Alteration of lung atrial natriuretic peptide receptors in genetic cardiomyopathy.

These studies were designed to characterize the atrial natriuretic peptide (ANF) receptor subtypes [guanylyl cyclase natriuretic peptide receptors (NPR-A, NPR-B) and NPR-C] in lungs of normal hamsters and to evaluate alterations in receptor kinetics in genetic cardiomyopathy (CMO), a model of human congestive heart failure. Lung membranes were obtained from normal and CMO 200-to 230-day-old hamsters. Cross-linking and competitive binding receptor assays using 125I-labeled human ANF showed that lung membranes exhibit NPR, mainly guanylyl cyclase NPR-A and clearance NPR-C receptors. Stimulation of guanylyl cyclase by ANF and C-type natriuretic peptide (CNP) confirmed the presence of NPR-A and NPR-B. The maximum binding capacity of total ANF binding sites (442 +/- 68 vs. 271 +/- 57 fmol/mg protein, P < 0.05) was reduced, but dissociation constant (0.26 +/- 0.04 vs. 0.41 +/- 0.08 nM) was not altered in CMO animals. Similar reductions were observed in the binding sites for brain natriuretic peptide (BNP; 438 +/- 83 vs. 236 +/- 53 fmol/mg protein) and CNP (321 +/- 80 vs. 165 +/- 56 fmol/mg protein, P < 0.05) which may reflect a decline in NPR-A and NPR-B and/or NPR-C. Acid wash improved binding of 125I-labeled rat ANF to lung membranes of both normal and CMO hamsters, but the tendency towards reduced binding in CMO hamsters did not reach statistical significance, implying that downregulation may not have been due only to prior occupancy of the receptors. Transcripts of NPR-A, NPR-B, and NPR-C receptors in hamster lungs were detected by quantitative polymerase chain reaction. Compared with normal controls, the CMO hamster lung NPR-A mRNA was reduced by 50%, but NPR-B mRNA and NPR-C mRNA were not altered. Moreover, CMO hamster lungs showed less activation of guanylyl cyclase by ANF. These studies demonstrate that lung NPR are downregulated in hamster CMO.

Enhanced natriuretic response to intrarenal infusion of atrial natriuretic factor during ovine pregnancy.

Volume-loading and intravenous infusion studies have suggested that the natriuretic response of atrial natriuretic factor (ANF) is blunted or unaltered during pregnancy, but these findings may be influenced by changes in peripheral metabolism or clearance. To better define the direct renal actions of ANF, three doses (0.3, 1.5, and 3.0 pmol.kg-1.min-1) of ANF were infused directly into the renal artery of nonpregnant (n = 7) and 115- to 135-day pregnant (n = 6) unilaterally nephrectomized conscious sheep. Each dose was administered on a separate day according to a protocol that consisted of two 20-min control periods, two 20-min ANF infusions, and three 30-min recovery periods. The levels of basal mean arterial pressure averaged 86 +/- 25 mmHg and were similar and constant throughout all protocols in nonpregnant and pregnant ewes. Glomerular filtration rate (GFR) and renal blood flow (RBF) were increased during pregnancy, but GFR, RBF, and fractional proximal tubular reabsorption were not significantly altered by infusion of ANF. Nonpregnant sheep exhibited a selective renal response that was limited to an increase in urinary sodium excretion (UNaV). Pregnant sheep exhibited a selective renal response that included increases in UNaV and urine flow rate produced by suppression of distal sodium and, presumably, fluid reabsorption. The natriuretic and diuretic response of pregnant sheep was much greater than that of nonpregnant sheep despite a smaller proportional increment in intrarenal ANF concentration. These data suggest that the ovine renal response to ANF is limited to actions on the distal tubule that are enhanced during pregnancy.

Renal atrial natriuretic factor receptors in hamster cardiomyopathy.

Hamsters with cardiomyopathy (CMO), an experimental model of congestive heart failure, display stimulated renin-angiotensin-aldosterone and enhanced sympathetic nervous activity, all factors that lead to sodium retention, volume expansion and subsequent elevation of plasma atrial natriuretic factor (ANF) by the cardiac atria. However, sodium and water retention persist in CMO, indicating hyporesponsiveness to endogenous ANF. These studies were undertaken to fully characterize renal ANF receptor subtypes in normal hamsters and to evaluate whether alterations in renal ANF receptors may contribute to renal resistance to ANF in cardiomyopathy. Transcripts of the guanylyl cyclase-A (GC-A) and guanylyl cyclase B (GC-B) receptors were detected by quantitative polymerase chain reaction (PCR) in renal cortex, and outer and inner medullas. Compared to normal controls, the cardiomyopathic hamster's GC-A mRNA was similar in cortex but significantly increased in outer and inner medulla. Levels of GC-B mRNA were not altered by the disease. On the other hand, competitive binding studies, autoradiography, and affinity cross-linking demonstrated the absence of functional GC-B receptors in the kidney glomeruli and inner medulla. Also, C-type natriuretic peptide (CNP), the natural ligand for the GC-B receptors, failed to stimulate glomerular production of its second messenger cGMP. In CMO, sodium and water excretion were significantly reduced despite elevated plasma ANF (50.5 +/- 11.1 vs. 309.4 +/- 32.6 pg/ml, P < 0.001). Competitive binding studies of renal glomerular ANF receptors revealed no change in total receptor density, Bmax (369.6 +/- 27.4 vs. 282.8 +/- 26.2 fmol/mg protein), nor in dissociation constant, Kd (647.4 +/- 79.4 vs. 648.5 +/- 22.9 pM). Also, ANF-C receptor density (254.3 +/- 24.8 vs. 233.8 +/- 23.5 fmol/mg protein), nor affinity were affected by heart failure. Inner medullary receptors were exclusively of the GC-A subtype with Bmax (153.2 +/- 26.4 vs. 134.5 +/- 21.2 fmol/mg protein) and Kd (395.7 +/- 148.0 vs. 285.8 +/- 45.0 pM) not altered by cardiomyopathy. The increase in ANF-stimulated glomerular cGMP production was similar in normal and CMO hamsters (94- vs. 75-fold). These results demonstrate that renal ANF receptors do not contribute to the attenuated renal responses to ANF in hamster cardiomyopathy.

Regulation of renal atrial natriuretic peptide receptors in pregnant sheep.

These studies were designed to characterize the atrial natriuretic peptide (ANF) receptor subtypes (guanylyl cyclase GC-A and GC-B and ANF-C) in normal sheep kidneys and to evaluate alterations in receptor kinetics during pregnancy. Kidneys were obtained from 12 nonpregnant and 12 pregnant sheep during late gestation and maintained on a 100 mmol/day salt intake. Competition binding receptor assays using [125I]human ANF showed that inner medullary membranes are exclusively of the GC-A subtype. The maximum binding capacity (Bmax, 109 +/- 12 vs. 89 +/- 18 fmol/mg protein) and dissociation constant (Kd, 240 +/- 70 vs. 324 +/- 99 pM) are not altered by pregnancy. Specific binding of glomerular membranes to [125I]Tyr-C-type natriuretic peptide, which shows the highest affinity toward GC-B receptors, was observed, but this binding was abolished when ANF-C receptors were saturated with excess C-ANF-(101-121), suggesting that [125I]Tyr-C-type natriuretic peptide binding was mediated by ANF-C receptors. Binding of [125I]human ANF to glomerular membranes revealed that glomerular ANF receptor number was reduced during pregnancy (1040 +/- 212 vs. 335 +/- 42 fmol/mg protein; P = 0.001), but binding affinity was not changed. The reduced number was mainly due to a decrease in ANF-C receptor density (832 +/- 213 vs. 260 +/- 31 fmol/mg protein; P = 0.005). Autoradiography of whole kidney frozen sections produced similar findings. These studies demonstrate that GC-B receptors are absent from renal glomeruli and inner medulla, and that ANF receptor subtypes are differentially regulated in the pregnant sheep kidney, suggesting a role for ANF in the altered volume and pressure homeostasis of pregnancy.

Reversible tracheal obstruction in the fetal sheep: effects on tracheal fluid pressure and lung growth.

Congenital diaphragmatic hernia (CDH) continues to carry high morbidity and mortality. A number of treatment modalities including extracorporeal membrane oxygenation and in utero repair have improved the mortality rate only minimally. With this condition, there is often insufficient lung mass at birth and persistent pulmonary hypertension postnatally. Experimental fetal tracheal ligation has been shown to increase lung growth in utero and to reduce the hernial contents in CDH. The purpose of this study was to determine the effect of reversible fetal tracheal occlusion on tracheal pressure and lung development. Nine fetal sheep were divided into two groups. Group 1 had intratracheal balloons placed, and the balloons were left inflated for 21 to 28 days. Group 2 consisted of littermates that served as controls. They either had uninflated balloons placed or were left unoperated. Tracheal pressure measurements were recorded periodically, and the amniotic fluid pressure served as a reference. The animals were killed near term, and the lungs, heart, and liver were weighed and corrected for body weight. Standard morphometry was used to compare the lungs further, and the lung DNA and protein content were measured. Tracheal damage from the balloon catheter also was assessed. The tracheal pressure was 3.85 (+/- .49 SE) mm Hg in experimental animals, and it averaged -0.27 (+/- .27 SE) mm Hg in controls (P < .0001). Tracheal occlusion increased lung weight and volume by two to three times (P < .0001 and P < .0006, respectively) while heart and liver weights remained similar to those of controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of atrial natriuretic factor by right and left atrial distension in pregnant sheep.

Previous studies of the atrial stretch-atrial natriuretic factor (ANF) relationship during pregnancy have employed volume expansion and measured only right atrial pressure (RAP). Consequently, we studied nonpregnant (n = 7) and 115- to 125-day pregnant (n = 7) sheep and assessed the ANF response to changes of RAP and left atrial pressure (LAP) induced by graded balloon inflation. Ewes prepared with vascular catheters and atrial balloons were studied after recovery from preparatory surgical procedures. The basal levels of mean arterial pressure (MAP, 83 +/- 3 mmHg), RAP (2.1 +/- 0.7 mmHg), LAP (4.7 +/- 0.9 mmHg), and heart rate (HR, 102 +/- 6 beats/min) were similar in nonpregnant and pregnant sheep. Pregnancy also resulted in elevation of ANF concentration from 25 +/- 6 to 57 +/- 4 fmol/ml. With right atrial distension, the RAP-ANF relationships were similar in both nonpregnant and pregnant sheep, with a 10-mmHg increase in RAP increasing ANF by an average of 95 +/- 9 fmol/ml. In nonpregnant sheep, the LAP-ANF relationship was more responsive than RAP-ANF because a 10-mmHg increase in LAP resulted in a 193 +/- 10 fmol/ml increase in ANF. Moreover, during pregnancy, the LAP-ANF relationship was significantly more sensitive because a 10-mmHg increase in LAP resulted in a 433 +/- 15 fmol/ml elevation of ANF. These data demonstrate that plasma ANF levels are more responsive to distension of the left atria than to the right. More importantly, the ANF response to left, but not right, atrial distension is enhanced by pregnancy.

Effects of angiotensin II on plasma atrial natriuretic factor in nonpregnant and pregnant ewes.

The release of atrial natriuretic factor (ANF) is primarily determined by atrial stretch, but may also be modulated by circulating angiotensin II (AngII). During pregnancy, the circulating concentrations of both ANF and AngII are increased. To further define possible effects of AngII on ANF release, four doses of AngII (0.5, 5, 20, 40 ng.kg-1.min-1) were intravenously infused into five nonpregnant and five pregnant (105-140 days of gestation) ewes alone and during the simultaneous infusion of sodium nitroprusside at doses sufficient to abolish the pressor effects of AngII. Mean arterial pressure (MAP) was increased from 80 +/- 2 to a maximum of 121 +/- 5 mmHg (1 mmHg = 133.3 Pa) in nonpregnant ewes (p < 0.01) and from 79 +/- 2 to 116 +/- 4 mmHg in pregnant ewes (p < 0.01) over the range of AngII infusion. MAP was unaltered during AngII plus nitroprusside infusion, averaging 78 +/- 3 mmHg in nonpregnant ewes and 80 +/- 2 mmHg in pregnant ewes. Basal ANF was higher (p < 0.01) in pregnant sheep than in nonpregnant sheep. With AngII infusion alone, plasma ANF was increased from 13 +/- 2 to 42 +/- 4 fmol/microL in nonpregnant ewes (p < 0.01) and from 23 +/- 5 to 72 +/- 16 fmol/microL in pregnant ewes (p < 0.01). However, during AngII plus nitroprusside infusion, the increases in plasma ANF observed were completely abolished in both nonpregnant and pregnant ewes.(ABSTRACT TRUNCATED AT 250 WORDS)

Intravertebral ANG II effects on plasma renin and Na excretion in dogs at constant renal artery pressure.

Studies were performed to determine whether intravertebral angiotensin II infusion (iva ANG II) decreases renin release by increasing renal perfusion pressure (RPP) and to investigate possible effects of iva ANG II on renal function. RPP was electronically servocontrolled in 12 conscious dogs equipped with chronic vascular catheters and a suprarenal aortic balloon constrictor while iva ANG II was infused bilaterally for 60 min at 0.33 ng.kg-1.min-1. Without servocontrol, iva ANG II increased mean arterial pressure (MAP) from 101 +/- 4 to 106 +/- 5 mmHg, urine flow (V) from 0.36 +/- 0.03 to 0.45 +/- 0.04 ml/min, and sodium excretion (UNaV) from 36.2 +/- 7.0 to 62.7 +/- 6.6 mumol/min. Plasma renin activity (PRA) decreased from 6.9 +/- 0.7 to 5.0 +/- 0.6 ng ANG I.ml-1.3 h-1. With servocontrol, iva ANG II increased MAP from 102 +/- 4 to 109 +/- 5 mmHg while RPP remained constant with a variation of less than +/- 1 mmHg. PRA did not change significantly (5.9 +/- 0.3 to 7.0 +/- 0.7 ng ANG I.ml-1.3 h-1). V decreased from 0.33 +/- 0.02 to 0.26 +/- 0.01 ml/min, and UNaV decreased from 49.0 +/- 5.7 to 29.7 +/- 4.4 mumol/min. The data provide evidence that iva ANG II decreases renin release by increasing RPP and stimulating the renal baroreceptor and/or the macula densa mechanisms. In addition, at constant RPP, ANG II exerts a central action to decrease UNaV.

Metabolic clearance of atrial natriuretic factor in ovine pregnancy.

Plasma atrial natriuretic factor (ANF) is normally released into the circulation primarily by volume expansion and atrial distension, but we have shown that plasma ANF is elevated in pregnant sheep before volume expansion. Because alterations in the metabolic clearance of ANF could lead to elevated plasma ANF levels, the present study was designed to determine the pharmacokinetics of plasma ANF in pregnant sheep. Chronically instrumented nonpregnant and pregnant sheep received intravenous injections of monoiodinated human ANF (125I-hANF). Plasma decay curves of 125I-hANF followed a biexponential function in both groups. High-performance liquid chromatography (HPLC) revealed the accumulation of smaller degradation products by 2 min postinjection, and by 30 min no intact ANF was present. Because HPLC identification of ANF and its metabolites was shown to be more efficient than precipitation with 10% trichloroacetic acid (TCA) or extraction by Sep-Pak cartridges, ANF kinetic parameters were calculated from HPLC-corrected plasma decay curves. Injected ANF was rapidly distributed in an initial distribution volume (IDV) that was expanded in pregnant sheep. Metabolic clearance rate (MCR) was greater in pregnant sheep (2.8 +/- 0.3 vs. 6.8 +/- 1.2 l/min, P = 0.002), while plasma half-life (t1/2) was not altered (2.2 +/- 0.5 vs. 2.4 +/- 0.4 min). The data demonstrate that during pregnancy, the t1/2 of ANF is not altered but the MCR of ANF is enhanced. These findings imply that plasma ANF is increased by mechanisms other than reduced clearance in pregnant sheep.

Effects of prostaglandin inhibition on the renal function curve during ovine pregnancy.

OBJECTIVE: This study was designed to assess the influence of prostaglandins on the steady-state relationship of mean arterial pressure and urinary sodium excretion. STUDY DESIGN: Ten nonpregnant and nine pregnant ewes were chronically prepared with indwelling catheters and maintained in metabolism cages. All the ewes received a continuous intravenous infusion of indomethacin (5 mg/kg/day) for 24 days. Step increases in sodium intake (20, 100, 400, and 1200 mmol per day), each maintained for 6 days, were administered as a fixed dietary (20 mmol) component with the balance infused intravenously. Mean arterial pressure was continuously monitored. The renal function curves were constructed from the steady-state mean arterial pressure-urinary sodium excretion relationships after sodium balance was attained. RESULTS: Indomethacin inhibited urinary excretion of prostaglandin E2 by 62% +/- 4% in nonpregnant and 47% +/- 6% in pregnant ewes, and 6 keto-prostaglandin F1 alpha, the stable metabolite of prostaglandin I2, by 55% +/- 4% and 65% +/- 6% in nonpregnant and pregnant ewes, respectively. Blood volume was not affected by indomethacin or salt in either group. Plasma angiotensin II was suppressed less in pregnant ewes (60% vs 80%) by increases in salt intake. Chronic treatment with indomethacin did not alter the renal function curves of either nonpregnant or pregnant ewes. CONCLUSIONS: These results suggest that prostaglandins do not influence the steady-state relationship between mean arterial pressure and urinary sodium excretion in either nonpregnant or pregnant sheep.

Renal perfusion pressure and renin secretion in bilaterally renal denervated sheep.

To further investigate the influence of renal nerves on renin secretion, the renin secretion responses to step reductions of renal perfusion pressure (RPP) were studied in conscious sheep with innervated kidneys (n = 5) and with bilaterally denervated kidneys (n = 5). The average basal level of RPP in sheep with denervated kidneys (82 +/- 4 mmHg; 1 mmHg = 133.3 Pa) was similar to that in sheep with innervated kidneys (83 +/- 3 mmHg). RPP was reduced in four sequential 15-min steps, to a final level of 54 +/- 2 mmHg in sheep with innervated kidneys and to 57 +/- 1 mmHg in denervated sheep. The renin secretion rate was increased as RPP was reduced in sheep with innervated kidneys. Baseline peripheral plasma renin activity was reduced and there was almost no response of renin secretion rate to reduction of RPP in sheep with denervated kidneys. Also, baseline renal blood flow, urine flow rate, sodium excretion rate, and potassium excretion rate were higher in sheep with denervated kidneys than those with innervated kidneys. Baseline plasma angiotensin II was similar in both groups of sheep. As RPP was decreased, plasma angiotensin II was increased in sheep with innervated kidneys, but was not RPP or renal denervation. In conclusion, hormonal factors, such as angiotensin II and atrial natriuretic factor, do not account for the dramatic suppression of renin secretion in response to the reduction of RPP in sheep with bilateral renal denervation. Renal nerves are a necessary component in the control of renin secretion during reduction of RPP and may contribute to the regulation of baseline plasma renin activity and sodium excretion rate in conscious ewes.

Atrial natriuretic factor in ovine pregnancy: plasma levels, molecular forms and biological activity.

Pregnancy is associated with hypervolemia and elevated plasma ANF, but the time course over which ANF increases and the mechanisms that control plasma ANF levels are unclear. Plasma ANF was measured in 12 nonpregnant and 16 pregnant sheep at various gestational ages. ANF was elevated at 30-35 days of pregnancy (15.7 +/- 1.4 vs. 22.9 +/- 3.4 fmol/ml, P = 0.04), but did not increase further with the advance of gestation. Tissue content of ANF was unchanged in the right atrium, left atrium, renal cortex, renal medulla, adrenals and lungs, but ovarian ANF content was increased during pregnancy (9.2 +/- 2.2 vs. 67.2 +/- 23.2 fmol/mg protein, P = 0.003). However, the ovarian tissue ANF concentration was less than 0.2% of that in the atria during pregnancy. HPLC of plasma from both nonpregnant and pregnant ewes revealed the presence of a single peak that elutes in parallel with synthetic human ANF. HPLC of atrial and lung tissue homogenates revealed multiple peaks that may represent different molecular forms of ANF. The biological activity of ANF in the plasma of pregnant ewes was reduced to 23% of nonpregnant levels. ANF in lung tissue was also biologically active, but that activity was reduced to 13% of nonpregnant levels. These data suggest that elevated plasma ANF in pregnancy is not secondary to increased atrial, renal, adrenal, ovarian or pulmonary contribution. Since we have previously shown that the metabolic clearance of ANF is not decreased, other extra-atrial sites may contribute to the increased plasma ANF during pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)

Indirect hypertensive actions of long-term intracarotid angiotensin II infusion during ovine pregnancy.

Angiotensin II (AngII) influences the regulation of mean arterial pressure (MAP) through numerous mechanisms, including an action of circulating AngII on the brain to alter autonomic activity. We have obtained evidence that the relative importance of this effect is increased during pregnancy. Consequently, these studies were undertaken to assess the effects of bilateral infusion of AngII (0.35 ng.kg-1.min-1.artery-1) into the internal carotid arteries (ica) of sheep for 13 days. Six non-pregnant (NP) and six 105- to 125-day pregnant (PG) ewes were maintained in large metabolism cages, where MAP was continuously monitored. By day 10 of ica AngII infusion in NP ewes, MAP was increased from 83.9 +/- 1.6 to 92.9 +/- 2.8 mmHg (1 mmHg = 133.3 Pa) (p = 0.001). Twenty-four hour urine volume (UV, 2664 +/- 341 to 1583 +/- 228 mL; p = 0.005) and sodium excretion (UNaV, 190 +/- 5 to 113 +/- 19 mmol/day; p = 0.005) were decreased. 51Cr-tagged blood volume (BV) was increased on day 13 (3643 +/- 187 to 4379 +/- 446 mL; p = 0.05). In contrast, by only day 6 of ica AngII infusion in PG ewes, MAP increased from 79.1 +/- 1.9 to 84.1 +/- 1.4 mmHg (p = 0.03) in association with a BV expansion from 3999 +/- 274 to 4207 +/- 275 mL. These changes were preceded by decreases in UV (2813 +/- 413 to 2198 +/- 362 mL; p = 0.01) and UNaV (190 +/- 15 to 118 +/- 26 mmol/day; p = 0.01). By day 13, MAP had plateaued at 93.0 +/- 1.2 mmHg.(ABSTRACT TRUNCATED AT 250 WORDS)

Influences of renal nerves and pregnancy on renin secretion in sheep.

To investigate the role of the renal nerves in the control of renin secretion during pregnancy, we prepared sheep with one denervated kidney, bilateral renal vein catheters, and an inflatable aortic occluder. The renin secretory responses to step reductions of renal perfusion pressure (RPP) of the innervated and denervated kidneys were compared in eight nonpregnant and nine pregnant (115-130 days gestation) sheep in a normal conscious state. Nonpregnant sheep had average basal levels of mean arterial pressure (MAP) of 91 +/- 3 mmHg, plasma renin activities (PRA) of 2.3 +/- 0.4 ng ANG I.ml-1.3 h-1, and plasma angiotensin (ANG) concentrations of 8.4 +/- 1.1 pg/ml. Pregnant sheep had reduced levels (P < 0.01) of MAP (76 +/- 3 mmHg) but elevated levels of PRA (3.8 +/- 0.4 ng ANG I.ml-1.h 3-1; P < 0.5) and ANG II (12.0 +/- 1.6 pg/ml; P = 0.08). Four successively reduced levels of RPP were observed for 15 min each in both groups of sheep. In both groups, RPP was reduced to a final level of 51 +/- 2 mmHg. The renal venous-arterial difference of PRA (v-aPRA) from the innervated kidneys was increased as RPP was reduced. However, there was little response of v-aPRA to RPP in denervated kidneys. In nonpregnant sheep, the relationship between RPP and renin secretion rate was characterized by an initially slow phase, over which small reductions of RPP resulted in small increases in renin secretion. As RPP fell, this relationship became progressively steeper in nonpregnant ewes.(ABSTRACT TRUNCATED AT 250 WORDS)

Tubular handling of fluid and electrolytes during ovine pregnancy.

Pregnancy is characterized by progressive water and sodium accumulation and increases in renal blood flow (RBF) and glomerular filtration rate (GFR). However, the influence of the different nephron segments on the increased tubular reabsorption is controversial. Consequently, four nonpregnant and five pregnant sheep were studied, after chronic instrumentation, to assess salt and water reabsorption in the proximal and distal tubules under basal and volume-loaded conditions. Lithium clearance was used as a marker for proximal tubular reabsorption. Volume loading was achieved by the rapid administration of 1,000 ml isotonic saline followed by 250 ml/h for 2 h. Under basal conditions with reference to the nonpregnant state, pregnant sheep had higher (P < 0.05) levels of right RBF (427 +/- 34 vs. 313 +/- 8 ml/min), GFR (133 +/- 7 vs. 94 +/- 9 ml/min), proximal tubular reabsorption (102 +/- 7 vs. 73 +/- 6 ml/min), distal nephron fluid delivery (31 +/- 2 vs. 20 +/- 2 ml/min), and fractional distal nephron reabsorption of fluid (92 +/- 2 vs. 87 +/- 1%) and sodium (98.8 +/- 0.3 vs. 97.0 +/- 0.7%). However, pregnant animals had significantly (P < 0.05) reduced fractional excretions of fluid (1.6 +/- 0.3 vs. 2.6 +/- 0.2%) and sodium (0.24 +/- 0.06 vs. 0.63 +/- 0.19%), but similar levels of filtration fraction, fractional proximal tubular reabsorption, urine flow, urinary sodium excretion, and osmolar and free water clearance. After saline loading, pregnant sheep excreted significantly (P < 0.05) less sodium (9.2 vs. 28.6%) and water (39.8 vs. 56.5%). Decreases in fractional proximal and distal nephron reabsorption of sodium and water after saline loading were attenuated in pregnant animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Steady-state arterial pressure-urinary output relationships during ovine pregnancy.

To determine the effects of long-term changes in sodium intake on mean arterial pressure (MAP) regulation during pregnancy, nonpregnant (n = 16) and 110- to 140-day pregnant (n = 13) ewes received total daily sodium intakes of 10, 30, 100, 400, and 1,200 mmol for 7 days. The sheep were housed in metabolism cages and MAP was monitored 24 h/day. Urinary sodium excretion (UNaV) followed changes in sodium intake, with steady-state levels being achieved with similar degrees of rapidity (2-3 days) in nonpregnant and pregnant sheep. At 10 mmol/day sodium intake, MAP was lower (79 +/- 1 vs. 82 +/- 2 mmHg; P < 0.01) and water intake (2,275 +/- 494 vs. 3,286 +/- 725 ml/day; P < 0.001) and 24-h urine volume (1,454 +/- 279 vs. 2,299 +/- 496 ml/day; P < 0.01) were greater in pregnant sheep. All of these variables exhibited direct relationships with increases in sodium intake. Plasma angiotensin II (pANG II) was increased in pregnancy (10.6 +/- 1.6 vs. 24.5 +/- 6.3 pg/ml; P < 0.001) at 10 mmol/day. Elevation of sodium intake suppressed pANG II to minimal levels in nonpregnant sheep, but to only 25% of the control level in pregnant sheep. During pregnancy, the renal function curve representing the steady-state MAP-UNaV relationship was shifted to lower MAP setpoint, but the sodium sensitivity of MAP was unchanged. Also, the inverse relationship of sodium intake and pANG II was blunted, suggesting a reduced role for ANG II in the maintenance of renal function during pregnancy.

Influence of pregnancy on mean systemic filling pressure and the cardiac function curve in guinea pigs.

To assess the degree of circulatory fullness and to evaluate the influence of peripheral and cardiac factors in the regulation of cardiac output during pregnancy, the following studies were conducted using pentobarbital-anesthetized, open-chest nonpregnant and late term pregnant guinea pigs. Mean circulatory filling pressure was taken as the equilibrium pressure when the pulmonary artery was constricted. Total vascular compliance was assessed by +/- 5-mL changes in blood volume performed while this constriction was maintained. A separate group of guinea pigs was prepared with a pulmonary artery electromagnetic flow probe and right atrial catheter. Rapid infusion of saline was used to increase right atrial pressure while the cardiac output was determined. Pregnancy was characterized by the following changes relative to nonpregnant controls: 51Cr-labelled RBC blood volume increased from 55 +/- 3 to 67 +/- 3 mL/kg; mean circulatory filling pressure increased from 7.1 +/- 0.2 to 8.0 +/- 0.5 mmHg (1 mmHg = 133.322 Pa); right atrial pressure decreased from 3.4 +/- 0.2 to 2.1 +/- 0.3 mmHg; and cardiac output increased from 71.8 +/- 3.9 to 96.8 +/- 3.3 mL.min-1.kg-1. Total vascular compliance was not changed (2.1 +/- 0.1 mL.kg-1.mmHg-1) and most of the expanded blood volume was accommodated as unstressed volume. The cardiac function curve was shifted upwards in pregnant animals. The resistance to venous return, as determined from the slope of the venous return curves, was not changed. These data suggest that the circulation of the pregnant guinea pig is slightly overfilled.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal nerve effects on renal adaptation to changes in sodium intake during ovine pregnancy.

To assess the possibility of an enhanced role of renal nerves in the control of urinary sodium excretion (UNaV) and fluid homeostasis during pregnancy, urine output, UNaV, and urinary potassium excretion were assessed hourly for 3 days before and for 6 days after a step reduction in total daily sodium intake from 400 to 40 mmol. Studies were performed in normal conscious sheep (4 nonpregnant and 4 pregnant). Each animal was prepared with a divided bladder so that urine could be collected simultaneously from one normally innervated and one denervated kidney. In nonpregnant ewes, ratios of the rates of excretion by denervated vs. innervated kidneys for UNaV averaged 1.00 +/- 0.07 under steady-state conditions at high levels of sodium intake. This ratio was not different at the low-sodium-intake state. In contrast, this ratio was 1.15 +/- 0.07 at high sodium intake and 1.13 +/- 0.03 at low sodium intake in pregnant ewes. The ratios at both steady-state intake levels were different (P less than 0.05) between nonpregnant and pregnant sheep. During the transition between sodium intake states, these ratios were unchanged in nonpregnant animals, whereas pregnant animals exhibited peak ratios of 2.20 +/- 0.39 (P less than 0.05), indicating sodium wasting by the denervated kidneys. In summary, the data suggest that renal nerve activity may not be completely suppressed by high sodium intakes in pregnant sheep. Furthermore, the renal nerves have an enhanced influence on sodium conservation during and after the transition from high- to low-sodium-intake states during pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)

Circulating hyaluronic acid in nonpregnant, pregnant, and postpartum guinea pigs: elevated levels observed at parturition.

OBJECTIVE: Dilatation of the uterine cervix at parturition is associated with an increase in cervical hyaluronic acid content. The objective is to test the hypothesis that circulating hyaluronic acid is increased at parturition. STUDY DESIGN: Serum hyaluronic acid levels from nonpregnant (n = 5), pregnant (n = 13), and postpartum (n = 4) adult Hartley guinea pigs were determined with a radiometric assay that utilizes iodine 125-labeled hyaluronic acid-binding protein. Results were analyzed for statistical significance with Student's paired t test and regression analysis. RESULTS: The serum hyaluronic acid level in nonpregnant animals was 238 +/- 88 ng/ml (mean +/- SEM). During pregnancy, serum hyaluronic acid levels were 127 +/- 12 and 126 +/- 34 ng/ml at 25 and 50 to 63 days' gestation, respectively. At parturition, hyaluronic acid levels increased fivefold to 765 +/- 111 ng/ml (p less than 0.001). Hyaluronic acid levels returned to antepartum values 2 days post partum (153 +/- 27 ng/ml). There was no significant difference between arterial and venous levels. CONCLUSION: Circulating hyaluronic acid levels increase significantly at parturition in the guinea pig.

Effect of baroreceptor denervation on stimulation of drinking by angiotensin II in conscious dogs.

Angiotensin II causes marked stimulation of drinking when it is injected centrally but is a relatively weak dipsogen when administered intravenously. However, it has been proposed that the dipsogenic action of systemically administered angiotensin II may be counteracted by the pressor action of the peptide. To test this hypothesis, the dipsogenic action of angiotensin II was investigated in dogs, in which low and high baroreceptor influences had been eliminated by denervation of the carotid sinus, aortic arch, and heart. In five sham-operated dogs, infusion of angiotensin II at 10 and 20 ng.kg-1.min-1 increased plasma angiotensin II concentration to 109.2 +/- 6.9 and 219.2 +/- 38.5 pg/ml and mean arterial pressure by 20 and 29 mmHg, respectively, but did not induce drinking. In four baroreceptor-denervated dogs, the angiotensin II infusions produced similar increases in plasma angiotensin II concentration and mean arterial pressure but, in contrast to the results in the sham-operated dogs, produced a dose-related stimulation of drinking. Water intake with the low and high doses of angiotensin II was 111 +/- 44 and 255 +/- 36 ml, respectively. The drinking responses to an increase in plasma osmolality produced by infusion of hypertonic sodium chloride were not different in the sham-operated and baroreceptor-denervated dogs. These results demonstrate that baroreceptor denervation increases the dipsogenic potency of intravenous angiotensin II and provides further support for the hypothesis that the dipsogenic action of intravenous angiotensin II is counteracted by the rise in blood pressure.