantially larger amount of salivary fluid than the rat. Interestingly, the total output of exocrine proteins, when normalized to body or gland wt, is roughly the same in both species. The possible pharmacologic explanation is related to our choice of pilocarpine as a secretogogue. This
309
agent is commonly employed to elicit salivary secretion and is generally considered to have a parasympathomimetic mode of action. However, Schneyer (1965) reported that pilocarpine has /I -adrenergic agonist properties as evidenced by the finding that /I-adrenergic antagonists could modify pilocarpineelicited secretory responses. Similarly, Bobyock et al. (1985) showed that propranolol markedly reduced amylase secretion from mouse parotid glands after pilocarpine injection. Also, Abe and Dawes (1982) reported that the /I-adrenergic agonist isoproterenol was (relative to pilocarpine) a much more potent stimulus for submandibular fluid secretion in the mouse than in the rat. Thus the observations reported here may be a reflection of the mixed agonist properties of pilocarpine and a more important role for fl-adrenoreceptors in fluid secretion in the mouse parotid gland than in the rat. This latter possibility is amenable to direct testing and requires investigation. Our observations also may be interesting with respect to model choice for studies of mechanisms involved in the transepithelial transport of water. Our data suggest that the mouse would be a useful model to study the regulatory and ion transport events which drive water movement. From recent studies, these events are primarily related to Cl- fluxes in acinar cells (Young, 1982; Martinez and Cassity, 1985; Kawaguchi et al., 1986). Independent of the explanation for our findings, the data suggest that the mouse parotid gland may be significantly enriched in the relevant ion transporters needed for water secretion, compared to the rat, upon which most studies thus far have focused. REFERENCES
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