Comp. Biochem. Physiol., 1975, Vol. 51A, pp. 17 to 20. Pergamon Press. Printed in Great Britain
CORTICOSTERONE
PLASMA
LEVELS IN DESERT
RODENTS*7
W. J. VANJONACK, I. M. SCOIT, M. K. YOUSE~AND H. D. JOHNSON Department of Dairy Husbandry,
and Department
University of Missouri-Columbia, Columbia, Missouri 65201; of Biological Science, University of Nevada, Las Vegas, Las Vegas, Nevada 89154, U.S.A. (Received 4 February
1974)
Abslraet-1. Plasma glucocorticoids were measured in six species of desert rodents. The data indicated that the primary glucocorticoid in all desert rodents is corticosterone. 2. Corticosterone plasma levels appeared to be related to ecological distribution of desert rodents. 3. It is suggested that lower corticosterone plasma levels may play an adaptive role in water conservation and lowered metabolic rates of desert rodents.
INTRODUCTION
light ether anesthetic. Determinations of glucocorticoids were made on all four samples; however, day 3 was used as a representative mean value. Samples obtained on the @st and second days showed great variability and thus were not used. The Iarge variations were apparently due to possible lengthy excitation and/or anesthetic effect. Corticosterone plasma levels were assayed in duplicate employing the dompetitive protein-binding technique (Murphy, 1967). Briefly, plasma samples of O-1 ml were first washed with petroleum ether and extracted with dichloromethane. Further extraction of corticosterone was made using carbon tetrachloride. The samples were corrected for recovery and expressed as ng/ml. All statistical analyses were performed by Student’s t-test.
THE RATIO
of metabolid rate; to body weight of most desert rodents is lower than the predicted ratio by Brody’s equation (Bradley & Yousef, 1972). In a recent study it was concluded that metabolic rate appears to be related to ecologic distributiori (Scott et al., 1972) apd no! related to body fat content. The lowered metabolic rates of rodents inhabiting the low desert represents an adaptive mechanism by which the animals conserve energy and water and tolerate heat stress. The mechanisms responsible for the low metabolic rate are not understood. Recently, Yousef & Johnson (1972) suggested the possible role of the endocrine system specifically the thyroid gland. The objective of the following study was to measure total plasma glucocorticoids and corticosterone in desert rodents representing a variety of habitats and then attempt to relate these values to metabolic rate, ecologic distribution, behavior and food habits.
RESULTS
Plasma corticosterone was found to comprise nearly 100 per cent (99.6 + @7 per cent) of the total glucocorticoids in all species studied. Other glucocorticoids may be present in the plasma (possibly low ng or pg levels) but corticosterone was found to be the primary glucocorticoid secreted. Plasma corticosterone levels differed among all species and actual values are shown in Fig. 1. Corticosterone levels were related to ecologic distribution as shown in Fig. 2. It is apparent that three of the species (Fig. 2) restricted to low desert had the lowest plasma corticosterone: desert kangaroo rat (30 + 3.1 ng/ml), Merriam kangaroo rat (60 + 12.0) and round-tail ground squirrel (74 + 8.2 ng/ml). The desert wood rat, which inhabits the arid woodland region, had a mean of 105 rt 15.4 ng/ml while the golden mantled ground squirrel, which inhabits the higher elevations, had a mean of 138 f 158. The antelope ground squirrel, which had a wide range of ecologic distribution, had the highest level (298 + 19.5) of corticosterone as compared to all other species. Differences among species in plasma concentration of corticosterone were evident in all samples.
MATERIALS AND METHODS Six species of desert rodents representing three families and ecological distribution ranging from low desert to montane forest were used. Ecologic distribution, food habits and body weight of the species studied are shown in Table 1. The rodents were live trapped in June 1971, and kept in the laboratory at 25 & 2°C for approximately 24 weeks prior to this study. The heteromyidae rodents were fed dry oats and all other animals were fed Purina mouse chow and apples as a water source.. Blood samples (0*3-0*5 ml) were obtained daily in 4 successive days in July (between 1000 and 1400 hr) via heart puncture using * Contribution from the Missouri Agricultural Experiment Station. Journal Series No. 6899. Approved by the Director. t This study was partly supported by NSF Grant GB-3528 to the Desert Research Institute. University of Nevada. 17
W. J. VANJONACK,I. M. Scxxr, M. K. YOUSEFANIBH. D. JOHNSON
18
Antelope ground squirrel
Golden mantled ground
rot
Dipo&& deserti’
Dbhmys Spam&&s merriomi te~tica&
Neotomo Spermophiius Ammw~w@nks ll?t~~OllS lepido
Fig. 1. Plasma corticosterone levels of desert rodents. *S.E. Table 1. Ecologic distribution and body weigbt of rodents used in this study Family, species and common name
No. of AhUllS
Average bodywe&&t
(9)
Ecologic distribution
Heteromyidae Dipodomys deserti @esert kangaroo rat)
Localized in sandy habitats in desert below 1200 m
9
110
11
35
12
105
9
80
Widespread 18OOm
11
88
Localized in sandy habitats in low desert below 900 m
12
150
Widespread in montane forest between 2100 and 3300 m
Dipoabmys merriami
(Merriam kangaroo rat)
Widespread 16OOm
in
desert
below
Cricetidae Neototna lepida (Desert wood rat)
Localized in rocky or wash habitats in desert below 1800m
Sciuridae Amnwspernwphilus Ieucurus
(Antelope ground squirrel)
in
desert below
Spernwphilus tereticaudua
(Round-tail ground squirrel) Spernwphilus lateralis
(Golden-mantled Ground squirrel)
However, corticosterone levels differed from one day to another within the same species. On day 3, variations were minimum and on day 1 variations were maximal. The difference in variability is probably due to behavior and to the degree of adaptation to bleeding. The relationship between behavior under laboratory conditions and per cent decrease in plasma level between day 1 and 3 is shown in Table 2. The behavior of the rodents is somewhat subjective and possibly can be affected by
the rodents’ circadian rhythms. The species that were rated as docile showed little change (3-5.6 per cent) in plasma levels, while the active or average species exhibited greater changes (35-48 per cent). The diurnal species (ground squirrels) had higher plasma levels (170 ng/ml) as compared to the nocturnal species (kangaroo rats and desert wood rat, 65 ng/ml). Within the species restricted to low desert the diurnal round-tail ground squirrel had a higher mean plasma value, 74+ 8.2, as compared to the
Corticosterone plasma levels in desert rodents
I!
Table 2. Plasma corticosterone levels and its relation to bebaviour Concentration of Corticosterone (ng/mB
Species Antelope ground squirrel Golden-mantled ground squirrel Desert wood rat Round-tail ground squirrel Desert kangaroo rat Merriam kangaroo rat * PcO.05.
t SE.
2982 19*5t 138+ 15.8 105+ 15.4 74 f 8.2 30+ 3.1 60* 12.0
%D=eday 1 vs day 3 Corticosterone (n&0
Rehaviour Very active Moderately active Moderately active Docile Docile Docile
37.5%* 35.0% NS 48.00/, 3.0 NS 5.6%$ NS 2.8% NS
$ Increase.
nocturnal kangaroo rats (60+ 12 and 3Ok 3-l). Possible circadian rhythms of glucocorticoids can be considered in these variations. No signiiicant relationship was found between body weight or sex of the rodents. DISCUSSION Studies on the endocrine system of desert rodents are very limited (Yousef & Johnson, 1972). We are not aware of any studies dealing with the secretory function of the adrenal cortex of desert rodents, The uniqueness of our study is not only to report on the plasma levels of glucocorticoids of six species of desert rodents but also to attempt to correlate plasma levels to ecologic distribution of the studied species. The finding that corticosterone is the primary glucocorticoid secreted by the adrenal cortex of desert rodents is in agreement with other studies on various rodents (Eleftheriou, 1964; Huibreytse et al., 1971). Our data support, but do not conclusively prove, that the adrenal cortical function of desert rodents is related to ecologic distribution. With the exception of one species, antelope ground squirrel, Antelope
rodents inhabiting low desert areas have lower plasma corticosterone than rodents found in high desert or montane areas. The low level of plasma corticosterone may represent an adaptive mechanism to desert heat. Also it explains, at least in part, the lowered metabolic rate of desert rodents since glucocorticoids are known to be calorigenic hormones in large and small mammals (Evans et al., 1957; Yousef & Johnson, 1967). Recently, it has been shown that continued heat exposure would decrease plasma glucocorticoids in small and large mammals (Kotby & Johnson, 1967). The decrease in plasma levels of glucocorticoids was shown by Lemaine et al. (1965) to be a specific adaptive response and not due to exhaustion of the adrenal cortex. Also, it is possible that the low levels of plasma corticosterone play a role in the ability of the animal to conserve water, since Levi et al. (1973) suggested that cortisol may be required for the inhibitory effect of norepinephrine on antidiuretic hormone (ADH). In conclusion, the low levels of plasma corticosterone in low desert rodents may represent an adaptive thermoregulatory mechanism resulting in decreased
ground squirrel
1
&
s tl
I
I 1 Golden mantled
m -Low Fig.
desert-
Arid woodland -Montane
forest-
2. Plasma corticosterone levels and ecologic distribution of desert rodents. *S.E.
20
W. J. VANJONACK,I. M. Scorn,, M. K. YOUSEFAND H. D. JOHNSON
endogenous heat production and conservation of water. It appears that the known lower metabolic rate of desert rodents is related to ecologic distribution and low secretory activity of the thyroid and adrenal cortex. SUMMARY
Six species of desert rodents, representing three families and a wide ecological distribution, were used to measure plasma glucocorticoids. Corticosterone was found to be the primary glucocorticoid. Species restricted to low desert habitat appear to have the lowest level of plasma glucocorticoids. Plasma corticosterone levels differed among the species studied, apparently due to behavior and ecologic distribution. It is suggested that a decrease in adrenal cortical activity may represent an adaptive mechanism in desert rodents. REFERENCES BRADLEY W. G. & YOUSEFM. K. (1972) Small mammals in the desert. In Physiological Adaptation (Edited by YOUSEFM. K., HORVATHS. & BULLARD R.) p. 127. Academic Press, New York. ELEFTHERIOU B. E. (1964) Bound and free corticosteroid in the plasma of two subspecies of deer mice after exposure to a low ambient temperature. J. Endocr. 31,75-80. EVANS E. S., ~ONTOPONLOUSA. N. & SIMPSONM. E. (1957) Hormonal factors influencing calorigenesis. Endocrinology 60,403-410.
HUIBREYTSEW. H., GUNVILLE R. & UNGAR F. (1971) Secretion of corticosterone in virro by normothermic and hibernating ground squirrels. Comp. Biochem. Physiol. 38A, 763-768. LEMAIN~R.. OLSEN.0. & BENCENYC. (1965) Le role des hormones cortico-surrenal darts l’adaptation a la chaleur. Arch. Sci. Physiol. 19,141-153. LEVI J., MASSRY S. G. & KLEEMANC. R. (1973) The requirement of cortisol for the inhibitory effect of norepinephrine on the antidiuretic action of vasopressin. l%oc. Sot. exp. Biol. Med. 142,687-690. KOTBYS. & JOHNSONH. D. (1967) Rat adrenal cortical activity during exposure to a high (34°C) ambient temperature. Life Sci. 6, 1121-l 132. MURPHY B. E. P. (1967) Some studies of the protein binding of steroids and their applications of the routine micro and ultra micro measurement of various steroids in body fluids by competitive protein-binding radioimmunoassay. J. clin. Endocr. 27,973-990. Scold I. M., YOUSEF M. K. & BRADLEYW. G. (1972) Body fat content and metabolic rate of rodents: desert and mountain. Proc. Sot. exp. Biol. Med. 141,818-821. YOUSEFM. K. & JOHNSONH. D. (1967) . , Calorigenesis of cattle as influenced by hydrowrtisone and environmental temperature. J. Anim. Sci. 26,1087-1093. YOUSEFM. K. &JOHN~~N H. D. (1972) Thyroid function and metabolic rate of desert rodents In Biometeorology (Edited by TROMPS. W. & BOUMAJ. J.), Vol. 5 Part I, p. 134. Swets & Zeitlinger N.V., Amsterdam. Key Word Index-Corticosterone; desert rodents; Dipodamys; Neotoma; Spermophilus; Ammospermophilus.
CORTICOSTERONE
PLASMA
LEVELS IN DESERT
RODENTS*7
W. J. VANJONACK, I. M. SCOIT, M. K. YOUSE~AND H. D. JOHNSON Department of Dairy Husbandry,
and Department
University of Missouri-Columbia, Columbia, Missouri 65201; of Biological Science, University of Nevada, Las Vegas, Las Vegas, Nevada 89154, U.S.A. (Received 4 February
1974)
Abslraet-1. Plasma glucocorticoids were measured in six species of desert rodents. The data indicated that the primary glucocorticoid in all desert rodents is corticosterone. 2. Corticosterone plasma levels appeared to be related to ecological distribution of desert rodents. 3. It is suggested that lower corticosterone plasma levels may play an adaptive role in water conservation and lowered metabolic rates of desert rodents.
INTRODUCTION
light ether anesthetic. Determinations of glucocorticoids were made on all four samples; however, day 3 was used as a representative mean value. Samples obtained on the @st and second days showed great variability and thus were not used. The Iarge variations were apparently due to possible lengthy excitation and/or anesthetic effect. Corticosterone plasma levels were assayed in duplicate employing the dompetitive protein-binding technique (Murphy, 1967). Briefly, plasma samples of O-1 ml were first washed with petroleum ether and extracted with dichloromethane. Further extraction of corticosterone was made using carbon tetrachloride. The samples were corrected for recovery and expressed as ng/ml. All statistical analyses were performed by Student’s t-test.
THE RATIO
of metabolid rate; to body weight of most desert rodents is lower than the predicted ratio by Brody’s equation (Bradley & Yousef, 1972). In a recent study it was concluded that metabolic rate appears to be related to ecologic distributiori (Scott et al., 1972) apd no! related to body fat content. The lowered metabolic rates of rodents inhabiting the low desert represents an adaptive mechanism by which the animals conserve energy and water and tolerate heat stress. The mechanisms responsible for the low metabolic rate are not understood. Recently, Yousef & Johnson (1972) suggested the possible role of the endocrine system specifically the thyroid gland. The objective of the following study was to measure total plasma glucocorticoids and corticosterone in desert rodents representing a variety of habitats and then attempt to relate these values to metabolic rate, ecologic distribution, behavior and food habits.
RESULTS
Plasma corticosterone was found to comprise nearly 100 per cent (99.6 + @7 per cent) of the total glucocorticoids in all species studied. Other glucocorticoids may be present in the plasma (possibly low ng or pg levels) but corticosterone was found to be the primary glucocorticoid secreted. Plasma corticosterone levels differed among all species and actual values are shown in Fig. 1. Corticosterone levels were related to ecologic distribution as shown in Fig. 2. It is apparent that three of the species (Fig. 2) restricted to low desert had the lowest plasma corticosterone: desert kangaroo rat (30 + 3.1 ng/ml), Merriam kangaroo rat (60 + 12.0) and round-tail ground squirrel (74 + 8.2 ng/ml). The desert wood rat, which inhabits the arid woodland region, had a mean of 105 rt 15.4 ng/ml while the golden mantled ground squirrel, which inhabits the higher elevations, had a mean of 138 f 158. The antelope ground squirrel, which had a wide range of ecologic distribution, had the highest level (298 + 19.5) of corticosterone as compared to all other species. Differences among species in plasma concentration of corticosterone were evident in all samples.
MATERIALS AND METHODS Six species of desert rodents representing three families and ecological distribution ranging from low desert to montane forest were used. Ecologic distribution, food habits and body weight of the species studied are shown in Table 1. The rodents were live trapped in June 1971, and kept in the laboratory at 25 & 2°C for approximately 24 weeks prior to this study. The heteromyidae rodents were fed dry oats and all other animals were fed Purina mouse chow and apples as a water source.. Blood samples (0*3-0*5 ml) were obtained daily in 4 successive days in July (between 1000 and 1400 hr) via heart puncture using * Contribution from the Missouri Agricultural Experiment Station. Journal Series No. 6899. Approved by the Director. t This study was partly supported by NSF Grant GB-3528 to the Desert Research Institute. University of Nevada. 17
W. J. VANJONACK,I. M. Scxxr, M. K. YOUSEFANIBH. D. JOHNSON
18
Antelope ground squirrel
Golden mantled ground
rot
Dipo&& deserti’
Dbhmys Spam&&s merriomi te~tica&
Neotomo Spermophiius Ammw~w@nks ll?t~~OllS lepido
Fig. 1. Plasma corticosterone levels of desert rodents. *S.E. Table 1. Ecologic distribution and body weigbt of rodents used in this study Family, species and common name
No. of AhUllS
Average bodywe&&t
(9)
Ecologic distribution
Heteromyidae Dipodomys deserti @esert kangaroo rat)
Localized in sandy habitats in desert below 1200 m
9
110
11
35
12
105
9
80
Widespread 18OOm
11
88
Localized in sandy habitats in low desert below 900 m
12
150
Widespread in montane forest between 2100 and 3300 m
Dipoabmys merriami
(Merriam kangaroo rat)
Widespread 16OOm
in
desert
below
Cricetidae Neototna lepida (Desert wood rat)
Localized in rocky or wash habitats in desert below 1800m
Sciuridae Amnwspernwphilus Ieucurus
(Antelope ground squirrel)
in
desert below
Spernwphilus tereticaudua
(Round-tail ground squirrel) Spernwphilus lateralis
(Golden-mantled Ground squirrel)
However, corticosterone levels differed from one day to another within the same species. On day 3, variations were minimum and on day 1 variations were maximal. The difference in variability is probably due to behavior and to the degree of adaptation to bleeding. The relationship between behavior under laboratory conditions and per cent decrease in plasma level between day 1 and 3 is shown in Table 2. The behavior of the rodents is somewhat subjective and possibly can be affected by
the rodents’ circadian rhythms. The species that were rated as docile showed little change (3-5.6 per cent) in plasma levels, while the active or average species exhibited greater changes (35-48 per cent). The diurnal species (ground squirrels) had higher plasma levels (170 ng/ml) as compared to the nocturnal species (kangaroo rats and desert wood rat, 65 ng/ml). Within the species restricted to low desert the diurnal round-tail ground squirrel had a higher mean plasma value, 74+ 8.2, as compared to the
Corticosterone plasma levels in desert rodents
I!
Table 2. Plasma corticosterone levels and its relation to bebaviour Concentration of Corticosterone (ng/mB
Species Antelope ground squirrel Golden-mantled ground squirrel Desert wood rat Round-tail ground squirrel Desert kangaroo rat Merriam kangaroo rat * PcO.05.
t SE.
2982 19*5t 138+ 15.8 105+ 15.4 74 f 8.2 30+ 3.1 60* 12.0
%D=eday 1 vs day 3 Corticosterone (n&0
Rehaviour Very active Moderately active Moderately active Docile Docile Docile
37.5%* 35.0% NS 48.00/, 3.0 NS 5.6%$ NS 2.8% NS
$ Increase.
nocturnal kangaroo rats (60+ 12 and 3Ok 3-l). Possible circadian rhythms of glucocorticoids can be considered in these variations. No signiiicant relationship was found between body weight or sex of the rodents. DISCUSSION Studies on the endocrine system of desert rodents are very limited (Yousef & Johnson, 1972). We are not aware of any studies dealing with the secretory function of the adrenal cortex of desert rodents, The uniqueness of our study is not only to report on the plasma levels of glucocorticoids of six species of desert rodents but also to attempt to correlate plasma levels to ecologic distribution of the studied species. The finding that corticosterone is the primary glucocorticoid secreted by the adrenal cortex of desert rodents is in agreement with other studies on various rodents (Eleftheriou, 1964; Huibreytse et al., 1971). Our data support, but do not conclusively prove, that the adrenal cortical function of desert rodents is related to ecologic distribution. With the exception of one species, antelope ground squirrel, Antelope
rodents inhabiting low desert areas have lower plasma corticosterone than rodents found in high desert or montane areas. The low level of plasma corticosterone may represent an adaptive mechanism to desert heat. Also it explains, at least in part, the lowered metabolic rate of desert rodents since glucocorticoids are known to be calorigenic hormones in large and small mammals (Evans et al., 1957; Yousef & Johnson, 1967). Recently, it has been shown that continued heat exposure would decrease plasma glucocorticoids in small and large mammals (Kotby & Johnson, 1967). The decrease in plasma levels of glucocorticoids was shown by Lemaine et al. (1965) to be a specific adaptive response and not due to exhaustion of the adrenal cortex. Also, it is possible that the low levels of plasma corticosterone play a role in the ability of the animal to conserve water, since Levi et al. (1973) suggested that cortisol may be required for the inhibitory effect of norepinephrine on antidiuretic hormone (ADH). In conclusion, the low levels of plasma corticosterone in low desert rodents may represent an adaptive thermoregulatory mechanism resulting in decreased
ground squirrel
1
&
s tl
I
I 1 Golden mantled
m -Low Fig.
desert-
Arid woodland -Montane
forest-
2. Plasma corticosterone levels and ecologic distribution of desert rodents. *S.E.
20
W. J. VANJONACK,I. M. Scorn,, M. K. YOUSEFAND H. D. JOHNSON
endogenous heat production and conservation of water. It appears that the known lower metabolic rate of desert rodents is related to ecologic distribution and low secretory activity of the thyroid and adrenal cortex. SUMMARY
Six species of desert rodents, representing three families and a wide ecological distribution, were used to measure plasma glucocorticoids. Corticosterone was found to be the primary glucocorticoid. Species restricted to low desert habitat appear to have the lowest level of plasma glucocorticoids. Plasma corticosterone levels differed among the species studied, apparently due to behavior and ecologic distribution. It is suggested that a decrease in adrenal cortical activity may represent an adaptive mechanism in desert rodents. REFERENCES BRADLEY W. G. & YOUSEFM. K. (1972) Small mammals in the desert. In Physiological Adaptation (Edited by YOUSEFM. K., HORVATHS. & BULLARD R.) p. 127. Academic Press, New York. ELEFTHERIOU B. E. (1964) Bound and free corticosteroid in the plasma of two subspecies of deer mice after exposure to a low ambient temperature. J. Endocr. 31,75-80. EVANS E. S., ~ONTOPONLOUSA. N. & SIMPSONM. E. (1957) Hormonal factors influencing calorigenesis. Endocrinology 60,403-410.
HUIBREYTSEW. H., GUNVILLE R. & UNGAR F. (1971) Secretion of corticosterone in virro by normothermic and hibernating ground squirrels. Comp. Biochem. Physiol. 38A, 763-768. LEMAIN~R.. OLSEN.0. & BENCENYC. (1965) Le role des hormones cortico-surrenal darts l’adaptation a la chaleur. Arch. Sci. Physiol. 19,141-153. LEVI J., MASSRY S. G. & KLEEMANC. R. (1973) The requirement of cortisol for the inhibitory effect of norepinephrine on the antidiuretic action of vasopressin. l%oc. Sot. exp. Biol. Med. 142,687-690. KOTBYS. & JOHNSONH. D. (1967) Rat adrenal cortical activity during exposure to a high (34°C) ambient temperature. Life Sci. 6, 1121-l 132. MURPHY B. E. P. (1967) Some studies of the protein binding of steroids and their applications of the routine micro and ultra micro measurement of various steroids in body fluids by competitive protein-binding radioimmunoassay. J. clin. Endocr. 27,973-990. Scold I. M., YOUSEF M. K. & BRADLEYW. G. (1972) Body fat content and metabolic rate of rodents: desert and mountain. Proc. Sot. exp. Biol. Med. 141,818-821. YOUSEFM. K. & JOHNSONH. D. (1967) . , Calorigenesis of cattle as influenced by hydrowrtisone and environmental temperature. J. Anim. Sci. 26,1087-1093. YOUSEFM. K. &JOHN~~N H. D. (1972) Thyroid function and metabolic rate of desert rodents In Biometeorology (Edited by TROMPS. W. & BOUMAJ. J.), Vol. 5 Part I, p. 134. Swets & Zeitlinger N.V., Amsterdam. Key Word Index-Corticosterone; desert rodents; Dipodamys; Neotoma; Spermophilus; Ammospermophilus.
