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Effect of Dietary Calcium on Blood Pressure of Turkeys
Effect of Dietary Calcium on Blood Pressure of Turkeys JOHN C. LEE, CHARLES J. McGRATH, A. T. LEIGHTON, JR., 1 and W. B. GROSS Comparative Cardiovascular Laboratory, Virginia-Maryland Regional College of Veterinary Medicine, and Department of Poultry Science, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (Received for publication August 8, 1983)
1984 Poultry Science 6 3 : 9 9 3 - 9 9 6 INTRODUCTION
In mature turkeys (Meleagris gallopavo), the resting arterial systolic pressure is normally above 200 mmHg, which is considerably higher than that of other vertebrates (Sturkie, 1979). A relationship between hypertension in turkeys and high incidence of aortic dissecting aneurysm and ruptures has been recognized (Krista et al., 1969). The exact pathogenesis of the hypertension in this species is unknown. The influence of dietary salt, particularly sodium, on the development of hypertension has been extensively studied in humans as well as experimental animals (Mogenson et al., 1983; Folkow, 1982). The role of calcium in regulation of cardiovascular function and many other intracellular functions has been well established, but surprisingly few studies have examined the possible etiologic role of calcium in hypertension. McCarron et al. (1982) and Stitt et al. (1973) have indicated that calcium homeostasis may be important in the development of hypertension. These studies specifically suggest that inadequate dietary calcium intake may be an important etiologic factor in the pathogenesis of essential hypertension. This finding raises the possibility that increased dietary calcium intake may attenuate develop-
1
Department of Poultry Science.
ment of hypertension. The purpose of this study was to determine whether high dietary calcium intake will affect blood pressure in turkeys. MATERIALS AND METHODS
Twenty Broad Breasted White (BBW) male turkeys between 45 and 52 weeks of age were studied. At 33 weeks of age, 10 turkeys continued to receive the standard mature turkey diet with normal calcium (.98%) and served as controls, and an additional 10 turkeys received the standard diet with twice the standard calcium (1.96%). The turkeys were housed at Virginia Tech Turkey Research Center. Feed and water were fed ad libitum. The turkeys were maintained on a 12-hr light cycle with a combination of natural and supplemental artificial lights. The direct arterial blood pressure was measured in unanesthetized turkeys by means of a polyethylene (PE 90) catheter in the left brachial artery following local anesthesia (2% procaine hydrochloride), using a Sanborn transducer, and recorded on a Grass Polygraph (Model 7). The heart rate was obtained from the arterial blood pressure pulse trace. Arterial blood samples were drawn for the determination of serum electrolytes (Photovolt Electrolyte Analyzer System, PVA-4 Dow Chemical) and numbers of lymphocytes and heterophils (Gross and Siegel, 1983). At the
993
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on June 8, 2015
ABSTRACT The influence of dietary calcium intake on blood pressure of turkeys was studied in 20 male Broad Breasted White turkeys between 45 and 52 weeks of age. The average systolic (P<.01), diastolic (P<.01), and mean (P<.005) blood pressures were significantly lower in turkeys fed high dietary calcium (1.96%) than the turkeys with normal calcium diet (.98%). Concomitantly, the mean total heart weight (P<.05), left ventricular weight (P-C025), and heterophil to lymphocyte ratio (P<.02) were also significantly lower in the high dietary calcium group. These results indicate that high dietary calcium not only attenuates high blood pressure in turkeys but also reduces the development of left ventricular hypertrophy, which is a consequence of arterial hypertension. In addition, high dietary calcium also reduces the heterophil to lymphocyte ratio, which indicates a reduction of stress. (Key words: dietary calcium, blood pressure, serum electrolytes, cardiac weights, heterophil to lymphocyte ratio, male turkeys)
994
LEE ET AL.
TABLE 1. Effect of dietary calcium on arterial blood pressure and heart rate (HR) in turkeys (mean ± SE) Arterial blood pressure calcium
No.
.98% 1.96%
10 10
HR
Systolic
Diastolic
Mean
137.3 ± 5.3 141.7 + 4.5
278.5 + 12.8 2 4 1 . 0 + 3.9
183.5 ± 5 . 6 163.5 ± 4.2
222.5 ± 8.1 193.0 ± 4 . 0
NS
<.01
<.01
<.005
(beats/min)
P
Dietary calcium
No.
.98% 1.96%
10 10
THW 2
LVF+S3
RVF4
LVF+S/RVF
16.6 ± .5 15.7 + .2
7 1 . 7 ± 4.5 6 0 . 7 ± 1.0
(g) 53.2 ± 4 . 2 4 2 . 7 ± .6
11.3 ± . 5 11.2 ± . 4
5.02 ± .41 3.74 ± .13
NS
<.05
<.025
NS
<.005
BW 1 (kg)
P 1
BW = Body weight.
2
THW = Total heart weight.
3
LVF+S = Left ventricular free wall plus septum weight.
"RVF = Right ventricular free wall weight.
conclusion of each experiment, the turkey was killed and the heart was immediately removed and cleaned. Each chamber of the heart was dissected and weighed (Lee and Downing, 1975). The experiments were always done in pairs (one from normal and one from high calcium group). All values given are mean ± SE. Student's t test was used to evaluate the significance between means. Differences were considered significant when the P value was less than 5% (Snedecor and Cochran, 1967). RESULTS Table 1 shows the effect of dietary calcium on arterial blood pressure and heart rate in turkeys. The average values of systolic and diastolic (P<.01) as well as mean (P<.005) blood pressure measurements were all significantly lower in the group with the high
2
Left ventricular ventricular free wall.
free
wall plus
septum/right
calcium diet (1.96%) than in the group with normal calcium diet (.98%). The difference in mean blood pressure between the two groups was about 30 mmHg. The mean heart rate was slightly higher in the high calcium group, but the difference was not significant. The mean total heart weight was found to be significantly (P<.05) lower in the high calcium dietary group than in the normal calcium group. This difference was mainly due to the lower left ventricular weight. The right ventricular free wall weight was almost identical in both groups. Thus, the LVF + S/RVF 2 ratio was significantly lower (P<.005) in high dietary calcium group (Table 2). These findings indicate that the level of dietary calcium intake not only influences the blood pressure of turkeys but also reduces left ventricular hypertrophy, which is presumably secondary to the change in arterial pressure load. Concentrations of serum calcium and other electrolytes are summarized in Table 3. The mean concentration of serum calcium was almost identical in both groups. No differences
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on June 8, 2015
TABLE 2. Effect of dietary calcium on heart weights (mean ± SE)
995
DIETARY CALCIUM AND BLOOD PRESSURE IN TURKEYS
TABLE 4. Effect of dietary calcium on heterophil to lymphocyte ratios
Dietary calcium
Heterophil to lymphocyte No.
.98% 1.96%
? o o
+i +i
»
£
NN
Z
between high and normal calcium dietary groups were significant for any of the other electrolytes. Table 4 shows the effect of dietary calcium on heterophil to lymphocyte ratios in turkeys. The average values of heterophil to lymphocyte ratios in normal and high dietary calcium was 1.33 (± .08 SE) and .91 (± .12 SE), respectively. The difference between the two groups was statistically significant (P<.02). DISCUSSION
^ z
JJ P» ci IT H m ^H ,-<
Z
t/1
Z
g oo g
00 •& Ox ON
The importance of supplemental calcium in chicken and turkey nutrition has been recognized for many years (Harms et ai, 1976). Most studies are concerned with the interrelationship of calcium with body growth, feed consumption, feed efficiency, and egg production. Relatively little attention, however, has been given to other beneficial actions of supplemental calcium in poultry. The results of the present study clearly demonstrate that high blood pressure in turkeys can be modulated with increased dietary calcium intake by some as yet undefined mechanisms. These observations are consistent with earlier reports in hypertensive rat models. High dietary calcium attenuates the development of hypertension in genetically hypertensive rats (Ayachi, 1979; McCarron, 1982) as well as in deoxycorticosterone- and salt-induced hypertension (Barry, 1977). Data from epidemiologic studies in humans have also suggested that high dietary calcium may afford some degree of protection in the development of high blood pressure. Studies of drinking water have shown that human populations drinking water with a high concentration of calcium also have lower blood pressure and less mortality related to cardiovascular disease (Masironi, 1970; Stitt et ai, 1973). It is interesting to note that female turkeys have
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on June 8, 2015
<
1.33 ± .08 .91 ± .12 <.02
996
LEE ET AL.
The authors acknowledge D. Phillip Sponenberg for his invaluable comments and M. Dominique Ashen for her excellent technical assistance.
This study was supported in part by BRSG Grant (NIH) and HATCH Grant (USDA) from Virginia Polytechnic Institute and State University. REFERENCES Ayachi, S., 1979. Increased dietary calcium lowers blood pressure in the spontaneously hypertensive rat. Metabolism 28:1234-1238. Barry, G. D., 1977. Effect of increased dietary calcium on the development of experimental hypertension. Fed. Proc. 36:5492. (Abstr.) Ferguson, T. M., D. H. Miller, J. W. Bradley, and R. L. Atkinson, 1969. Blood pressure and heart rate of turkeys 17-21 months of age. Poultry Sci. 48: 1478-1481. Folkow, B., 1982. Physiological aspects of primary hypertension. Physiol. Rev. 62:347—504. Gross, W. B., and H. S. Siegel, 1983. Evaluation of the heterophil/lymphocyte ratio as a measure of stress in chickens. Avian Dis. (in press). Harms, R. H., B. L. Damron, D. A. Roland, and L. M. Potter, 1976. NFIA Literature Review on Calcium in Broiler, Layer and Turkey Nutrition. Natl. Feed Ingred. Assoc. (NFIA), West Des Moines, IA. Krista, L. M., P. E. Waibel, R. N. Shoffner, and J. H. Sautter, 1969. Natural dissecting aneurysm (aortic rupture) and blood pressure in the turkey. Nature (London) 214:1162. Krista, L. M., P. E. Waibel, R. N. Shoffner, and J. H. Sautter, 1970. A study of aortic rupture and performance as influenced by selection for hypertension and hypotension in the turkey. Poultry Sci. 4 9 : 4 0 5 - 4 1 1 . Lee, J. C , and S. E. Downing, 1975. A comparison of ventricular weight and geometry in newborn, young and adult mammals. J. Appl. Physiol. 38:147-150. Masironi, R., 1970. Cardiovascular mortality in relation to radioactivity and hardness of local water supplies in the U.S.A. Bull. World Health Org. 43:687-697. McCarron, D. A., 1982. Blood pressure and calcium balance in the Wistar-Kyoto rat. Life Sci. 30: 683-689. McCarron, D. A., C. D. Morris, and C. Cole, 1982. Dietary calcium in human hypertension. Science 217:267-269. Mogenson, G. J., B. M. Box, and D. J. Philbrick, 1983. Nutrition and hypertension. Can. J. Physiol. Pharmacol. 61:260-270. Snedecor, G. W., and W. G. Cochran, 1967. Statistical Methods. 6th ed. Iowa State Univ. Press, Ames, IA. Stitt, F. W., M. D. Crawford, D. G. Clayton, and J. N. Morris, 1973. Clinical and biomedical indicators of cardiovascular disease among men living in hard and soft water areas. Lancet 1:122—126. Sturkie, P. D., 1979. Page 84 in Avian Physiology. 3rd ed. Springer-Velag, New York, NY.
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on June 8, 2015
considerably lower blood pressure than males, and mortality in females due to aortic rupture is also considerably less than in males (Ferguson et al, 1969; Krista et al, 1970). The reason for these differences is not known. In addition to the genetic considerations and many other factors, the results of the present study raise the possibility that the sex difference in arterial blood pressure in turkeys may be, in part, secondary to the differential calcium balance and dietary calcium contents, although this hypothesis has not been tested. The mechanisms by which increased dietary calcium intake modifies blood pressure are not fully understood, but they are likely to be multiple. In this study the mean concentration of serum calcium in turkeys was almost identical in both dietary calcium groups. Thus, the increased dietary calcium intake lowers the blood pressure in turkeys but appears to be independent of concentrations of extracellular calcium. However, precise delineation of the cellular mechanisms that underlie these observations will require more detailed understanding of the molecular and biochemical actions of calcium than is currently available. More recently, Gross and Siegel (1983) have shown that the heterophil to lymphocyte ratio may be a useful index of stress in poultry. The higher heterophil to lymphocyte ratios indicate higher corticosterone levels and higher levels of stress. In the present study, it is noteworthy that heterophil to lymphocyte ratios were significantly lower in high dietary calcium group than in the normal dietary calcium group. This suggests that increasing dietary calcium intake may also reduce the level of stress in turkeys. However, this possibility must await experimental confirmation. In summary, results of the present investigation have revealed a viable option for controlling high blood pressure in turkeys by dietary management. In addition, this study also provides new approaches to reduce susceptibility of turkeys to environmental stresses. ACKNOWLEDGMENTS
1984 Poultry Science 6 3 : 9 9 3 - 9 9 6 INTRODUCTION
In mature turkeys (Meleagris gallopavo), the resting arterial systolic pressure is normally above 200 mmHg, which is considerably higher than that of other vertebrates (Sturkie, 1979). A relationship between hypertension in turkeys and high incidence of aortic dissecting aneurysm and ruptures has been recognized (Krista et al., 1969). The exact pathogenesis of the hypertension in this species is unknown. The influence of dietary salt, particularly sodium, on the development of hypertension has been extensively studied in humans as well as experimental animals (Mogenson et al., 1983; Folkow, 1982). The role of calcium in regulation of cardiovascular function and many other intracellular functions has been well established, but surprisingly few studies have examined the possible etiologic role of calcium in hypertension. McCarron et al. (1982) and Stitt et al. (1973) have indicated that calcium homeostasis may be important in the development of hypertension. These studies specifically suggest that inadequate dietary calcium intake may be an important etiologic factor in the pathogenesis of essential hypertension. This finding raises the possibility that increased dietary calcium intake may attenuate develop-
1
Department of Poultry Science.
ment of hypertension. The purpose of this study was to determine whether high dietary calcium intake will affect blood pressure in turkeys. MATERIALS AND METHODS
Twenty Broad Breasted White (BBW) male turkeys between 45 and 52 weeks of age were studied. At 33 weeks of age, 10 turkeys continued to receive the standard mature turkey diet with normal calcium (.98%) and served as controls, and an additional 10 turkeys received the standard diet with twice the standard calcium (1.96%). The turkeys were housed at Virginia Tech Turkey Research Center. Feed and water were fed ad libitum. The turkeys were maintained on a 12-hr light cycle with a combination of natural and supplemental artificial lights. The direct arterial blood pressure was measured in unanesthetized turkeys by means of a polyethylene (PE 90) catheter in the left brachial artery following local anesthesia (2% procaine hydrochloride), using a Sanborn transducer, and recorded on a Grass Polygraph (Model 7). The heart rate was obtained from the arterial blood pressure pulse trace. Arterial blood samples were drawn for the determination of serum electrolytes (Photovolt Electrolyte Analyzer System, PVA-4 Dow Chemical) and numbers of lymphocytes and heterophils (Gross and Siegel, 1983). At the
993
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on June 8, 2015
ABSTRACT The influence of dietary calcium intake on blood pressure of turkeys was studied in 20 male Broad Breasted White turkeys between 45 and 52 weeks of age. The average systolic (P<.01), diastolic (P<.01), and mean (P<.005) blood pressures were significantly lower in turkeys fed high dietary calcium (1.96%) than the turkeys with normal calcium diet (.98%). Concomitantly, the mean total heart weight (P<.05), left ventricular weight (P-C025), and heterophil to lymphocyte ratio (P<.02) were also significantly lower in the high dietary calcium group. These results indicate that high dietary calcium not only attenuates high blood pressure in turkeys but also reduces the development of left ventricular hypertrophy, which is a consequence of arterial hypertension. In addition, high dietary calcium also reduces the heterophil to lymphocyte ratio, which indicates a reduction of stress. (Key words: dietary calcium, blood pressure, serum electrolytes, cardiac weights, heterophil to lymphocyte ratio, male turkeys)
994
LEE ET AL.
TABLE 1. Effect of dietary calcium on arterial blood pressure and heart rate (HR) in turkeys (mean ± SE) Arterial blood pressure calcium
No.
.98% 1.96%
10 10
HR
Systolic
Diastolic
Mean
137.3 ± 5.3 141.7 + 4.5
278.5 + 12.8 2 4 1 . 0 + 3.9
183.5 ± 5 . 6 163.5 ± 4.2
222.5 ± 8.1 193.0 ± 4 . 0
NS
<.01
<.01
<.005
(beats/min)
P
Dietary calcium
No.
.98% 1.96%
10 10
THW 2
LVF+S3
RVF4
LVF+S/RVF
16.6 ± .5 15.7 + .2
7 1 . 7 ± 4.5 6 0 . 7 ± 1.0
(g) 53.2 ± 4 . 2 4 2 . 7 ± .6
11.3 ± . 5 11.2 ± . 4
5.02 ± .41 3.74 ± .13
NS
<.05
<.025
NS
<.005
BW 1 (kg)
P 1
BW = Body weight.
2
THW = Total heart weight.
3
LVF+S = Left ventricular free wall plus septum weight.
"RVF = Right ventricular free wall weight.
conclusion of each experiment, the turkey was killed and the heart was immediately removed and cleaned. Each chamber of the heart was dissected and weighed (Lee and Downing, 1975). The experiments were always done in pairs (one from normal and one from high calcium group). All values given are mean ± SE. Student's t test was used to evaluate the significance between means. Differences were considered significant when the P value was less than 5% (Snedecor and Cochran, 1967). RESULTS Table 1 shows the effect of dietary calcium on arterial blood pressure and heart rate in turkeys. The average values of systolic and diastolic (P<.01) as well as mean (P<.005) blood pressure measurements were all significantly lower in the group with the high
2
Left ventricular ventricular free wall.
free
wall plus
septum/right
calcium diet (1.96%) than in the group with normal calcium diet (.98%). The difference in mean blood pressure between the two groups was about 30 mmHg. The mean heart rate was slightly higher in the high calcium group, but the difference was not significant. The mean total heart weight was found to be significantly (P<.05) lower in the high calcium dietary group than in the normal calcium group. This difference was mainly due to the lower left ventricular weight. The right ventricular free wall weight was almost identical in both groups. Thus, the LVF + S/RVF 2 ratio was significantly lower (P<.005) in high dietary calcium group (Table 2). These findings indicate that the level of dietary calcium intake not only influences the blood pressure of turkeys but also reduces left ventricular hypertrophy, which is presumably secondary to the change in arterial pressure load. Concentrations of serum calcium and other electrolytes are summarized in Table 3. The mean concentration of serum calcium was almost identical in both groups. No differences
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on June 8, 2015
TABLE 2. Effect of dietary calcium on heart weights (mean ± SE)
995
DIETARY CALCIUM AND BLOOD PRESSURE IN TURKEYS
TABLE 4. Effect of dietary calcium on heterophil to lymphocyte ratios
Dietary calcium
Heterophil to lymphocyte No.
.98% 1.96%
? o o
+i +i
»
£
NN
Z
between high and normal calcium dietary groups were significant for any of the other electrolytes. Table 4 shows the effect of dietary calcium on heterophil to lymphocyte ratios in turkeys. The average values of heterophil to lymphocyte ratios in normal and high dietary calcium was 1.33 (± .08 SE) and .91 (± .12 SE), respectively. The difference between the two groups was statistically significant (P<.02). DISCUSSION
^ z
JJ P» ci IT H m ^H ,-<
Z
t/1
Z
g oo g
00 •& Ox ON
The importance of supplemental calcium in chicken and turkey nutrition has been recognized for many years (Harms et ai, 1976). Most studies are concerned with the interrelationship of calcium with body growth, feed consumption, feed efficiency, and egg production. Relatively little attention, however, has been given to other beneficial actions of supplemental calcium in poultry. The results of the present study clearly demonstrate that high blood pressure in turkeys can be modulated with increased dietary calcium intake by some as yet undefined mechanisms. These observations are consistent with earlier reports in hypertensive rat models. High dietary calcium attenuates the development of hypertension in genetically hypertensive rats (Ayachi, 1979; McCarron, 1982) as well as in deoxycorticosterone- and salt-induced hypertension (Barry, 1977). Data from epidemiologic studies in humans have also suggested that high dietary calcium may afford some degree of protection in the development of high blood pressure. Studies of drinking water have shown that human populations drinking water with a high concentration of calcium also have lower blood pressure and less mortality related to cardiovascular disease (Masironi, 1970; Stitt et ai, 1973). It is interesting to note that female turkeys have
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on June 8, 2015
<
1.33 ± .08 .91 ± .12 <.02
996
LEE ET AL.
The authors acknowledge D. Phillip Sponenberg for his invaluable comments and M. Dominique Ashen for her excellent technical assistance.
This study was supported in part by BRSG Grant (NIH) and HATCH Grant (USDA) from Virginia Polytechnic Institute and State University. REFERENCES Ayachi, S., 1979. Increased dietary calcium lowers blood pressure in the spontaneously hypertensive rat. Metabolism 28:1234-1238. Barry, G. D., 1977. Effect of increased dietary calcium on the development of experimental hypertension. Fed. Proc. 36:5492. (Abstr.) Ferguson, T. M., D. H. Miller, J. W. Bradley, and R. L. Atkinson, 1969. Blood pressure and heart rate of turkeys 17-21 months of age. Poultry Sci. 48: 1478-1481. Folkow, B., 1982. Physiological aspects of primary hypertension. Physiol. Rev. 62:347—504. Gross, W. B., and H. S. Siegel, 1983. Evaluation of the heterophil/lymphocyte ratio as a measure of stress in chickens. Avian Dis. (in press). Harms, R. H., B. L. Damron, D. A. Roland, and L. M. Potter, 1976. NFIA Literature Review on Calcium in Broiler, Layer and Turkey Nutrition. Natl. Feed Ingred. Assoc. (NFIA), West Des Moines, IA. Krista, L. M., P. E. Waibel, R. N. Shoffner, and J. H. Sautter, 1969. Natural dissecting aneurysm (aortic rupture) and blood pressure in the turkey. Nature (London) 214:1162. Krista, L. M., P. E. Waibel, R. N. Shoffner, and J. H. Sautter, 1970. A study of aortic rupture and performance as influenced by selection for hypertension and hypotension in the turkey. Poultry Sci. 4 9 : 4 0 5 - 4 1 1 . Lee, J. C , and S. E. Downing, 1975. A comparison of ventricular weight and geometry in newborn, young and adult mammals. J. Appl. Physiol. 38:147-150. Masironi, R., 1970. Cardiovascular mortality in relation to radioactivity and hardness of local water supplies in the U.S.A. Bull. World Health Org. 43:687-697. McCarron, D. A., 1982. Blood pressure and calcium balance in the Wistar-Kyoto rat. Life Sci. 30: 683-689. McCarron, D. A., C. D. Morris, and C. Cole, 1982. Dietary calcium in human hypertension. Science 217:267-269. Mogenson, G. J., B. M. Box, and D. J. Philbrick, 1983. Nutrition and hypertension. Can. J. Physiol. Pharmacol. 61:260-270. Snedecor, G. W., and W. G. Cochran, 1967. Statistical Methods. 6th ed. Iowa State Univ. Press, Ames, IA. Stitt, F. W., M. D. Crawford, D. G. Clayton, and J. N. Morris, 1973. Clinical and biomedical indicators of cardiovascular disease among men living in hard and soft water areas. Lancet 1:122—126. Sturkie, P. D., 1979. Page 84 in Avian Physiology. 3rd ed. Springer-Velag, New York, NY.
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on June 8, 2015
considerably lower blood pressure than males, and mortality in females due to aortic rupture is also considerably less than in males (Ferguson et al, 1969; Krista et al, 1970). The reason for these differences is not known. In addition to the genetic considerations and many other factors, the results of the present study raise the possibility that the sex difference in arterial blood pressure in turkeys may be, in part, secondary to the differential calcium balance and dietary calcium contents, although this hypothesis has not been tested. The mechanisms by which increased dietary calcium intake modifies blood pressure are not fully understood, but they are likely to be multiple. In this study the mean concentration of serum calcium in turkeys was almost identical in both dietary calcium groups. Thus, the increased dietary calcium intake lowers the blood pressure in turkeys but appears to be independent of concentrations of extracellular calcium. However, precise delineation of the cellular mechanisms that underlie these observations will require more detailed understanding of the molecular and biochemical actions of calcium than is currently available. More recently, Gross and Siegel (1983) have shown that the heterophil to lymphocyte ratio may be a useful index of stress in poultry. The higher heterophil to lymphocyte ratios indicate higher corticosterone levels and higher levels of stress. In the present study, it is noteworthy that heterophil to lymphocyte ratios were significantly lower in high dietary calcium group than in the normal dietary calcium group. This suggests that increasing dietary calcium intake may also reduce the level of stress in turkeys. However, this possibility must await experimental confirmation. In summary, results of the present investigation have revealed a viable option for controlling high blood pressure in turkeys by dietary management. In addition, this study also provides new approaches to reduce susceptibility of turkeys to environmental stresses. ACKNOWLEDGMENTS