Renal responses of normal and preascitic broilers to systemic hypotension induced by unilateral pulmonary artery occlusion

Renal Responses of Normal and Preascitic Broilers to Systemic Hypotension Induced by Unilateral Pulmonary Artery Occlusion MARY F. FORMAN1 and ROBERT F. WIDEMAN, JR. Department of Poultry Science, University of Arkansas, Fayetteville, Arkansas, 72701

INTRODUCTION Symptoms of the pathophysiological progression leading to pulmonary hypertension syndrome (PHS; ascites) include increases in the hematocrit, pulmonary arterial pressure (pulmonary hypertension), and right:total ventricular weight ratio (RV:TV); decreases in the heart rate (bradycardia) and mean systemic arterial pressure (systemic hypotension); and saturation of hemoglobin with oxygen in arterial blood (hypoxemia) (Cueva et al., 1974; Sillau and Montalvo, 1982; Huchzermeyer and DeRuyck, 1986; Guthrie et al., 1987; Hernandez, 1987; Peacock et al., 1989, 1990; Maxwell, 1991; Mirsalimi and Julian, 1991; Julian and Mirsalimi, 1992; Julian, 1993; Odom, 1993; Wideman and Bottje, 1993; Lubritz and McPherson, 1994; Owen et al., 1994; Enkvetchakul et al., 1995; Lubritz et al., 1995; Owen et al., 1995; Wideman and Kirby, 1995a,b; Fedde and Wideman, 1996; Shlosberg et al., 1996; Roush et al., 1996, 1997; Kirby et al., 1997; Olkowski et al., 1997; Wideman, 1999). Characteristic changes in most of these indices have been used repeatedly to accurately differentiate normal broilers from susceptible (preascitic) broilers

Received for publication April 19, 1999. Accepted for publication August 11, 1999. 1 To whom correspondence should be addressed: mforman@comp. uark.edu

that subsequently will develop PHS (Roush et al., 1996, 1997; Kirby et al., 1997; Wideman et al., 1997, 1998c). However, relatively little consideration has been given to the onset and persistence of systemic hypotension in association with pulmonary hypertension and hypoxemia in fastgrowing broilers (Peacock et al., 1989). In clinically healthy domestic fowl, a direct association between pulmonary hypertension and the contemporaneous onset of systemic hypotension can be demonstrated when large acute increases in pulmonary vascular resistance are induced by hypoxic or pharmacologic pulmonary vasoconstriction or by mechanical occlusion of one pulmonary artery. Evidently, the relatively weak right ventricle normally is incapable of developing sufficient pressure to propel the entire cardiac output through an elevated pulmonary vascular resistance, resulting in an initial reduction in cardiac output that substantially accounts for the corresponding systemic hypotension (Besch and Kadono, 1978; Wideman and Bottje, 1993; Owen et al., 1995; Wideman et al., 1996a,b; 1998a,b,c). When chronically challenged, the right ventricle hypertrophies to further elevate the pulmonary arterial pressure. Consequently, the persistence of systemic hypotenAbbreviation Key: PAH = para-aminohippuric acid; PHS = pulmonary hypertension syndrome; RV:TV = right:total ventricular weight ratio.

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ABSTRACT During the pathophysiological progrespressure (91 vs 100 mm Hg) and percentage saturation sion of pulmonary hypertension syndrome (PHS; ascites), of hemoglobin with oxygen (73 vs 84%), higher hematobroilers concurrently develop systemic hypotension (low crits (35 vs 30%), heavier right ventricles (3.44 vs 2.32 g), mean systemic arterial pressure) that may initiate renal and higher right:total ventricular weight ratios (0.32 vs retention of water and solute, contributing to fluid accu0.24) than normal broilers. Body weights (2,445 vs 2,429 mulation in the abdominal cavity (ascites). In male Single g, respectively), left ventricle plus septum weights (7.16 Comb White Leghorns, glomerular filtration is autoreguvs 7.19 g), and heart rates (349 vs 341 beats/min) were lated over a systemic arterial pressure range of 110 to 60 similar. Preascitic broilers exhibited larger (P ≤ 0.05) demm Hg, and corresponding reductions in urine flow are pendent reductions in glomerular filtration, urine flow, attributed to a phenomenon known as pressure natriureosmolal clearance, and solute excretion and had a higher sis. Acute unilateral pulmonary artery occlusion was used free water clearance than normal broilers in response to in the present study to reduce systemic arterial pressure pulmonary artery occlusion. The differences observed betoward the lower autoregulatory limit for glomerular filtween normal and preascitic broilers demonstrate that tration, and to evaluate kidney function in normal and systemic hypotension can trigger renal mechanisms conpreascitic broilers. Preascitic broilers characteristically extributing to fluid and solute retention during develophibited lower (P ≤ 0.05) values for mean systemic arterial ment of PHS. (Key words: ascites, pulmonary hypertension, renal function, systemic hypotension) 1999 Poultry Science 78:1773–1785

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MATERIALS AND METHODS Two hatches of male by-product chicks from the Hubbard2 6610-HiY breeder pullet line (hatch dates: February 2

Hubbard ISA, Walpole, NH 03608. Sigma Chemical Co., St. Louis, MO 63178-9916. Interstate Drug Exchange, Inc., Amityville, NY 11701. 5 Arista Surgical Supply Co., Inc., New York, NY 10010-1898. 6 Intramedic Co., Parsippany, NJ 07054. 7 World Precision Instruments, Sarasota, FL 34230. 8 Biopac Systems, Inc., Santa Barbara, CA 93117. 3 4

17, 1997 and April 23, 1997) were reared on fresh wood shavings litter in environmental chambers (8 m2 floor space). They were brooded at 32, 30, and 27 C during Weeks 1, 2, and 3, respectively, and, thereafter, the temperature was maintained at 24 C. Throughout the experiment, the birds were fed a corn-soybean meal-based ration formulated to meet NRC (1984) standards for all ingredients, including 22.7% CP, 3,059 kcal/kg ME, 1.5% arginine, 1.43% lysine, and 0.24% sodium. Feed and water were provided for ad libitum consumption. When the birds reached 36 to 53 d of age, the largest clinically healthy individuals with bright red combs (indicative of normal arterial blood oxygenation) were selected for the normal group (n = 14 and 6 for Hatches 1 and 2, respectively). Large birds exhibiting obvious cyanosis indicative of preascitic hypoxemia were selected for the preascitic group (n = 8 and 16 for Hatches 1 and 2, respectively). The surgical protocol for acute pulmonary occlusion was described previously (Wideman and Kirby, 1995b; Wideman et al., 1996a,b, 1998b,c). The birds were anesthetized to a surgical plane with an intramuscular injection of allobarbital3 (5,5-diallyl-barbituric acid; 15 mg/kg body weight). They were fastened in dorsal recumbency on a heated surgical board that was thermostatically regulated to maintain a surface temperature of 35 C. One end of the surgical board was elevated to a 20 degree head-up angle. A 2% lidocane4 solution was infiltrated intracutaneously as a supplemental local anesthetic along the midline of the thoracic inlet. A midline incision was made, the crop and trachea were retracted laterally, and the clavicular air sac was opened. A snare, consisting of a loop of braided 2-O silk suture thread5 passed through a 6-cm length of PE290 polyethylene tubing,6 was placed loosely around the left pulmonary artery. The skin of the thoracic inlet was closed with stainlesssteel wound clips. The left brachial artery and left cutaneous ulnar vein were cannulated with PE50 polyethylene tubing6 filled with 0.9% sodium chloride containing 200 IU heparin.3 The arterial cannula was advanced to a position near the descending aorta, and, except when used for arterial blood sampling, the distal end of the cannula was attached to a BLPR blood pressure transducer7 for continuous monitoring of systemic arterial pressure. The blood pressure transducer was interfaced through a Transbridge preamplifier7 to a Biopac MP100 data acquisition system8 using Acknowledge software.8 Heart rate (beats per minute) was calculated by counting systolic peaks over time in the systemic mean arterial pressure recording. A solution of 25 g mannitol,3 3 g inulin,3 and 3 g para-aminohippuric acid3 (PAH)/L water was injected as a priming bolus of 5 mL into the venous cannula to rapidly elevate plasma inulin and PAH levels. The same solution then was infused i.v. at a constant rate of 0.05 mL/kg body weight per min to maintain stable plasma levels of appropriate markers (inulin and PAH) for assessing renal function throughout the experiment (Wideman et al., 1983, 1985, 1994; Wideman and Gregg, 1988; Vena et al., 1990; Wideman, 1990). The probe of

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sion during chronic pulmonary hypertension has been attributed, in part, to systemic precapillary arteriole dilation (a reduction in total peripheral resistance) in response to hypoxemia. Evidently the broiler heart is incapable of maintaining a normal systemic arterial pressure when maximal tissue demands for blood flow and oxygen delivery are confounded by an inadequate pulmonary capacity (Peacock et al., 1989; Wideman and Bottje, 1993). Systemic hypotension triggers a variety of intrarenal mechanisms that normally serve to increase the blood volume and thereby restore the blood pressure by stimulating the kidneys of domestic fowl to retain fluid and osmotically active solute (Wideman and Gregg, 1988; Vena et al., 1990; Wideman, 1991; Wideman et al., 1993; Glahn et al., 1993). Reducing the renal arterial pressure across the entire range from hypertension to profound hypotension causes proportional reductions in urine flow and solute excretion through a variety of phenomena known cumulatively as “pressure natriuresis.” Independent of pressure natriuresis, the glomerular filtration rate is autoregulated (remains constant) over an arterial pressure range of 110 to approximately 60 mm Hg in Single Comb White Leghorn roosters. However, at arterial pressures below the autoregulatory range, reductions in glomerular filtration contribute directly to large dependent reductions in urine flow with urine flow ceasing at arterial pressures below 50 mm Hg. When the renal arterial perfusion pressure approaches the lower autoregulatory limit for glomerular filtration, the renin-angiotensin-aldosterone system is activated and may contribute to solute and fluid retention (Wideman et al., 1993). In broilers that are preascitic and hypotensive, excessive renal retention of solute and fluid theoretically should expand the blood volume, contribute to the vascular congestion associated with incipient right-sided congestive heart failure, and ultimately would serve as the source of ascitic fluid (Wideman and Bottje, 1993). The present study was designed to evaluate the arterial pressure and renal function of normal and preascitic broilers during control urine collection intervals and during an interval of acute systemic hypotension induced by unilateral pulmonary artery occlusion. We tested the hypotheses that preascitic broilers should exhibit an existing systemic hypotension and lower rates of urine flow and solute excretion when compared with normal broilers and that similar differences should be revealed when acute unilateral pulmonary artery occlusion triggers reductions in arterial pressure approaching or exceeding the lower autoregulatory limit for glomerular filtration.

RENAL RESPONSES OF NORMAL AND PREASCITIC BROILERS

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Criticare Systems, Inc., Milwaukee, WI 53226. Bob Smith Industries, Atascadero, CA 94322. 11 Wescor, Inc., Logan, UT 84312. 12 Instrumentation Laboratory, Lexington, MA 02173. 10

group. Only birds with higher RV:TV ratios were included in the preascitic group. Timed urine samples were collected in preweighed tubes for gravimetric determination of urine volume. Whole urine osmolality was determined immediately after sample collection using a vapor pressure osmometer.11 Aliquots of whole urine were mixed with equal volumes of 0.5 M LiOH3 in a separate tube to dissolve uric acid precipitates and release trapped solutes for subsequent analysis. Heparinized blood samples were centrifuged immediately after collection. A portion of the arterial blood was used for duplicate hematocrit determinations using heparinized capillary tubes and a microhematocrit centrifuge. Plasma and urine sample tubes were sealed with parafilm, double wrapped in plastic, stored at −4 C, and thawed immediately prior to analysis. Plasma samples were analyzed for colloid and crystalloid osmolality11 after thawing. Plasma and urine concentrations of sodium and potassium were measured by flame photometry,12 and plasma concentrations of inulin and PAH were measured by spectrophotometry (Brun, 1957; Waugh, 1977). The urine flow rate was calculated for each kidney as the volume collected per kilogram body weight per minute. Glomerular filtration rate (milliters per kilogram BW per minute) was calculated from the clearance of inulin, which equals the urine to plasma inulin concentration ratio multiplied by the urine flow rate. The absolute rate of sodium excretion (micromoles per kilogram BW per minute) was calculated as the urinary concentration of sodium (micromoles per milliter) multiplied by the urine flow rate. The filtered load of sodium (micromoles per kilogram BW per minute) was calculated as the glomerular filtration rate multiplied by the plasma concentration of sodium (micromoles per milliter). The fraction of the filtered load of sodium excreted in the urine was calculated as the absolute rate of sodium excretion divided by the filtered load of sodium. The effective renal plasma flow rate (milliters per kilogram BW per minute) was calculated as the clearance of PAH, which equals the urine to plasma PAH concentration ratio multiplied by the urine flow rate. The rate at which plasma was cleared of osmotically active solute was calculated as the osmolal clearance rate (milliters per kilogram BW per minute), which equals the urine to plasma crystalloid osmolality ratio multiplied by the urine flow rate. The contribution of the kidneys to diluting or concentrating the plasma by net recovery (negative values; the formation of hyperosmotic or concentrated urine with respect to plasma) or net elimination (positive values; the formation of hypoosmotic or dilute urine with respect to plasma), respectively, of solute-free water is known as the free water clearance (milliters per kilogram BW per minute) and was calculated as the difference between urine flow rate and the osmolal clearance rate. All data are presented as single kidney values. The glomerular filtration rate did not differ (P = 0.3618) when broilers from separate hatches were compared within their respective normal or preascitic groups; conse-

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a pulse oximeter9 was positioned on the right wing to illuminate the tissue between the radius and ulna for continuous and noninvasive measurements of the percentage saturation of hemoglobin with oxygen (Peacock et al., 1990; Julian and Mirsalimi, 1992; Wideman and Kirby, 1995b; 1996; Wideman et al., 1998a,b). In preparation for urine collection, the cloaca was everted, the ureteral openings were exposed, the cloacal folds were sutured open with 2-O silk suture,5 and a ureteral cannula was glued over each ureteral orifice with cyano-acrylate10 to permit separate collection of the urine from each kidney (Wideman and Braun, 1982; Wideman et al., 1983, 1985). Following completion of the surgical procedures, a stabilization period of 40 min was allowed to elapse, then nine consecutive urine samples of 10 min in duration were collected (urine samples 1 to 9). Arterial blood samples (1 mL each) were withdrawn after urine samples 1, 2, 4, 5, 7, and 8 (arterial samples A to F, respectively). Preliminary experiments had shown that preascitic broilers with a percentage saturation of hemoglobin with oxygen ≤76% could not survive unilateral pulmonary occlusion. Furthermore, preascitic broilers also exhibited an immediate cessation of urine flow followed by death within 15 min if the systemic arterial pressure remained below 50 mm Hg after the pulmonary artery snare was fully tightened. Consequently, preascitic broilers with a percentage saturation of hemoglobin with oxygen ≤76% during urine sample periods 1 to 3 were assigned to a time control group in which the pulmonary artery snare had been implanted but was not tightened (preascitic control; n = 11). Preascitic broilers with a percentage saturation of hemoglobin with oxygen >76% were assigned to the preascitic snare group (n = 13) in which the snare was tightened for 30 min while urine samples 4 to 6 were collected. Then, the snare was released. To maintain viability and permit continued urine sample collection, the snare initially was fully tightened and then gradually relaxed to the extent necessary to maintain a minimum systemic arterial pressure between 55 and 60 mm Hg. Normal broilers were randomly assigned to a time control group (normal control; n = 9) or to a snare group (normal snare; n = 11) without regard for their initial systemic arterial pressure. In all normal broilers, it was consistently possible to tighten the snare rapidly and fully without triggering profound hypotension, cessation of urine flow, or mortality. At the end of each experiment, birds were killed with an i.v. overdose of anesthetic or 2 M potassium chloride. The heart was removed, dissected, and weighed for calculation of the RV:TV ratios. Broilers are considered to have pulmonary hypertension when their RV:TV ratios are 0.28 or higher (Cueva et al., 1974; Hurchzermeyer et al., 1988; Peacock et al., 1989); therefore, only birds with RV:TV ratios of ≤0.274 were accepted for the normal

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FORMAN AND WIDEMAN, JR. TABLE 1. Comparisons of body weight, ventricular weights, right:total ventricular weight ratio (RV:TV), percentage saturation of hemoglobin with oxygen in arterial blood, and mean systemic arterial pressure in all normal vs all preascitic broilers during the initial sample intervals1 Variable

Normal group (n = 20)

Body weight, g Right ventricle, g Right ventricle/kg body weight Left ventricle + septum, g Left ventricle + septum/kg body weight Total venticle, g RV:TV Saturation of hemoglobin with O2, %2 Mean systemic arterial pressure, mm Hg2 Heart rate, beats/min

2,429 2.32 0.00096 7.19 0.00296 9.5 0.244 ± 83.8 100.3 349

± 83 ± 0.14 ± 0.00001 ± 0.28 ± 0.00001 ± 0.39 0.007 ± 0.3 ± 0.3 ± 1

Preascitic group (n = 24) 2,445 3.44 0.00141 7.16 0.00295 10.6 0.323 72.7 90.6 341

± ± ± ± ± ± ± ± ± ±

76 0.13 0.00001 0.25 0.00001 0.35 0.006 0.3 0.3 1

P 0.8867 0.0001 0.0048 0.9518 0.8383 0.0434 0.0001 0.0001 0.0001 0.3943

Data are means ± SEM. Combined values for the presnare sample periods.

1 2

oxygen (Figure 1) and mean systemic arterial pressure (Figure 2) remained lower in preascitic broilers than in normal broilers throughout the experimental protocol. Tightening the pulmonary artery snare reduced the percentage saturation of hemoglobin with oxygen and mean systemic arterial pressure in both normal and preascitic broilers. The systemic arterial pressure declined to 74.2 ± 2.8 mm Hg with the snare fully tightened in normal broilers, and the systemic arterial pressure was maintained at 59.8 ± 2.6 mm Hg by partially relaxing the snare in preascitic broilers (Figure 2). The heart rate was lower

RESULTS Comparisons of the presnare values for all normal vs all preascitic broilers are shown in Table 1. The normal and preascitic broilers did not differ in body weight, left ventricle plus septum weight, left ventricle plus septum weight normalized for body weight, or heart rate. When compared with normal broilers, the preascitic broilers had higher values for right ventricle weight, right ventricle weight normalized for body weight, total ventricular weight, and RV:TV ratios. The percentage saturation of hemoglobin with oxygen and the mean systemic arterial pressure were lower in preascitic broilers than in normal broilers (Table 1). When preascitic and normal broilers were separated into their respective time control or snare groups, the percentage saturation of hemoglobin with

FIGURE 1. The upper graph represents the percentage saturation of hemoglobin with oxygen (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in the time control groups that did not undergo unilateral pulmonary artery occlusion during consecutive 10-min sample intervals (1 to 9). The lower graph represents the percentage saturation of hemoglobin with oxygen for normal (n = 11) and preascitic (n = 13) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min during sample intervals 4 to 6 (snare tightened). Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

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quently, data from both hatches were combined for final statistical comparisons. Body weights and heart weights were compared by the GLM procedure (SAS Institute, 1990). The systemic arterial pressure and renal function variables were divided into a presnare interval (urine Samples 1 to 3; plasma Samples A and B), a snare interval (urine Samples 4 to 6; plasma Samples C and D), and a postsnare interval (urine Samples 7 to 9; plasma Samples E and F). These intervals were analyzed by two-factor ANOVA with test statements for within-treatment and between-treatment variance using PROC GLM (SAS Institute, 1990). The Least Significant Difference test (SAS Institute, 1990) was used as a mean separation technique to calculate within-treatment differences between mean presnare sample intervals and individual sample intervals while the snare was tightened (Milliken and Johnson, 1984). The Least Significant Difference test also was used as a mean separation technique to determine differences between the presnare, snare, and postsnare intervals for experimental variables (SAS Institute, 1990). Relationships between systemic mean arterial pressure and renal function variables were evaluated by linear regression using SigmaStat威 Statistical Software (Jandel Scientific, 1994). All differences were considered significant at P ≤ 0.05.

RENAL RESPONSES OF NORMAL AND PREASCITIC BROILERS

FIGURE 3. The upper graph represents heart rate (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in the time control groups that did not undergo unilateral pulmonary artery occlusion during consecutive 10-min sample intervals 1 to 9. The lower graph represents heart rate for normal (n = 11) and preascitic (n = 13) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min, during 10-min sample intervals 1 to 9. Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

in preascitic than in normal broilers only while the snare was tightened (Figure 3). Renal function variables are illustrated in Figures 4 to 9. The glomerular filtration rate did not differ between normal and preascitic broilers as long as the pulmonary artery snare remained loose (Figure 4). Tightening the snare caused the glomerular filtration rate to decline in both groups; however, the glomerular filtration rate was lower in the preascitic group than in the normal group as long as the pulmonary artery snare remained tightened (Figure 4). The effective renal plasma flow rate did not differ between normal and preascitic broilers in the time control groups but was lower in the preascitic broilers than in normal broilers in the snare groups during urine Samples 1 to 6 (Figure 5). Tightening the snare caused the effective renal plasma flow to decline in both groups, and the renal plasma flow remained lower in preascitic group than in the normal group as long as the pulmonary artery snare was tightened (Figure 5). The urine flow rate (Figure 6) and osmolal clearance rate (Figure 7) were lower in the preascitic time control group than in the normal time control group throughout the experimental protocol. The same variables did not differ between the snare groups during the pre- or postsnare intervals; however, tightening the snare reduced the urine flow rate and the osmolal clearance rate to a greater extent in the preascitic than in the normal broilers (Figures 6 and 7). During the initial sample intervals, the free water clearance rate was higher (less negative) in preascitic controls when compared with normal controls (Figure 8). The free

FIGURE 4. The upper graph represents glomerular filtration rate (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in time control groups that did not undergo unilateral pulmonary artery occlusion during 10-min sample intervals 1 to 9. The lower graph represents glomerular filtration rate for normal (n = 14) and preascitic (n = 11) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min, during 10-min sample intervals 4 to 6 (snare tightened). Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

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FIGURE 2. The upper graph represents mean systemic arterial pressure (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in the time control groups that did not undergo unilateral pulmonary artery occlusion during consecutive 10-min sample intervals 1 to 9. The lower graph represents mean systemic arterial pressure for normal (n = 11) and preascitic (n = 13) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min, during 10-min sample intervals 1 to 9. Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

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FIGURE 6. The upper graph represents urine flow rate (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in the time control groups that did not undergo unilateral pulmonary artery occlusion during 10-min sample intervals 1 to 9. The lower graph represents urine flow rate for normal (n = 11) and preascitic (n = 13) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min during 10-min sample intervals 4 to 6 (snare tightened). Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

FIGURE 8. The upper graph represents the free water clearance (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in the time control groups that did not undergo unilateral pulmonary artery occlusion during 10-min sample intervals 1 to 9. The lower graph represents the free water clearance for normal (n = 11) and preascitic (n = 13) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min during 10-min sample intervals 4 to 6 (snare tightened). Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

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FIGURE 5. The upper graph represents effective renal plasma flow (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in the time control groups that did not undergo unilateral pulmonary artery occlusion during 10-min sample intervals 1 to 9. The lower graph represents effective renal plasma flow for normal (n = 11) and preascitic (n = 13) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min during 10-min sampling intervals 4 to 6 (snare tightened). Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

FIGURE 7. The upper graph represents the clearance of osmolal solute (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in the time control groups that did not undergo unilateral pulmonary artery occlusion during 10-min sample intervals 1 to 9. The lower graph represents the clearance of osmolal solute for normal (n = 11) and preascitic (n = 13) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min during 10-min sample intervals 4 to 6 (snare tightened). Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

RENAL RESPONSES OF NORMAL AND PREASCITIC BROILERS

water clearance rate did not differ between the snare groups during the presnare sample intervals. Tightening the snare increased the free water clearance rate to less negative values in both groups, with a greater change occurring in preascitic than in normal broilers throughout the period of pulmonary artery occlusion (Figure 8). The sodium excretion rate was higher in preascitic controls than in normal controls during the initial sample intervals (Figure 9). Sodium excretion did not differ between the snare groups during the initial sample intervals, sodium excretion declined only in preascitic broilers while the snare was tightened, and sodium excretion increased in the preascitic broilers when compared with normal broilers after the snare was released (Figure 9). Additional kidney function values are summarized in Table 2. When compared with the initial sample intervals, pulmonary artery occlusion caused similar reductions in urine osmolality in normal and preascitic broilers (Table 2). The urine sodium concentration initially was higher in preascitic broilers than in normal broilers in the time control groups, but thereafter the urine sodium concentration did not differ. In the snare groups, pulmonary artery occlusion did not reduce urine sodium concentrations when compared with the respective initial values; however, the urine sodium concentration in the normal snare group gradually declined to levels that were lower than those for the preascitic group during the occlusion and postocclusion intervals. The percentage contribution of sodium to the total urine osmolality [(urine sodium concentration/urine osmolality) × 100] was low in all groups (<6%) and did not differ among the time control and snare groups during the initial sample intervals. During

the occlusion and postocclusion intervals, the fraction of urine osmolality attributable to sodium declined in the normal snare group to a level that was lower than that in the preascitic snare group. The filtered load of sodium was lower in time control preascitic broilers than in normal control broilers during the final sample intervals. Pulmonary artery occlusion caused a greater reduction in the filtered load of sodium in preascitic broilers than in normal broilers, whereas the filtered load of sodium increased to a higher value in preascitic broilers than in normal broilers after the snare was released. Fractional sodium excretion declined over time in preascitic broilers in both groups but was higher in preascitic broilers than in normal broilers during the final sample intervals. The urine potassium concentration initially was lower in preascitic broilers than in normal time control broilers. Pulmonary artery occlusion reduced the urine potassium concentration in preascitic broilers (Table 2). Hematocrit and plasma values during the initial, pulmonary artery occlusion, and postocclusion sample intervals are shown in Table 3. The hematocrit remained consistently higher in preascitic broilers than in normal broilers throughout the experimental protocol. Plasma osmolality was higher in preascitic broilers than in normal broilers of both groups during intervals 7 to 9. No consistent intergroup differences were detected for colloid osmotic pressure or plasma sodium concentration. The plasma potassium concentration increased in normal and preascitic broilers during pulmonary artery occlusion, and plasma potassium was lower in preascitic broilers than in normal broilers during samples 7 to 9 (Table 3). The relationships between mean systemic arterial pressure vs percentage saturation of hemoglobin with oxygen or renal function parameters during urine sample intervals 1 to 6 are shown in Table 4 for the normal and preascitic time control groups. Only the mean systemic arterial pressure (93.2 ± 1.4 mm Hg, samples 1 to 3; 89.0 ± 1.4 mm Hg, samples 4 to 6) was positively correlated with the percentage saturation of hemoglobin with oxygen for broilers in the normal control group. For the preascitic control group, the mean systemic arterial pressure (83.4 ± 2.3 mm Hg, samples 1 to 3; 76.9 ± 2.3 mm Hg, samples 4 to 6) was positively correlated with glomerular filtration rate, urine flow rate, and sodium excretion rate. For the normal and preascitic snare groups, the relationships between mean systemic arterial pressure vs the percentage saturation of hemoglobin with oxygen or renal function parameters during urine sample intervals 1 to 6 are shown in Table 5. In the normal snare group, the mean systemic arterial pressure (106.1 ± 2.8 mm Hg, samples 1 to 3; 74.2 ± 2.8 mm Hg, samples 4 to 6) was positively correlated with the percentage saturation of hemoglobin with oxygen, glomerular filtration rate, effective renal plasma flow rate, sodium excretion rate, and osmolal clearance rate. In the preascitic snare group, mean systemic arterial pressure (97.1 ± 2.6 mm Hg, samples 1 to 3; 59.8 ± 2.6 mm Hg, samples 4 to 6) was positively correlated with the percentage saturation of hemoglobin with oxygen, glomerular filtration rate, effective renal plasma

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FIGURE 9. The upper graph represents the rate of sodium excretion (mean ± SEM) for normal (n = 9) and preascitic (n = 11) broilers in the time control groups that did not undergo unilateral pulmonary artery occlusion during 10-min sample intervals 1 to 9. The lower graph represents the rate of sodium excretion for normal (n = 11) and preascitic (n = 13) broilers in the pulmonary artery snare groups that did undergo unilateral pulmonary artery occlusion for 30 min during 10-min sample intervals 4 to 6 (snare tightened). Different letters (a,b) designate differences (P ≤ 0.05) between the means for group values during sample intervals 1 to 3, 4 to 6, or 7 to 9. Asterisks (*) designate differences (P ≤ 0.05) between the means for sample intervals 4 to 6 compared with the means for sample intervals 1 to 3.

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TABLE 2. Urine values in normal and preascitic broilers during ininitial sample intervals 1 to 3, during sample intervals 4 to 6 while a snare around one pulmonary artery remained loose (time control groups) or was tightened (snare groups), and during sample intervals 7 to 9 when the snare again was loose in all birds1 Time control groups Samples

Normal (n = 9)

Urine osmolality, mOsm/L

1 4 7 1 4 7 1 4 7 1 4 7 1 4 7

424 430 491 10.4 9.9 7.2 2.3 2.3 1.7 358.9 331.3 350.6 0.88 0.81 0.53

Urine sodium, mmol/L (Urine sodium/urine osmolality) × 100 Filtered load of sodium, µM × kg−1 × min−1

Fractional sodium excretion

Urine potassium, mmol/L

to to to to to to to to to to to to to to to

3 6 9 3 6 9 3 6 9 3 6 9 3 6 9

1 to 3 4 to 6 7 to 9

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Preascitic (n = 11) 10a 10a 10a 2.8b 2.8a 2.8a 0.7a 0.7a 0.7a 11.3a 11.3a 11.5a 0.17a 0.17a 0.17b

111.1 ± 7.7a 115.2 ± 7.7a 121.8 ± 7.7a

441 441 432 22.3 15.8 13.7 5.4 3.7 3.3 339.2 353.1 315.0 1.65 1.08 0.98

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Normal (n = 11) 9a 9a 9a 2.5a 2.5a* 2.5a 0.7a 0.7a 0.7a 10.2a 10.2a 10.2b 0.16a 0.16a* 0.16a

83.8 ± 7.0b 95.2 ± 7.0a 115.4 ± 7.0a

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

443 409 416 16.9 15.1 14.5 4.0 3.8 3.7 351.9 265.2 331.1 1.50 1.58 1.72

Preascitic (n = 13) 9x 9x* 9x 2.5x 2.5y 2.5y 0.7x 0.7y 0.7y 10.2x 10.2x* 10.2y 0.16x 0.16x 0.16y

112.9 ± 7.0x 94.7 ± 7.0x 116.2 ± 7.0x

434 403 397 21.1 21.7 25.5 5.1 5.6 6.7 343.7 184.6 366.9 1.87 1.73 3.15

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

8x 8x* 8x 2.3x 2.3x 2.3x 0.6x 0.6x 0.6x 9.4x 9.4y* 9.4x 0.14x 0.14x* 0.14x

105.4 ± 6.4x 86.6 ± 6.4x* 106.9 ± 6.4x

Means across a single row with different superscripts differ (P ≤ 0.05) for comparisons between time control groups. Means across a single row with different superscripts differ (P ≤ 0.05) for comparisons between snare groups. 1 Data are means ± SEM. *Means within a single column and variable differ (P ≤ 0.05) for comparisons of samples 1 to 3 vs 4 to 6. a,b

x,y

flow rate, urine flow rate, sodium excretion rate, and osmolal clearance rate.

this objective, it was essential to differentiate correctly the normal (clinically healthy) from preascitic broilers on the day of each kidney function experiment. Visual evidence of cyanosis proved to be a reliable screening tool for selecting broilers for the preascitic group that had a higher hematocrit, a lower percentage saturation of hemoglobin with oxygen, a lower mean systemic arterial

DISCUSSION One objective of this study was to compare kidney function in normal and preascitic broilers. To accomplish

TABLE 3. Hematocrit and plasma values in normal and preascitic broilers during initial sample intervals 1 to 3, during sample intervals 4 to 6 while a snare around one pulmonary artery remained loose (time control groups) or was tightened (snare groups), and during sample intervals 7 to 9 when the snare again was loose in all birds1 Time control groups Variable

Samples

Normal (n = 9)

Hematocrit, %

1 4 7 1 4 7 1 4 7 1 4 7 1 4 7

29.3 28.8 29.7 300 300 300 6.9 6.7 6.6 140.1 140.2 139.4 3.73 3.76 3.84

Plasma osmolality, mOsm/L

Plasma colloid osmotic pressure, mm Hg

Plasma sodium, mmol/L

Plasma potassium, mmol/L

to to to to to to to to to to to to to to to

3 6 9 3 6 9 3 6 9 3 6 9 3 6 9

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Snare groups

Preascitic (n = 11) 0.7b 0.7b 0.8b 3a 3a 3b 0.2a 0.2a 0.2a 1.1a 1.1a 1.0b 0.08a 0.08a 0.08a

35.0 34.1 33.1 305 301 307 7.1 6.9 7.1 142.1 141.9 142.3 3.74 3.84 3.73

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Normal (n = 11) 0.7a 0.7a 0.7a 2a 2a 2a 0.2a 0.2a 0.2a 1.0a 1.0a 0.9a 0.07a 0.07a 0.07b

30.7 30.6 30.0 297 299 295 7.2 7.3 6.8 141.7 142.4 141.7 3.75 3.96 3.94

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Preascitic (n = 13) 0.7y 0.7y 0.7y 2x 2x 2y 0.2x 0.2x 0.2y 1.0x 1.0x 0.9x 0.07x 0.07x* 0.07x

Means across a single row with different superscripts differ (P ≤ 0.05) for comparisons between time control groups. Means across a single row with different superscripts differ (P ≤ 0.05) for comparisons between snare groups. 1 Data are means ± SEM. *Meanas within a single column and variable differ (P ≤ 0.05) for comparisons of samples 1 to 3 vs samples 4 to 6. a,b

x,y

36.0 35.5 35.2 304 308 304 7.6 7.6 7.8 140.6 142.4 141.0 3.76 4.10 3.67

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.6x 0.6x 0.6x 2x 2x 2x 0.2x 0.2x 0.2x 0.9x 0.9x 0.9x 0.07x 0.07x* 0.07y

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Variable

Snare groups

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RENAL RESPONSES OF NORMAL AND PREASCITIC BROILERS TABLE 4. Linear regression equations, Pearson correlation coefficients (r), coefficients of determination (r2) and probability (P) values for relationships between the mean systemic arterial pressure during sample intervals 1 to 6 vs percentage saturation of hemoglobin with oxygen, glomerular titration rate, renal plasma flow, urine flow rate, absolute sodium excretion, or osmolar clearance in normal control and preascitic control groups that did not undergo unilateral pulmonary artery occlusion Linear regression equation

n

r

r2

P

SATO22 Normal Preascitic

MAP2 = 0.363 (SATO2) + 59.3 MAP = 0.187 (SATO2) + 66.1

54 66

0.283 0.151

0.0803 0.0229

0.0378 0.2252

GFR2 Normal Preascitic

MAP = 1.560 (GFR) + 94.7 MAP = 7.830 (GFR) + 61.0

54 66

0.138 0.566

0.0190 0.3200

0.3207 0.0001

MAP = 0.043 (RPF) + 90.6 MAP = 0.220 (RPF) + 78.5

54 66

0.016 0.078

0.0002 0.0061

0.9080 0.5320

MAP = 136.50 (UFR) + 86.7 MAP = 320.90 (UFR) + 73.0

54 66

0.188 0.258

0.0354 0.0663

0.1732 0.0368

ENa2 Normal Preascitic

MAP = −1.25 (ENa) + 91.2 MAP = 13.3 (ENa) + 77.0

54 66

0.048 0.337

0.0023 0.1140

0.7316 0.0056

COSM2 Normal Preascitic

MAP = 160.40 (COSM) + 82.5 MAP = 331.40 (COSM) + 66.9

54 66

0.227 0.242

0.0517 0.0548

0.2541 0.1755

RPF2 Normal Preascitic UFR2 Normal Preascitic

1 Normal group = Birds with a right ventricular to total ventricular ratio (RV:TV) ≤0.275; preascitic group = birds with an RV:TV ≥0.275. 2 MAP = Mean systemic arterial blood pressure (mm Hg), SATO2 = percentage saturation of hemoglobin with oxygen (percentage), GFR = glomerular filtration rate (milliliters per kilogram per minute), ERPF = effective renal plasma flow (milliliters per kilogram per minute), UFR = urine flow rate (milliliters per kilogram per minute), ENa = absolute excretion of Na (micromoles per kilogram per minute), COSM = osmolal clearance rate (milliliters per kilogram per minute).

TABLE 5. Linear regression equations, Pearson correlation coefficients (r), coefficients of determination (r2), and probability (P) values for relationships between the mean systemic arterial pressure during sample intervals 1 to 6 vs percentagae saturation of hemoglobin with oxygen, glomerular filtration rate, renal plasma flow, urine flow rate, absolute sodium excretion, or osmolar clearance in normal snare and preascitic snare groups that did not undergo unilateral pulmonary artery occlusion Group1

Linear regression equation

n

r

r2

P

MAP2 = 1.11 (SATO2) + 4.76 MAP = 1.44 (SATO2) − 18.8

66 72

0.654 0.812

0.4160 0.6600

0.0001 0.0001

MAP = 13.2 (GFR) + 61.5 MAP = 16.5 (GFR) + 50.5

66 72

0.466 0.601

0.2170 0.3610

0.0001 0.0001

MAP = 2.3 (RPF) + 72.6 MAP = 4.2 (RPF) + 57.1

66 72

0.268 0.464

0.0720 0.2150

0.0293 0.0001

MAP = 307.60 (UFR) + 82.5 MAP = 1204.3 (UFR) + 54.8

66 72

0.170 0.614

0.0291 0.3770

0.1712 0.0001

MAP = 13.9 (ENa) + 84.0 MAP = 10.6 (ENa) + 73.9

66 72

0.331 0.283

0.1090 0.0804

0.0067 0.0158

MAP = 238.1 (COSM) + 81.8 MAP = 959.1 (COSM) + 53.3

66 72

0.284 0.606

0.0805 0.3670

0.0210 0.0001

2

SATO2 Normal Preascitic GFR2 Normal Preascitic RPF2 Normal Preascitic UFR2 Normal Preascitic ENa2 Normal Preascitic COSM2 Normal Preascitic

1 Normal group = Birds with a right ventricular to total ventricular ratio (RV:TV) ≤ 0.275; preascitic group = birds with an RV:TV ≥ 0.275. 2 MAP = Mean systemic arterial blood pressure (mm Hg), SATO2 = percentage saturation of hemoglobin with oxygen (percentage), GFR = glomerular filtration rate (milliliters per kilogram per minute), ERPF = effective renal plasma flow (milliliters per kilogram per minute), UFT = urine flow rate (milliliters per kilogram per minute), ENa = absolute excretion of Na (micromoles per kilogram per minute), COSM = osmolal clearance rate (milliliters per kilogram per minute).

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Group1

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though linear regression analysis indicated the lowest systemic arterial pressures were associated with the lowest glomerular filtration rates in preascitic but not normal time control broilers. The degree of preexisting systemic hypotension in preascitic time control broilers evidently was sufficient to trigger an overall reduction in urine flow and to reduce the rate at which the kidneys cleared osmotically active solutes from the plasma. In concordance with the combined phenomena known as pressure natriuresis, linear regression analysis also demonstrated a positive relationship between systemic arterial pressure, urine flow rate, and sodium excretion in preascitic time control but not in normal time control broilers. These observations conform with the hypothesis that the kidneys of preascitic broilers responded to the progressive development of systemic hypotension by retaining fluid (lower urine flow rate) and osmotically active solute (lower osmolal clearance rate), which, in turn, would eventually contribute to blood volume expansion, central venous congestion, and ascitic fluid accumulation. The specific patterns of sodium and free water excretion observed in the time control groups were complex. Both the normal and preascitic broilers excreted urine with a comparatively low sodium concentration throughout the experiment; however, the urine sodium concentration was paradoxically higher in preascitic broilers than in normal broilers during the initial sample intervals (Table 2). In turn, the elevated urine sodium concentration accounted for the higher initial absolute rate of sodium excretion (Figure 9) despite a lower urine flow rate (Figure 6) in preascitic broilers when compared with normal broilers. These differences in urinary sodium content and excretion rates cannot be attributed to contemporaneous differences in the rate at which sodium entered the tubules by glomerular filtration (filtered load) or to the fraction of the filtered sodium that was excreted in the urine (Table 2). The concentration of sodium in the urine of preascitic control broilers constituted a minor fraction (5.4%) of the total urine osmolality (Table 2) and consequently did not substantially compromise the higher overall retention of osmotically active solute by preascitic broilers when compared with normal broilers during the initial sample intervals (Figure 7). No group differences were detected in the absolute urine osmolality (Table 2) or in the urine to plasma osmotic ratios (1.36 vs 1.37 for normal vs preascitic) during the initial sample intervals. However, small quantitative differences in free water clearance rates suggest the urinary concentrating mechanism was slightly less effective in preascitic broilers than in normal broilers. The less negative free water clearance rate did not substantially compromise the tendency for preascitic control broilers to maintain a lower urine flow rate at an equivalent glomerular filtration rate when compared with normal control broilers. Overall, the causes for these slight initial differences in sodium excretion and free water clearance are unknown, but they suggest the possibility that interactions among neural, myogenic, and hormonal mechanisms designed to maintain blood volume and pressure homeostasis may trigger paradoxical

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pressure, and a higher RV:TV ratio when compared with broilers selected for the normal group on the basis of a bright red comb. Surgical protocols for directly measuring the pulmonary arterial pressure and blood flow have been used previously (Wideman et al., 1997, 1998a), but were not attempted during the present study because of the complexity of preparing marginally viable preascitic broilers for urine sample collection and unilateral pulmonary artery occlusion. Nevertheless, the final distributions of RV:TV ratios demonstrate conclusively that broilers included in the normal category (RV:TV = 0.210 to 0.274) did not have pulmonary hypertension, whereas the higher RV:TV ratios for broilers included in the preascitic category reflect a preexisting pulmonary hypertension (Burton et al., 1968; Huchzermeyer and DeRuyck, 1986; Peacock et al., 1989, 1990; Wideman and Bottje, 1993; Owen et al., 1995; Wideman and Kirby, 1995a,b; Wideman et al., 1996a,b, 1998a,b,c). Elevated RV:TV ratios, pulmonary hypertension, hypoxemia, increased hematocrits, and systemic hypotension are concurrent symptoms associated with the pathophysiological progression leading to ascites (see Introduction), and combinations of these indices are highly predictive of individual birds that will progressively develop PHS (Roush et al., 1996, 1997; Kirby et al., 1997; Wideman et al., 1998c). Bradycardia was not characteristic of the preascitic birds in the present study. Presumably, reductions in heart rate occur later during the pathophysiological progression, after generalized cardiac dilation, and a proportionally longer ventricular filling cycle have developed (Wideman et al., 1998c; Kirby et al., 1999). Differences between normal and preascitic broilers in the present study cannot be directly attributed to differences in anesthesia, surgical preparation, or intravenous infusion, all of which were maintained as consistent as possible across the individual experiments. Of the preascitic broilers evaluated in this study, only those having the highest (most normal) initial values for blood oxygenation could survive inclusion in the snare group (see Materials and Methods). Comparisons of multiple renal function variables during the initial sample intervals for the preascitic snare and normal snare groups revealed differences solely in effective renal plasma flow. Preascitic broilers with the most pronounced initial systemic hypoxemia were unable to survive with one pulmonary artery occluded and were assigned to the preascitic time control group in which the snare was not tightened. For comparison, clinically healthy broilers also were assigned to a normal time control group without consideration for their initial blood oxygenation or systemic arterial pressure. Therefore, differences in initial renal function variables between the time control groups can be attributed, at least in part, to the amplified preexisting differences in blood oxygenation and, presumably, further progression by the preascitic broilers toward the onset of ascites. Accordingly, similar glomerular filtration rates for both time control groups throughout sample intervals 1 to 9 indicate the systemic arterial pressure of preascitic broilers had not consistently fallen below the range of glomerular filtration rate autoregulation, al-

RENAL RESPONSES OF NORMAL AND PREASCITIC BROILERS

the kidneys. The existing systemic arterial hypotension in the preascitic group might have caused a redistribution of renal arterial blood flow toward the medullary regions of the kidneys, partially sustaining the flow and pressure necessary for glomerular filtration, solute, and fluid reabsorption by the juxtamedullary (mammalian-type) nephrons, independent of a concurrent hypotensive reduction in filtration by intermediate- and superficial-cortical (reptilian-type) nephrons (Wideman, 1988). Direct reductions in renal arterial perfusion pressure, while systemic arterial pressure remained normal, previously did not affect total renal plasma flow because of compensatory increments in renal portal flow (Wideman and Gregg, 1988; Vena et al., 1990, Wideman, 1991, Glahn et al., 1993). However, the generalized systemic hypotension induced by unilateral pulmonary artery occlusion did reduce effective renal plasma flow modestly in both the normal and preascitic broilers in the present study. Presumably, the generalized decline in systemic arterial pressure reduced the blood flow to the lower extremities, thereby reducing the venous return available to support an additional inflow of renal portal blood. Closure of the renal portal valve following the decline in mean systemic arterial pressure would help to maintain renal blood flow, provide nutritional support for the kidney cells, and facilitate ongoing excretion of waste products such as uric acid (Wideman, 1988), but the renal portal valve generally is thought to remain closed in sedentary birds under normal conditions (Rennick and Gandia, 1954). The pathophysiology of PHS in growing broilers includes a) pulmonary hypertension as reflected by increased RV:TV ratios and specific electrocardiographic changes, b) systemic hypoxemia as reflected by a reduced percentage saturation of hemoglobin with oxygen and an increased hematocrit, and c) systemic hypotension attributed to a high pulmonary vascular resistance coupled with a low total peripheral resistance triggered by the onset of hypoxemia (see Introduction). In this experiment, broilers assigned to the preascitic groups were hypoxemic, had ventricular weight ratios ≥ 0.28 that were indicative of pulmonary hypertension, and the kidneys of those preascitic individuals with the most profound preexisting systemic hypotension retained osmotically active solute and fluid. When pulmonary vascular resistance was increased acutely by tightening a snare around one pulmonary artery in normal broilers with ventricular weight ratios ≤0.27, they also immediately became hypoxemic and hypotensive, and their kidneys retained solute and fluid. These experiments document the onset of solute and fluid retention accompanying the development of systemic hypotension early during the pathophysiological progression leading to PHS in broilers. The contribution of excessively retained solute and fluid to the subsequent evolution of right-sided congestive heart failure and ascites can be alleviated either by using pulmonary vasodilators to diminish the pressure that must be developed by the right ventricle or by reducing the renal responsiveness to systemic hypotension through the use of

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adjustments in sodium and free water retention as preascitic broilers progress toward right-sided cardiac congestion (Vena et al., 1990; Wideman et al., 1993). For example, right-sided congestion leading to increased filling (stretch) of the systemic veins and right atrium may reflexively reduce the circulating levels of arginine vasotocin, the avian antidiuretic hormone. Reduced levels of arginine vasotocin could, in turn, prevent the kidneys from recovering maximal amounts of solute-free water and may lead to a modest increase in sodium excretion. A similar mechanism may account for the reduction in urine osmolality and the increase in free water clearance after the pulmonary artery snare was tightened in normal and preascitic broilers (Robinzon et al., 1988; Bottje et al., 1989). To the extent that the differences in renal function between preascitic and normal broilers in the time control groups reflect the responses accompanying the progressive development of systemic hypotension, then qualitatively similar changes in renal function would be expected to develop immediately after acute systemic hypotension was induced in the normal and preascitic snare groups. Partially tightening the pulmonary artery snare reduced the systemic arterial pressure to a lower absolute level in preascitic broilers (59.8 ± 2.6 mm Hg) than did fully tightening the snare in normal broilers (74.2 ± 2.8 mm Hg). The magnitude of the induced systemic hypotension was accompanied by proportional contemporaneous reductions in the glomerular filtration rate, filtered load of sodium, urine flow rate, osmolal clearance rate, urine osmolality, and sodium excretion rate. Positive correlations between these renal function variables and systemic arterial pressure were confirmed further by linear regression analysis of sample intervals 1 to 6 for normal and preascitic broilers in the snare groups. The independent roles of pressure natriuresis and glomerular filtration rate autoregulation cannot be differentiated based on the responses to tightening the pulmonary artery snare because the systemic arterial pressure fell below a critical lower limit for glomerular filtration rate autoregulation in both groups. Pending direct comparisons, the lower limit for glomerular filtration rate autoregulation may be somewhat higher in broilers (approximately 75 mm Hg, present study) than the 60 mm Hg limit previously reported for male Single Comb White Leghorn domestic fowl (Wideman and Gregg, 1988; Vena et al., 1990; Wideman, 1991; Wideman et al., 1992, 1993; Glahn et al., 1993), and this difference would make broilers more susceptible than male Single Comb White Leghorn domestic fowl to fluid retention and ascites over an equilivant range of systemic hypotension. Overall, these data are consistent with the hypothesis that the systemic hypotension developing during the pathophysiological progression leading to PHS can cause the kidneys of susceptible broilers to retain solute and fluid, thereby accelerating the onset of central venous congestion and ascites. Systemic hypotension produced specific changes in effective renal blood flow that may reflect overall autoregulatory responses by the arterial and portal vasculature of

1783

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a diuretic such as furosemide (Wideman et al., 1995a,b, 1996a).

ACKNOWLEDGMENTS This research was supported by BARD Project Number US-2736. We acknowledge the assistance of Dr. Ronald W. McNew of the Agriculture Statistics Laboratory, University of Arkansas, Fayetteville, AR 72701 in the analysis of the data.

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