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Daily rhythm of plasma insulin in japanese quail (Coturnix c. japonica) fed Ad libitum
Camp. Eiochem. Physiol.
Vol. 104A,No. 1, pp.
143-145,
1993
0300-9629/93$6.00+ 0.00 0 1992Pergamon Press Ltd
Printed in Great Britain
DAILY RHYTHM OF PLASMA INSULIN IN JAPANESE QUAIL (COTURNIX C. JAPONICA) FED AD LIBITUM BRUCE L. TEDFORD and ALBERT H. MEIER Department
of Zoology and Physiology, Louisiana State University, Baton Rouge, LA 70803, U.S.A. (Tel. 504 388-1132) (Received 28 April 1992; accepted 5 June 1992)
Abstract-l. Plasma levels of immuno-reactive insulin (IRI) of 6 groups of 5-week-old quail, held on a 16L: SD photoregime, were measured every 4 hr, for 24 hr. 2. Concentrations of plasma IRI varied during the 24 hr period (Anova, P < 0.003). Insulin levels were high (mean = 857 pg/ml) from 6 a.m. (light onset) until 6 p.m. (4 hr before light offset) and low
(mean = 185pg/ml) at 10p.m. and 2 a.m. 3. IRI concentrations at 6 a.m., 10 a.m., 2 p.m. and 6 p.m. were significantly different from values at 2 a.m. and 10p.m. by Wailer grouping. 4. Low IRI levels at light offset (10 p.m.-when the birds had full crops and were in an absorptive state) and high levels at light onset (6 a.m.-before feeding resumed) indicate that feeding does not fully account for the IRI rhythm.
INTRODUCTION
Circadian rhythms of plasma insulin concentration have been demonstrated in several mammalian species, including humans (for review see Mejean et al., 1988), dogs (Fischer et al., 1985), Syrian hamsters (de Souxa and Meier, 1987), rats (Bellinger ef al., 1975) and mice (Pessacq et al., 1976). Avian species have received little attention, and daily rhythms of insulin have not yet been reported in birds. Most of the avian work focuses on insulin responses to the animal’s prandial state, especially starvation/refeeding experiments. A few studies on daily variations of several indices of carbohydrate regulation have been reported in the chicken. Sollburger (1964) observed a circadian liver glycogen rhythm in chicks and Twiest and
Smith (1970) reported a circadian variation of plasma glucose levels. Such metabolic rhythms might reflect rhythms of insulin and/or rhythms of other glucoregulatory hormones. The present study examined plasma insulin concentrations throughout a l-day period in Japanese quail. MATERIALS AND METHODS Thirty-six, sexually mature (5-week-old) Japanese quail (Cofurnix c. japonica) were purchased from the Poultry Science Dept., Louisiana State University. Birds were held on continuous light (L:L) in an environmentally controlled room, maintained at 22°C with food and water available ud lib. from hatch until 5 weeks of age, at which time they were transferred to a 16L:8D photoregime (lights on at 6a.m.). After 2 weeks acclimation, blood samples were collected into non-heparinized syringes by cardiac puncture and then transferred to test tubes. Six different birds were sampled every 4 hr for 24 hr (2 a.m., 6 a.m., 10 a.m., 2 p.m., 6p.m. and 10 p.m.-36 birds in total). Care was taken to minimize disturbance to birds not being sampled. The samples at light onset (6a.m.) were taken CLIP(A)
104/1-J
before the birds began diurnal (photophase) feeding. Blood samples were placed on ice and allowed to clot before centrifugation. The serum was frozen and stored at -20°C until assayed. Plasma levels of quail insulin were determined by radioimmunoassay, described by McMurtry er al. (1983), using purified chicken insulin (Litron Laboratories, Rochester, NY) as the standard and for iodination and guinea-pig antichicken insulin serum as the first antibody. Inhibition curves obtained by serial dilution of Japanese quail plasma were found to be parallel to chicken insulin standard curves, suggesting that the quail plasma insulin inhibits binding in the same manner as the standard solutions. All samples were run in duplicate. The data were analysed using the SAS General Linear Models Procedure (SAS, 1985). Variations among treatment groups were examined by analysis of variance. Comparisons between groups were made with the Wailer-Duncan k-ratio t-test. RESULTS Concentrations
of plasma insulin varied during the
24 hr period (Anova, P < 0.003). Insulin levels were high from 6 a.m. (at light onset) until 6 p.m. (4 hr before light offset) (Fig. 1). Mean insulin levels from 6 a.m. to 6 p.m. (mean = 857 pg/ml) were more than four times as high as the mean values at 10 p.m. and 2 a.m. (mean = 185 pg/ml). Insulin concentrations at 6 a.m., 10 a.m., 2 p.m. and 6 p.m. did not significantly differ from each other as shown by Waller grouping. The 2 a.m. and the 10 p.m. values did not differ from each other, but both differed significantly, by Waller grouping, from those of all other sampling times (P < 0.05).
Feed consumption in these birds was diurnal as evidenced by the observation of full crop sacs at all times of the day, except the 2 a.m. and 6 p.m. time slots. It is noteworthy that crop sacs were especially full at light offset (10 p.m.). 143
BRUCEL. TED~~RDand ALBERTH. MEER
144
Fig. 1. Plasma insulin concentrations over a 24 hr time period. The solid bars denote the dark period. Values marked with different superscripts are significantly different from each other (P < 0.05).
DISCUSSION
Japanese quail fed ad lib. exhibited a robust circadian variation of plasma insulin, with levels high from 6 a.m. to 6 p.m. and low from 10 p.m. to 2 a.m. High levels during most of the photophase might seem to support the conclusion of Raheja (1973) who suggested that plasma insulin variations in chickens are directly related to feeding. Indeed, insulin levels in birds are generally elevated after feeding (Anthony et al., 1990; Krestel-Rickert et al., 1986; Simon and Leclercq, 1982; Simon and Rosselin, 1979). Furthermore, Japanese quail are diurnal feeders, consuming food throughout the photophase (with a marked increase in feeding just before light offset) and halting consumption during the scotophase (Siopes T. D., personal communication). The increase in feeding near the end of the photophase allows the quail to fill their crops with food which is then digested during the next several hours of darkness. This pattern mirrors that observed in chickens (Scanes et al., 1987; Chandrabose, 1970). However, if feeding patterns were to account fully for the daily variations in plasma insulin levels, then one would expect to observe elevated insulin levels at 10 a.m., 2 p.m., 6p.m. and especially at lOp.m., and see depressed levels at 2 a.m. and 6 p.m. (plasma samples were drawn at light onset before diurnal feeding resumed). This was not the case. Insulin levels were depressed at 10 p.m. despite the fact that the quails’ crops were full of food and the birds were clearly in an absorptive state. Conversely, plasma insulin levels were high at 6 a.m. even though no food had been consumed for 10 hr beforehand. These “food-deprived” birds (at 6 a.m.) should be metabolically equivalent to fasted birds, and as such might be expected to have low plasma insulin levels (Anthony et al., 1990; Krestel-Rickert et al., 1986; McMurtry J. P., personal communication; Simon and Leclercq, 1982; Simon and Rosselin, 1979). Insulin rhythms that do not parallel feeding rhythms have been described in other animals. Hulcher et al. (1985) measured insulin levels in two strains of twicedaily meal-fed monkeys (Cercopethicus uethiops). The vervet strain displayed a large insulin peak following the 2p.m. feeding, but no such peak was associated with the 8.30 a.m. meal. In fact, the lowest daily insulin level occurred 30 min after feeding in the
grivet strain monkeys. In ad lib. fed mice, Gagliardino and Hemandez (1971) and Roesler et al. (1985), independently demonstrated plasma insulin rhythms wherein insulin levels rose, and subsequently peaked, during the period when the mice consume relatively little feed (photophase), and were at their lowest levels during the scotophase, when mice naturally fed. A similar pattern of plasma insulin levels in rats was described by Jolin and Montes (1973). Our findings in quail also indicate that factors, in addition to feeding, are responsible for the plasma insulin rhythm. It is well known in mammals that pancreatic insulin release. is not controlled by blood glucose levels alone. The rise in plasma insulin levels prior to any rise in plasma levels of absorbed nutrients, known as the preabsorptive insulin response (PIR) (Rohner-Jeanrenaund et al., 1983; Steffens and Strubbe, 1983), is just one example. Rich autonomic innervation of the endocrine pancreas (both parasympathetic and sympathetic) affect insulin release (Luiten et al., 1987). Neural regulation of pancreatic sensitivity to stimuli (e.g. glucose) via the vagus, and controlled by a hypothalamic pacemaker, allow for circadian regulation of insulin secretion (Strubbe et al., 1986; Marcilloux et al., 1985). It has been demonstrated in rats that the circadian rhythm of feeding does not necessarily drive the IRI rhythm (Ikonomov et al., 1985). Under conditions of continuous darkness (DD), the feeding rhythm of these rats persisted while the daily variations of IRI was lost. It was demonstrated (de Souza and Meier, 1987) that plasma insulin rhythms in Syrian hamsters (Mesocricetus aurutw) shifted in phase seasonally with respect to the daily photoperiod, whereas feeding patterns did not. This finding suggests that, although the feeding and IRI rhythms may both be entrained by the light/dark cycle, the phase relationships are not tixed and one rhythm does not necessarily drive the other. Because there are also rhythms of metabolic responses to insulin, a shift in phase of the insulin rhythm can change the net effect insulin has on liver lipid and/or carbohydrate metabolism (Cincotta and Meier, 1984). The ability of an animal to alter its food intake or the way it metabolically responds to food intake is very important. Whereas many hormones work together, and are necessary for the proper regulation of feeding, body wt and body composition, insulin is perhaps the most prominent. Insulin receptors in the brain, specifically in the ventromedial hypothalamic area (VMH) and the lateral hypothalamic area (LHA), are involved in the regulation of body wt and food intake (for review see Steffens et al., 1990). Injections of exogenous insulin in mammals can induce increases in body wt (Lotter and Woods, 1977), increases in body fat stores in animals under food intake control (Torbay et al., 1985), and changes in feeding (Panksepp et al., 1975), although insulin injections have much greater lipogenic and hypoglycemic effects at one time of day than at other times (Cincotta and Meier, 1984). The daily fluctuations in insulin levels we observed could well be expressions of circadian neural control centers that regulate body wt, fat stores or feeding, in part, by modulating the phase relationship of
Quail insulin rhythm the insulin rhythm and the rhythms of metabolic responses to insulin. The insulin rhythm may he an important mediator for neuroendocrine regulation of metabolism. Robust circadian variations in lipogenic and hypoglycemic responsiveness to insulin occur in hamsters in addition to circadian rhythms of plasma insulin concentration (Cincotta and Meier, 1984). Furthermore, the phase of the insulin rhythm varies so that the daily insulin peak coincides with the daily interval of lipogenic responsiveness in fat animals and not in lean animals (de Souza and Meier, 1987). These findings and those of the present study indicate clearly that the daily variations in plasma insulin concentration are not merely homeostatic reactions reflective of feeding. Acknowledgements-The authors are indebted to Dr J. P. McMurtty (U.S.D.A., A.R.S., Nonruminant Animal Nutrition Laboratory, Beltsville, MD) for assaying the quail plasma samples for IRI.
REFERENCES
Anthony N. B., Vasilatos-Younken R., Bacon W. L. and Lilbum M. S. (1990) Secretory pattern of growth hormone, insulin, and related metabolites in growing male turkeys: effects of overnight fasting and refeeding. Poultry Sci. 69, Sol-81 1. Bellinger L. L., Mendel V. E. and Moberg G. P. (1975) Circadian insulin, GH, prolactin, corticosterone and glucose rhythms in fed and fasted rats. Horm. Metub. Res. 7, 132-135. Chandrabose K. A. (1970) Effect of hypophysectomy on some aspects of lipid metabolism. Ph.D. thesis, Cornell University, Ithaca, NY. Cincotta A. H. and Meier A. H. (1984) Circadian rhythms of lipogenic and hypoglycaemic responses to insulin in the golden hamster (Mesocricetus uuratus). J. Endocr. 103, 141-146.
de Soura C. J. and Meier A. H. (1987) Circadian and seasonal variations of plasma insulin and cortisol concentrations in the Syrian hamster, Mesocricetus auratus. Chronobiol. Inr. 4, 141-151.
Fischer U., Freyse E.-J., Albrecht G., Gebel G., Heil M. and Unger W. (1985) Daily glucose and insulin rhythms in diabetic dogs on the artificial beta cell. Exp. clin. Endocr. 55, 27-37.
Gagliardino J. J. and Hemandez R. E. (1971) Circadian variation of the serum glucose and immunoreactive insulin levels. Endocrinology 88, 1529-l 53 1. Hulcher F. H., Reynolds J. and Rose J. C. (1985) Circadian rhythm of HMG-CoA reductase and insulin in African green monkeys. Biochem. Int. 10, 177-185. Ikonomov 0. C., Stoynev A. G., Shisheva A. C. and Tarkolev N. T. (1985) Effect of constant light and darkness on the circadian rhythms in rats. II. Plasma mnin activity and insulin concentration. Acta Physiol. Phurmac. Bulg. 11, 55-61. Jolin T. and Montes A. (1973) Daily rhythm of plasma glucose and insulin levels in rats. Horm. Res. 4, 153156. Krestel-Rickert D. H., Baile C. A. and Buonomo F. C. (1986) Changes in insulin, glucose and GH concentrations in fed chickens. Physiol. Behuo. 37, 361-363.
Latter E. C. and Woods S. C. (1977) Injection of insulin and changes of body weight. Physiol. Behao. 18, 293297.
145
Luiten P. G. M., ter Horst G. J. and Steffens A. B. (1987) The hypothalamus, intrinsic connections and outflow pathways to the endocrine system in relation to the control of feeding and metabolism. Prog. Neurobiol. 25, l-54.
Marcilloux J.-C., Simon J. and Auffray P. (1985) Effects of V.M.H. lesions on plasma insulin in the goose. Physiol. Behav. 35, 725-728.
McMurtry J. P., Rosebrough R. W. and Steele N. C. (1983) A homologous radioimmunoassay for chicken insulin. Poultry Sci. 62, 697-701.
Mejean L., Bicakova-Rocher A., Kolopp M., Villaume C., Levi F., Debry G., Reinberg A. and Drouin P. (1988) Circadian and ultradian rhythms in blood glucose and plasma insulin of healthy adults. Chronobiol. fnt. 5, 227-236.
Panksepp J., Pollack A., Krost K., Meeker R. and Ritter M. (1975) Feeding in response to repeated protamine zinc insulin injections. Physiol. Behuv. 14, 487-493. Pessacq M. T., Rebolledo 0. R., Mercer R. G. and Gagliardino J.-J. (1976) Effect of fasting on the circadian rhythm of serum insulin levels. Chronobiologiu 3, 20-26. Raheja K. L. (1973) Comparison of diurnal variations in plasma glucose, cholesterol, triglyceride, insulin and in liver glycogen in younger and older chicks (GaNuc domesticus). Comp. Biochem. Physiol. 44A, 1009-1014. Roesler W. J., Helgason C., Gulka M. and Khandelwal R. L. (1985) Aberrations in the diurnal rhythms of plasma glucose, plasma insulin, liver glycogen, and hepatic glycogen synthase and phosphorylase activities in-genetically diabetic (dbldb) mice. Horm. Merab. Res. 17. 572-575. Rohner-Jeanrenaud F., Bobbioni E., Ionescu ‘E., Sauter J. F. and Jeanrenaud B. (1983) Central nervous system regulation of insulin secretion. In Aduances in Metabolic Disorders (Edited by Szabo A. J.), Vol. 10, pp. 193-200. Academic Press, New York. SAS Institute (1985) The GLM Procedure. In SAS/STAT Guide for Personal Computers. 6th edition, __ pp. 183-260, SAS Institute, Cary, NC. Scanes C. G.. Camnbell R. and Grimineer P. (1987) Control of energy balance during egg productTon in the laying hen. J. Nurr. 117, 605-611. Simon J. and Leclercq B. (1982) Longitudinal study of adiposity in chickens selected for high or low abdominal fat content: further evidence of a glucose-insulin imbalance in the fat line. J. Nutr. 112, 1961-1973. Simon J. and Rosselin G. (1979) Effect of intermittent feeding on glucose-insulin relationship in the chicken. J. Nutr. 109, 631-641. Sollburger A. (1964) The control of circadian glycogen rhythms. Ann. N. Y. Acad. Sci. 117, 5 19-554. Steffens A. B. and Strubbe J. H. (1983) CNS regulation of glucagon secretion. In Advances in Metabolic Disorders (Edited by Sxabo A. J.), Vol. 10, pp. 221-257. Academic Press, New York. Steffens A. B., Strubbe J. H., Balkan B. and Scheurink A. J. W. (1990) Neuroendocrine mechanisms involved in regulation of body weight, food intake and metabolism. Neurosci. Biobehav. Rev. 14. 305-313.
Strubbe J. H., Keijser J., Dijkstra T. and Prins A. J. A. (1986) Interaction between circadian and caloric control of feeding behavior in the rat. Physiol. Behuv. 36, 489-493.
Torbay N., Bracco E. F., Geliebter A., Stewart I. M. and Hashim S. A, (1985) Insulin increases body fat despite control of food intake and physical activity. Am. J. Physiol. 248, R120-R124. Twiest G. and Smith C. J. (1970) Circadian rhythm in blood glucose level of chickens. Comp. Biochem. Physiol. 32, 371-375.
Vol. 104A,No. 1, pp.
143-145,
1993
0300-9629/93$6.00+ 0.00 0 1992Pergamon Press Ltd
Printed in Great Britain
DAILY RHYTHM OF PLASMA INSULIN IN JAPANESE QUAIL (COTURNIX C. JAPONICA) FED AD LIBITUM BRUCE L. TEDFORD and ALBERT H. MEIER Department
of Zoology and Physiology, Louisiana State University, Baton Rouge, LA 70803, U.S.A. (Tel. 504 388-1132) (Received 28 April 1992; accepted 5 June 1992)
Abstract-l. Plasma levels of immuno-reactive insulin (IRI) of 6 groups of 5-week-old quail, held on a 16L: SD photoregime, were measured every 4 hr, for 24 hr. 2. Concentrations of plasma IRI varied during the 24 hr period (Anova, P < 0.003). Insulin levels were high (mean = 857 pg/ml) from 6 a.m. (light onset) until 6 p.m. (4 hr before light offset) and low
(mean = 185pg/ml) at 10p.m. and 2 a.m. 3. IRI concentrations at 6 a.m., 10 a.m., 2 p.m. and 6 p.m. were significantly different from values at 2 a.m. and 10p.m. by Wailer grouping. 4. Low IRI levels at light offset (10 p.m.-when the birds had full crops and were in an absorptive state) and high levels at light onset (6 a.m.-before feeding resumed) indicate that feeding does not fully account for the IRI rhythm.
INTRODUCTION
Circadian rhythms of plasma insulin concentration have been demonstrated in several mammalian species, including humans (for review see Mejean et al., 1988), dogs (Fischer et al., 1985), Syrian hamsters (de Souxa and Meier, 1987), rats (Bellinger ef al., 1975) and mice (Pessacq et al., 1976). Avian species have received little attention, and daily rhythms of insulin have not yet been reported in birds. Most of the avian work focuses on insulin responses to the animal’s prandial state, especially starvation/refeeding experiments. A few studies on daily variations of several indices of carbohydrate regulation have been reported in the chicken. Sollburger (1964) observed a circadian liver glycogen rhythm in chicks and Twiest and
Smith (1970) reported a circadian variation of plasma glucose levels. Such metabolic rhythms might reflect rhythms of insulin and/or rhythms of other glucoregulatory hormones. The present study examined plasma insulin concentrations throughout a l-day period in Japanese quail. MATERIALS AND METHODS Thirty-six, sexually mature (5-week-old) Japanese quail (Cofurnix c. japonica) were purchased from the Poultry Science Dept., Louisiana State University. Birds were held on continuous light (L:L) in an environmentally controlled room, maintained at 22°C with food and water available ud lib. from hatch until 5 weeks of age, at which time they were transferred to a 16L:8D photoregime (lights on at 6a.m.). After 2 weeks acclimation, blood samples were collected into non-heparinized syringes by cardiac puncture and then transferred to test tubes. Six different birds were sampled every 4 hr for 24 hr (2 a.m., 6 a.m., 10 a.m., 2 p.m., 6p.m. and 10 p.m.-36 birds in total). Care was taken to minimize disturbance to birds not being sampled. The samples at light onset (6a.m.) were taken CLIP(A)
104/1-J
before the birds began diurnal (photophase) feeding. Blood samples were placed on ice and allowed to clot before centrifugation. The serum was frozen and stored at -20°C until assayed. Plasma levels of quail insulin were determined by radioimmunoassay, described by McMurtry er al. (1983), using purified chicken insulin (Litron Laboratories, Rochester, NY) as the standard and for iodination and guinea-pig antichicken insulin serum as the first antibody. Inhibition curves obtained by serial dilution of Japanese quail plasma were found to be parallel to chicken insulin standard curves, suggesting that the quail plasma insulin inhibits binding in the same manner as the standard solutions. All samples were run in duplicate. The data were analysed using the SAS General Linear Models Procedure (SAS, 1985). Variations among treatment groups were examined by analysis of variance. Comparisons between groups were made with the Wailer-Duncan k-ratio t-test. RESULTS Concentrations
of plasma insulin varied during the
24 hr period (Anova, P < 0.003). Insulin levels were high from 6 a.m. (at light onset) until 6 p.m. (4 hr before light offset) (Fig. 1). Mean insulin levels from 6 a.m. to 6 p.m. (mean = 857 pg/ml) were more than four times as high as the mean values at 10 p.m. and 2 a.m. (mean = 185 pg/ml). Insulin concentrations at 6 a.m., 10 a.m., 2 p.m. and 6 p.m. did not significantly differ from each other as shown by Waller grouping. The 2 a.m. and the 10 p.m. values did not differ from each other, but both differed significantly, by Waller grouping, from those of all other sampling times (P < 0.05).
Feed consumption in these birds was diurnal as evidenced by the observation of full crop sacs at all times of the day, except the 2 a.m. and 6 p.m. time slots. It is noteworthy that crop sacs were especially full at light offset (10 p.m.). 143
BRUCEL. TED~~RDand ALBERTH. MEER
144
Fig. 1. Plasma insulin concentrations over a 24 hr time period. The solid bars denote the dark period. Values marked with different superscripts are significantly different from each other (P < 0.05).
DISCUSSION
Japanese quail fed ad lib. exhibited a robust circadian variation of plasma insulin, with levels high from 6 a.m. to 6 p.m. and low from 10 p.m. to 2 a.m. High levels during most of the photophase might seem to support the conclusion of Raheja (1973) who suggested that plasma insulin variations in chickens are directly related to feeding. Indeed, insulin levels in birds are generally elevated after feeding (Anthony et al., 1990; Krestel-Rickert et al., 1986; Simon and Leclercq, 1982; Simon and Rosselin, 1979). Furthermore, Japanese quail are diurnal feeders, consuming food throughout the photophase (with a marked increase in feeding just before light offset) and halting consumption during the scotophase (Siopes T. D., personal communication). The increase in feeding near the end of the photophase allows the quail to fill their crops with food which is then digested during the next several hours of darkness. This pattern mirrors that observed in chickens (Scanes et al., 1987; Chandrabose, 1970). However, if feeding patterns were to account fully for the daily variations in plasma insulin levels, then one would expect to observe elevated insulin levels at 10 a.m., 2 p.m., 6p.m. and especially at lOp.m., and see depressed levels at 2 a.m. and 6 p.m. (plasma samples were drawn at light onset before diurnal feeding resumed). This was not the case. Insulin levels were depressed at 10 p.m. despite the fact that the quails’ crops were full of food and the birds were clearly in an absorptive state. Conversely, plasma insulin levels were high at 6 a.m. even though no food had been consumed for 10 hr beforehand. These “food-deprived” birds (at 6 a.m.) should be metabolically equivalent to fasted birds, and as such might be expected to have low plasma insulin levels (Anthony et al., 1990; Krestel-Rickert et al., 1986; McMurtry J. P., personal communication; Simon and Leclercq, 1982; Simon and Rosselin, 1979). Insulin rhythms that do not parallel feeding rhythms have been described in other animals. Hulcher et al. (1985) measured insulin levels in two strains of twicedaily meal-fed monkeys (Cercopethicus uethiops). The vervet strain displayed a large insulin peak following the 2p.m. feeding, but no such peak was associated with the 8.30 a.m. meal. In fact, the lowest daily insulin level occurred 30 min after feeding in the
grivet strain monkeys. In ad lib. fed mice, Gagliardino and Hemandez (1971) and Roesler et al. (1985), independently demonstrated plasma insulin rhythms wherein insulin levels rose, and subsequently peaked, during the period when the mice consume relatively little feed (photophase), and were at their lowest levels during the scotophase, when mice naturally fed. A similar pattern of plasma insulin levels in rats was described by Jolin and Montes (1973). Our findings in quail also indicate that factors, in addition to feeding, are responsible for the plasma insulin rhythm. It is well known in mammals that pancreatic insulin release. is not controlled by blood glucose levels alone. The rise in plasma insulin levels prior to any rise in plasma levels of absorbed nutrients, known as the preabsorptive insulin response (PIR) (Rohner-Jeanrenaund et al., 1983; Steffens and Strubbe, 1983), is just one example. Rich autonomic innervation of the endocrine pancreas (both parasympathetic and sympathetic) affect insulin release (Luiten et al., 1987). Neural regulation of pancreatic sensitivity to stimuli (e.g. glucose) via the vagus, and controlled by a hypothalamic pacemaker, allow for circadian regulation of insulin secretion (Strubbe et al., 1986; Marcilloux et al., 1985). It has been demonstrated in rats that the circadian rhythm of feeding does not necessarily drive the IRI rhythm (Ikonomov et al., 1985). Under conditions of continuous darkness (DD), the feeding rhythm of these rats persisted while the daily variations of IRI was lost. It was demonstrated (de Souza and Meier, 1987) that plasma insulin rhythms in Syrian hamsters (Mesocricetus aurutw) shifted in phase seasonally with respect to the daily photoperiod, whereas feeding patterns did not. This finding suggests that, although the feeding and IRI rhythms may both be entrained by the light/dark cycle, the phase relationships are not tixed and one rhythm does not necessarily drive the other. Because there are also rhythms of metabolic responses to insulin, a shift in phase of the insulin rhythm can change the net effect insulin has on liver lipid and/or carbohydrate metabolism (Cincotta and Meier, 1984). The ability of an animal to alter its food intake or the way it metabolically responds to food intake is very important. Whereas many hormones work together, and are necessary for the proper regulation of feeding, body wt and body composition, insulin is perhaps the most prominent. Insulin receptors in the brain, specifically in the ventromedial hypothalamic area (VMH) and the lateral hypothalamic area (LHA), are involved in the regulation of body wt and food intake (for review see Steffens et al., 1990). Injections of exogenous insulin in mammals can induce increases in body wt (Lotter and Woods, 1977), increases in body fat stores in animals under food intake control (Torbay et al., 1985), and changes in feeding (Panksepp et al., 1975), although insulin injections have much greater lipogenic and hypoglycemic effects at one time of day than at other times (Cincotta and Meier, 1984). The daily fluctuations in insulin levels we observed could well be expressions of circadian neural control centers that regulate body wt, fat stores or feeding, in part, by modulating the phase relationship of
Quail insulin rhythm the insulin rhythm and the rhythms of metabolic responses to insulin. The insulin rhythm may he an important mediator for neuroendocrine regulation of metabolism. Robust circadian variations in lipogenic and hypoglycemic responsiveness to insulin occur in hamsters in addition to circadian rhythms of plasma insulin concentration (Cincotta and Meier, 1984). Furthermore, the phase of the insulin rhythm varies so that the daily insulin peak coincides with the daily interval of lipogenic responsiveness in fat animals and not in lean animals (de Souza and Meier, 1987). These findings and those of the present study indicate clearly that the daily variations in plasma insulin concentration are not merely homeostatic reactions reflective of feeding. Acknowledgements-The authors are indebted to Dr J. P. McMurtty (U.S.D.A., A.R.S., Nonruminant Animal Nutrition Laboratory, Beltsville, MD) for assaying the quail plasma samples for IRI.
REFERENCES
Anthony N. B., Vasilatos-Younken R., Bacon W. L. and Lilbum M. S. (1990) Secretory pattern of growth hormone, insulin, and related metabolites in growing male turkeys: effects of overnight fasting and refeeding. Poultry Sci. 69, Sol-81 1. Bellinger L. L., Mendel V. E. and Moberg G. P. (1975) Circadian insulin, GH, prolactin, corticosterone and glucose rhythms in fed and fasted rats. Horm. Metub. Res. 7, 132-135. Chandrabose K. A. (1970) Effect of hypophysectomy on some aspects of lipid metabolism. Ph.D. thesis, Cornell University, Ithaca, NY. Cincotta A. H. and Meier A. H. (1984) Circadian rhythms of lipogenic and hypoglycaemic responses to insulin in the golden hamster (Mesocricetus uuratus). J. Endocr. 103, 141-146.
de Soura C. J. and Meier A. H. (1987) Circadian and seasonal variations of plasma insulin and cortisol concentrations in the Syrian hamster, Mesocricetus auratus. Chronobiol. Inr. 4, 141-151.
Fischer U., Freyse E.-J., Albrecht G., Gebel G., Heil M. and Unger W. (1985) Daily glucose and insulin rhythms in diabetic dogs on the artificial beta cell. Exp. clin. Endocr. 55, 27-37.
Gagliardino J. J. and Hemandez R. E. (1971) Circadian variation of the serum glucose and immunoreactive insulin levels. Endocrinology 88, 1529-l 53 1. Hulcher F. H., Reynolds J. and Rose J. C. (1985) Circadian rhythm of HMG-CoA reductase and insulin in African green monkeys. Biochem. Int. 10, 177-185. Ikonomov 0. C., Stoynev A. G., Shisheva A. C. and Tarkolev N. T. (1985) Effect of constant light and darkness on the circadian rhythms in rats. II. Plasma mnin activity and insulin concentration. Acta Physiol. Phurmac. Bulg. 11, 55-61. Jolin T. and Montes A. (1973) Daily rhythm of plasma glucose and insulin levels in rats. Horm. Res. 4, 153156. Krestel-Rickert D. H., Baile C. A. and Buonomo F. C. (1986) Changes in insulin, glucose and GH concentrations in fed chickens. Physiol. Behuo. 37, 361-363.
Latter E. C. and Woods S. C. (1977) Injection of insulin and changes of body weight. Physiol. Behao. 18, 293297.
145
Luiten P. G. M., ter Horst G. J. and Steffens A. B. (1987) The hypothalamus, intrinsic connections and outflow pathways to the endocrine system in relation to the control of feeding and metabolism. Prog. Neurobiol. 25, l-54.
Marcilloux J.-C., Simon J. and Auffray P. (1985) Effects of V.M.H. lesions on plasma insulin in the goose. Physiol. Behav. 35, 725-728.
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