The effects of dietary sodium restriction on fluid and electrolyte metabolism in the chicken (Gallus domesticus)

Camp.

Biochem.

Physd.,

1977, Vol. %A,

pp. 311 to 317. Pergamon

Press. Printed in Great Briroin

THE EFFECTS OF DIETARY SODIUM RESTRICTION ON FLUID AND ELECTROLYTE METABOLISM IN THE CHICKEN (GALLUS DOMEsTICUS) KAREN M. HARRIS* AND T. I. Kormt Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, AR 72201, U.S.A. (Received 10 January 1977) Abstract-l.

The effect of chronic dietary sodium restriction, with and without administration of furosemide, was examined in the domestic fowl (Go//us domesticus). 2. Food intake was not altered by low sodium diet and/or furosemide treatment but gains in body weight were less in birds maintained on the deficient diet. 3. Little changes were noted in plasma electrolytes. 4. Exchangeable sodium and the volume of extracellular fluid were lowered by sodium deficient diet and/or fuorosemide treatment but plasma volume and hematocrit remained unchanged. 5. The results suggest that chickens can maintain vascular volume better than mammals in response to dietary sodium restriction

INTRODUCTION

Studies conducted with a number of avian species indicate that certain members of this vertebrate class (viz. pigeons, ducks and chickens) are better able to withstand the harmful effects of hemorrhage than mammals (Djojosugito et al., 1968; Kovach & Szasz, 1968; Wyse & Nickerson, 1971). The greater tolerance to blood loss in birds is partly attributable to the fact that extravascular fluid is mobilized more rapidly and in relatively greater volume than in mammals. In ducks the greater capacity of fluid mobilization is correlated with a much larger capillary area and a greater fall in capillary hydrostatic pressure in skeletal muscle consequent to hemorrhage (Djojosugito et a/., 1968). The results of such acute studies have led to the suggestion that the physiological mechanisms that maintain volume homeostasis in birds may differ from mammals (Wyse & Nickerson, 1971). Since vascular volume is closely related to sodium balance and its consequent effects on extracellular fluid volume (ECF’V) in mammals (Romero et al., 1968) it seemed of interest to examine the effects of chronic dietary sodium restriction on fluid and electrolyte metabolism in an avian species. The present experiments were designed for this purpose. MILTHODS him&

White Leghorn pullets of uniform genetic stock were obtained at approximately 13 weeks of age. The birds were * Supported by United States Public Health Service Predoctoral Fellowship 5 FOl GM 49672. Submitted by K.M.H. to the Graduate School, University of Arkansas in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physiology. Present address of K.M.H.: 12520 S. W. Faircrest, Portland, Oregon 97225, U.S.A. t To whom inquiries should be addressed.

housed in an environmentally controlled room (14 hr light: 10 hr dark) and were adapted to individual screen-bottomed cages and handling (for measurements of body weight and food and water intake) for at least a week before being placed on the experimental diets. A total of 37 birds were used in the present study. Their average age when sacrificed was 20 + 1.5 weeks (S.E.) with a range of 18-22 weeks. Groups weighed an average of about 0.8 kg [range, 0.75 f 0.05 (SE.) to 0.86 f 0.08 kg] when placed on the experimental diets. The composition of the two purified diets used in these studies is presented in Table 1. After mixing, the diets were moistened with deionized water and formed into pellets by passage through a meat grinder. The pellets were then dried in a forced draft oven at 50°C. Differences in electrolyte composition between the diets were made at the expense of cellulose and the chloride content of the sodium deficient diet was reduced in order to minimize acid-base disturbances (Hurwitz et al., 1973; Nesheim et al., 1964). The concentration of the major ions in the control diet were: (%) Ca, 1; P, 0.6; Na, 0.2; K, 0.2; and Cl, 0.4. The sodium-deficient diet contained the same amounts of Ca, P, and K as above but the Cl level was 0.3% and the sodium content was less than 0.02% by analysis (mean: 0.017%). Experimental

In one series of experiments two groups of pullets (7 birds per group) were maintained on the purified diets for a period of 6 weeks. In several birds from each group body weights were measured twice weekly and food and water intakes measured dally except for weekends. At the end of the 6-week feeding period the animals were fasted for 18 hr, weighed and anesthetized with sodium pentobarbital (30mg/kg body weight, i.v.). Cannulae were placed in the wing vein (PE 50 or 90) and femoral artery (PE 50) and both ureters were occluded as previously described (Harris & Koike, 1975). Plasma volume (PV) and ECFV were estimated by determining the volumes of distribution of Evans Blue (Harvey Laboratories, Inc., Philadelphia, PA) and 12’1-labeled sodium iothalamate (Glofil, Abbott Laboratories, Chicago, IL). Results of a previous study indicated that the volume of distribution of labeled iothalamate was a reasonable estimate of ECFV in this species (Harris & 311

KAREN M. HARRIS AND T. 1. KOIKE

312

Table

1. Composition

of diets Control

Cornstarch Isolated soybean protein DL-methionine Refined soy oil Vitamin mixture* Mineral mixture (Na-free)? NaCl KC1 KHCO, CaHPO, CaCl*

taco, Cellulose

Sodium

deficient

0,

II

6;; 22.0 0.4 3.0

62.: 27.0 0.4 3.0

1.o

I .o

0.34 0.51

0.34 0.3x

0.512 2.64 0.141 0.435 6.12

2.64 0. I xx 0.38 1.37

*Supplies the following per 1OOg of diet: Vitamin A acetate, 990 U; DL alpha tocopherol acetate; 1.1 U, riboflavin, 1.0 mg; Ca pantothenate, 2.0 mg; nicotinic acid, 3.3 mg ; folic acid, 0.3 mg ; thiamine. HCI, 1.O mg; pyridoxine’ HCl, 0.9 mg ; menadione sodium bisulfite, 0.55 mg; vitamin D,, 110 U; vitamin B, 2, 1.0 pg; and biotin. 4.4 pg. t Supplies the following per IOOg of diet: MgS0,,7Hz0, 612 mg; ferric citrate, 20mg; KIO,, l.Omg; ZnCO,, 13 mg; CuSO,. l.Omg; MnS04.HZ0. 25 mg; NaMO,.2H,O, 0.8 mg; Na,SeO,, 20 pg; cobalt acetate, 180 pg. Koike, 1975). At zero time l.Oml 0.15 M NaCl containing and 4.5 mg Evans Blue were about 1 PCi ‘251-iothalamate injected into the wing vein followed by 1.O ml 0.15 M NaCl. Heparinized blood samples (1 ml each) were obtained from the arterial cannula at 3, 10, 20, 40 and 60min. At the end of the 60min sample terminal venous and arterial blood samples (5 ml each) were obtained. Both kidneys were removed, cleaned of adhering tissue and weighed. The volume of distribution of Evans Blue was computed by dividing the amount of dye injected by the extrapolated plasma concentration at zero time. Vascular mixing was found to occur within IOmin in agreement with results obtained in younger birds by Hegsted et nl. (1951). ‘Z51-iothalamate space was calculated using the 20, 40 and 60min plasma concentrations of the labeled compound. The mean slope of the regression line through the three points did not differ significantly from zero. As previously described, the amount of radioactivity present in the two kidneys at the end of the experiment was subtracted from the amount injected in calculating the ‘251-iothalamate space (Harris & Koike, 1975). Anaerobic pH and hematocrit were determined on the terminal arterial blood sample at the end of the experiment. The remainder of the blood sample was centrifuged and the plasma frozen for subsequent determination of sodium and potassium concentrations, osmolality, and in some cases, Ca and Cl concentrations.

Trentmrnt with furosrmidr In a second series of experiments two groups of six pullets each were placed on the experimental diets for the same period of time as above. In addition, a diuretic. furosemide (Lasix, Hoechst Pharmaceuticals, Inc.) was administered to both control and deficient groups. Each bird received an intramuscular injection of diuretic (lOmg/ injection) for 3 consecutive days beginning 14 days prior to sacrifice. Collection of droppings were made from several birds in each group to assess sodium loss. Twenty-four hour samples were obtained on the day preceding and during the 3 days of diuretic administration. The droppings were collected in enamel trays containing 0.1 N HCl. Each 24 hr collection was transferred with deionized water rinses into a graduated cylinder and mixed. The volume was recorded.

the mixture filtered of the filtrate.

and sodium

determmed

on an aliquot

In a third series of experiments Na, and CL, were determined on birds that had been maintained on the diets and administered the diuretic as described above (Trearment with Furosemide). At the end of the 6 weeks feeding period the birds were weighed, food and water withdrawn. and 1 ml of 0.154 M NaCl containing approximately 1 pCi “Na (New England Nuclear) was injected intravenously. Blood samples (2 ml each) were obtained in heparinized syringes (ammonium heparin) at 18, 20, 22. and 24 hr subsequent to injection of the Isotope for determination 01 plasma specific activities. Droppings were collected during the 24-hr period following injection to correct the body content of 22Na for the amount excreted. The amount of ‘*Na injected was determined by measuring the radioactivity contained in a volume identical to that administered. Exchangeable sodium was calculated by dividing the cpm of 22Na injected (corrected for the total cpm excreted) by the specific activity (counts/mm ml-‘)/(p-equiv!ml) of the plasma sodium at zero time. The latter was obtained by plotting the specific activity of each plasma sample as a function of time and extrapolating the sample regression line to zero time. The mean slope of the regression lines for both groups of birds was zero. When expressed as a percent of the 18-hr value. the mean specific activity (& SE.) at 20, 22. and 24 hr for birds on the control diet were 99 f 1.5. 98 + 1.6, and 100 k 0.7. respectively. The analogous values for birds on the sodium deficient diet were 99 k 2.1, 103 i 2.4 and lo0 k 0.6, respectively. Ezchangeable sodium is expressed as m-equivjkg body weight. The 24-hr V,, value was calculated by dividing the amount of “Na in the body by the amount found in the plasma sample (counts/min ml _ I) and is expressed as pcrcent of the body weight (7; B. wt). In a few birds in this series bone samples were obtamed from control and deficient groups. The tibia was removed after sacrifice, scraped clean with a razor blade and split longitudinally. Medullary and cortical bone were separated. dried to constant weight at 106°C (Woodbury, 1956) and subjected to Kjeldahl digestion for analysis of sodium (Sanui & Pace. 1972).

Sodium deficiency in chickens

313

Fig. 1. Average weekly food intake and body weight of pullets on the control (closed circles) and sodium-deficient (open circles) diets. Averages (f SE.) were computed from observations over the 6-week period on 3-4 birds without diuretic treatment (panel A) and 7-9 birds administered the diuretic (panel B). Analytical

Evans Blue was determined on plasma samples diluted with 0.15 M NaCl at a wavelength of 623 rnp. The amount of 22Na and ‘251-iothalamate in plasma or kidney samples was estimated using a Nuclear Chicago Autogamma counter. Sodium, potassium and calcium were determined by atomic absorption spectroscopy (Perkin Elmer, model 290), chloride with a Buchler-Cotlove chloridometer and osmolality on an Advanced Wide-range osmometer (Advanced Instruments, model 68-31 WAS). The pH of arterial blood was measured anaerobically with a Radiometer pH meter (model 27 GM) at 42°C. Packed cell volume was determined on a micro-hematocrit centrifuge (International Equipment Co., model MB) after centrifugation for 5 min. Statistical

analysis

Differences between means were evaluated using the Student’s unpaired t-test. RESULTS The average weekly food intakes and body weights following placement of the birds on the experimental diets are depicted in Fig. 1 and the average daily food and water intakes and the total body weight gained

over the entire 6-week feeding period are summarized in Table 2. Food, but not water, intake was significantly depressed in both control and deficient groups by diuretic treatment (Fig. IS). During the week prior to furosemide administration control and deficient groups ate an average of 44 + 3.9 (SE.) and 51 f 4.4 g/kg body weight/day, respectively, while intakes during the 3 days of diuretic treatment were 22.1 + 2.4 and 20.9 + 2.1 (P < 0.005 in each case). However, when averaged over the 6-week feeding period no significant differences in daily food intake were observed among the 4 groups (Table 2). The total body weight gained by the two groups on the deficient diet was significantly lower than birds on

the control diet treated with furosemide (Table 2). Water intake differed only between the two groups on the control diet (Table 2). With one exception, no differences were noted in plasma electrolyte composition, hematocrit or arterial pH among the 4 groups at the termination of the 6-week feeding period (Table 3). Plasma sodium concentration of the control group without diuretic treatment was significantly higher than the control and deficient groups administered the diuretic. The volumes of distribution of Evans Blue and ’ 25Eiothalamate, used as estimates of PV and ECFV, respectively, are summarized in Table 4. The volume of distribution of ’ 25I-iothalamate in the group maintained on the control diet, not administered the diuretic, was significantly greater than both control and sodium-deficient groups treated with furosemide (P < 0.01 and 0.001, respectively). Of the two groups on the sodium restricted diet 1251-iothalamate space was significantly lower in the diuretic-administered group (P < 0.005). When compared to the birds on the control diet without furosemide administration the volume of distribution of 1251-iothalamate of the control group with diuretic treatment and the deficient groups without and with diuretic treatment were 27, 10 and 32% lower, respectively. In contrast, PV did not differ significantly between the groups on the control and sodium-deficient diets. A difference in PV which paralleled the difference in the volume of distribution of ’ 251-iothalamate, however, was noted in the two groups on the deficient diets. Differences in interstitial fluid volume (ISFV), taken as the difference between 1251-iothalamate and Evans Blue spaces. among the 4 groups mirrored the differences in ECFV (Table 4). The magnitude of the differences in ISFV were somewhat larger than those observed with ECFV. Thus, the estimated ISFV of the control group given furosemide and the deficient groups without

KARL+ M. HARRISAUD T. I. K~IKI

314

Table 2. Average (i S.E.) daily food and water intakes. water Intake weight gain over the 6.week feeding period. Numbers within parentheses per group

Diet

Group

Diuretic __

A B

Control

c

Sodiumdeficient

D

Water

Food intake (g’kg:‘day)

(ml/kg;day)

per gram food consumed and refer to the number of animal\

intake* (ml;g food)

Weight gatn-I (kg,

33 & 1.7 (4) 12 * 3.1 (9)

67 + X.0(4) 124 i_ 13.2 (6,

1.6 & 0.7 (3) 7.‘) * 0.5 (6)

0.25 * 0.0.X (1)

+

+

41 ) 2.2 (3) 47 f 2.4 (7)

75 * 1X.8(3) 120 + 21.9(4)

2.0 i 0.5 (3) 2.4 * 0.04 (-I)

0.11 * O.ICX)(3) 0.23 * 0.032 (7)

I I.34 + 0.020(‘Ii

*Water intake expressed as ml/kg/day (P < 0.01) and ml/g food consumed (P < 0.0.5) was significantly greater in group B than A. t Weight gains of the following were statistically chfferent: B > C (P < 0.M)and D (P < 0.01).

Table 3. Plasma

Diet

Group

A

D

*A z B

D

Diet

Table

147 * l.4(3)’ 140 * ‘.I (6)

3.2 f 0.2 (3) 3.2 k 0.16(6)

12.5 * 1.5(7) 11.8 ) 1.57(6)

31x * 3.6(7) 372 + _ IO.? (6)

_

I43 + 4.5 (3)

3.6 k 0.3 (3)

12.6 f

?I? + 2.4(-i)

+

140 k I .8 (6)

3.6 f 0.14(6)

IO.7 + 0.67 (6)

Diuretic

P (m-eqL/l)

P c., (mg”,,)

P 0\111 (mOsm/kg)

I 7(7)

3% i 9.5 (6)

Diet Control Na Deficient

Hct (5”)

Art. pH

_ +

II’) I: 2.9(4)

25.5 * 1.5 (7) 73.7 * 0.9 (3)

7.370 * 0.0193 (7) 7.391 + 0.0085 (4)

_

IIX kO.Y(4)

26.2 i

1.0(6)

7.302 i 0.0377 (6)

+ I .0X (6)

7.381 f 0.0140 (6)

+

4. Volumes

Group A B C D

+

_

Control Control Sodium deficient Sodium deficient

B C

P, (m-equivjl)

(P < 0.05) and D (P < 0.01)

Group

A

diuretic

P h.l (m-equiv;l)

Diuretic

Control Control Sodium deficient Sodium deficient

B C

composition of pullets on control and sodium deficient diets with and without treatment. Mean + S.E.; () refers to number of animals

24.1

of extracellular fluid compartments of birds on control and low sodium and without diuretic treatment (means k S.E.)*

Diuretic

N

_ + _ +

5 6 4 6

Plasma volume 4.x 4.7 5.2 4.7

k * + k

0.3 0.7 0.1 0. I

Body fluid compartments Extracellular fluid volume

diets with

(“,, B. wt) Intcrstitlal fluid volume

22.x + 1.4 16 7 + I.’ 20.5 + I.1

I x.0 + I 6 12.0 f I 1 15.3 + I.7

15.5* 0.6

10.x f 0.5

*For plasma volume, C > D (P < 0.01): for extracellular fluid volume, A > B (P < 0.0)and D (P < 0.001); C > D (P < 0.005): for interstitial fluid volume. A > B (P < 0.025) and D (P -c0.005). C > D (P < 0.01). and with diuretic treatment were lower by 33, I5 and 40% respectively, compared to the value of the group on the control diet. In groups administered furosemide and maintained on the control and sodium deficient diets for 6 weeks NaE and 1/22,, were ll”{, lower in the deficient group (Table 5). The difference was statistically significant in the case of V~~,d (P < 0.02) but not NaE (P < 0.10). While measurements were not made on birds that were maintained on the purified diets and not administered the diuretic. NaE and L’::,, on similarly-aged

pullets that had been maintained on a commercial growing ration averaged 42.9 k I.25 m-equivjkg body weight and 27.0 + 0.45”, body weight, respectively (Table 5). Differences in Na, and L,, that were noted between this group and the groups on the purified diets were not significant statistically. DlSCUSSION

Growth in chickens can be depressed because of aversion to purified diets (Taylor VI al.. 1970) as well

Sodium deficiency in chickens

315

Table 5. Exchangeable sodium (Na,) and 24-hr volume of distribution of “Na (t&J on birds maintained on purified diets for 6 weeks on a group of birds maintained on a commercial ration*

Diet

Bird

Commercial ration

8-l 8-2 8-3 8-4 8-5 8-6

Purified, control

21 25 27 29 31 33

Purified, sodium deficient

6 22 30 32 34

Body weight (kg)

NaE (m-equiv/kg B. wt)

vz&. (% B. wt)

1.05 1.14 1.13 0.88 1.27 1.25 Mean f S.E. 1.20 1.34 1.39 0.91 1.08 1.30 Mean k SE. 1.03 1.10 1.15 1.01 0.93 Mean k SE.

41.9 37.9 41.7 45.6 44.3 46.1 42.9 + 1.25 40.9 42.1 47.6 5o.a 51.4 43.0 46.0 + 1.88 36.3 38.0 44.7 45.5 40.3 41.0 f 1.81

21.6 25.0 26.8 28.3 27.0 27.3 27.0 k 0.45 26.1 28.6 30.9 30.6 30.9 25.0 28.7 + 0.46 23.9 23.9 27.9 27.5 25.0 25.6 f 0.87

* The commercial ration was Purina@ Laboratory Chick S-G chow@. The approximate stated amounts of Na, K and Cl in the commercial diet were 0.14, 1.02, and 0.24x, respectively. The protocol used in the group on the commercial diet differed in that PE cannulae were placed in a jugular vein and dorsal carotid artery 3 days prior to injection of ‘*Na and food and water were provided ad libitum during the 24 hr subsequent to the injection of the isotope, as to imbalances or deficits in nutrients. An apparent temporal decrease in weekly food consumption, perhaps related to the reduced palatibility of the purified diets, was noted over the 6-week period (Fig. I). However, since the diets were isocaloric and average daily food intakes over the entire 6-week feeding period were similar among the 4 groups the differences in body weight gain noted in the present study can be attributed to factors other than caloric intake (Table 2). While the differences were significant only in the 2 groups fed the sodium deficient diets, furosemide administration was associated with a greater intake of water when compared to uninjected birds on either diet (Table 2). The differences in intake were not due to differences in food consumption nor to differences in electrolyte levels in the control and deficient diets since food intakes were similar in all groups and greater water intakes were noted in diuretic treated birds on both diets (Table 2). In laying hens (Cohen et al., 1972) or broiler chicks (Hurwitz et al., 1973) the ratio of sodium to chloride in the diet influences acid-base balance. The alterations in blood pH related to changes in dietary Na:CI ratios are accompanied by reciprocal changes in plasma chloride and bicarbonate levels in hens (Cohen et al., 1972) and suppression of growth in chicks (Hurwitz et al., 1973). Although Na:CI ratios of the control and deficient diets used in our experiments were 0.5 and 0.06, respectively, the disparity was not accompanied by differences in either arterial pH or plasma chloride concentrations (Table 3). Nesheim et al. (1964) have shown that optimum growth in chicks requires a balance of dietary sodium and potassium to chloride and sulfate which is pre-

sumably related to the maintenance of acid-base balance. The lack of difference in arterial pH in the present experiments (Table 3) would suggest that the difference in the relative amounts of sodium and potassium to acid anions in the control and sodium deficients diets (Table 1) were not large enough to cause acid-base imbalances under the conditions used in these experiments. In several mammalian species a decrease in PV occurs in response to low sodium diets with or without combined diuretic treatment (Romero et al., 1968; Brown et al., 1971; Higgins, 1971; Weiner et al., 1971). Similar changes in vascular volume are observed in response to thermal stress or exercise (Kozlowski & Saltin, 1964) or water deprivation (Schultze et al., 1972). The reductions in plasma volume are associated with reductions in ECFV (Kozlowski & Saltin, 1964; Romero et al., 1968; Schultze et al., 1972) and exchangeable body sodium (Brown et al., 1971). The results of the present study would suggest that the changes in extracellular fluid compartments differ in chickens and mammals following dietary sodium restriction. No apparent changes in PV are observed in chickens despite differences in ECFV of l&32% (Table 4). While PV changes in a parallel manner with ISFV or ECFV in cats (Schultze et al., 1972) we found no correlation between PV and ECFV (r = 0.17) or ISFV (r = 0.05) in chickens (Fig. 2). Thus, the apparent losses of fluid associated with diuretic treatment and/or low sodium diets could be attributed primarily to losses from the interstitial fluid compartment. The reasons for the reduced volume of distribution of lZSI-iothalamate in birds on the control diet administered furosemide compared to the uninjected

KAREN M. HARRIS AND T. I. KOIKE

316

. .

Lx

.

:o

ns

0

0 0 0

0

0

.

.

.

>

0

a

.

L

I

I

5

I

1

I5 ISFV.

%

I 25

B. W t

Fig 2. Lack of correlation between plasma volume (PV) and interstitial fluid volume (ISFV) in birds maintained on the control and sodium deficient diets with and without diuretic treatment. Groups on control and deficient diets are indicated by circles and squares, respectively. Closed symbols indicate groups treated with furosemide.

group are not apparent (Table 4). The mean (+ S.E.) 24-hr excretion of sodium in control and sodium-deficient birds on the day prior to the initial injection of diuretic was 2.76 f 0.88 (N = 5) and 0.48 + 0.20 (N = 5) m-equiv, respectively. Sodium lost during the 3 days of diuretic treatment totalled 7.70 + 2.4 and 5.60 &- 1.9 m-equiv in control and deficient groups even though food intakes were depressed by about a half. Thus, diuretic treatment caused a net loss of sodium in both groups. Food intake in control and deficient groups had returned to normal during the final week on the diets (37 + 3.9 and 44 + 2.3 g/kg B. wt/day, respectively). One possibility might be that the net sodium lost during diuretic treatment was not replenished in the control group prior to sacrifice. Sodium excretion during the 24 hr prior to sacrifice was 3.85 f 0.75 m-equiv in 5 birds on the control diet and 0.13 f 0.05 m-equiv in 3 birds on the deficient diet. The estimated intakes of sodium (computed from the average food intake during the final week and the dietary sodium content) for these control were and deficient birds 3.61 + 0.65 and 0.35 + 0.05 m-equiv/day, respectively. While the 24-hr sodium excretion and intake in the controls do not differ significantly, a small daily negative balance might conceivably have added to the sodium losses incurred by diuretic treatment. Differences in Na,, while suggestive, do not resolve the question. In a separate series of experiments in which furosemide was administered, NaE in birds maintained on the control diet was 12% greater than the deficient group and 7% greater than in birds maintained on a commercial growing ration (Table 5). The sodium content of the control, commercial and deficient diets was 0.2, 0.14, and 0.017%, respectively. Thus, the differences noted between groups fed the 3 diets, while not statistically significant, suggests that Na, correlates with dietary sodium intake in chickens. Such an observation has been reported in humans (Jagger et al., 1963) and in rats (Edmonds, 1960). In rats bone serves as a sodium reservoir during acute or chronic sodium depletion (Bergstrom & Wallace. 1954). In 4 control and 2 deficient birds in which the tibia was removed for analysis, cortical bone con-

tained 0.7 f 0.17 (S.E.) and 0.2 & 0.12 m-equik sodium per gram dried weight, respectively (P < 0.1) while medullary bone contained 0.2 k 0.01 and 0 (not detectable) (P < 0.005) m-equiv sodium per gram dried weight. In laying hens, cortical bone serves as a calcium store (Hurwitz & Bar, 1971). In the present experiments sodium depletion resulted in reductions in bone sodium which would suggest that both medullary and cortical bone may serve to dampen the effects of sodium depletion in female chickens. From the effects of sodium deficient diet on growth (Table 2), volumes of distribution of ‘2”I-iothalamate (Table 4) and “Na (Table 5), and Nas (Table 5) it is evident that a reduction in sodium intake in chickens is associated with sodium depletion and contraction of ECFV. PV. as indicated by the lack of changes in Evans Blue space, and hematocrit was unaltered (Tables 3 and 4). The reasons for the absence of changes in PV are not apparent but may relate to qualitative differences in the forces that determine the distribution of fluid between the vascular and interstitial compartments (Djojosugito rt ul.. 1968; Wyse & Nickerson, 1971). The maintenance of PV in response to reduced intake of sodium in chickens undoubtedly depends on the extent to which body sodium is depleted. Hypovolemia. indirectly assessed by hematocrit. is readily induced in rapidly growing birds on a chloride-(Leach & Nesheim, 1963) or sodium-deficient (Lumijarvi & Vohra, 1976) diet. It would be interesting to examine the mechanisms for the apparent stability of PV over certain ranges of sodium depletion invoked by restricting sodium intake in the chicken. SUMMARY

The effects of chronic dietary sodium restriction, with and without the administration of the diuretic. furosemide, have been examined in the domestic fowl (Gallus domesticus). Food intakes were similar on groups on the control and sodium-deficient diets but birds maintained on the deficient diet had lower body weight gains. No differences were observed in arterial pH, plasma osmolality or in the plasma concentrations of potassium. chloride and calcium. Sodium levels were higher in birds on the control diet compared to groups on the control or sodium-deficient diets administered the diuretic. The low sodium diet and/or diuretic treatment were associated with reductions in the volume of extracellular (ECFV) and interstitial fluid (ISFV). Plasma volumes (PV) and hematocrit, however, remained unchanged. A lack of correlation between PV and ECFV or ISFV indicate that the chicken has the ability to maintain vascular volume over ranges of sodium depletion that are associated with hypovolemia in mammals.

REFERENCES BERGSTROMW. H. & WALLACE W. W. (1954)

sodium and 867-873.

potassium

reservoir.

BROWN W. J., BROWN. F. K. &

J.

Clbl.

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