Effects of pregnancy on inulin and para-aminohippurate clearances in the anaesthetized rat

The role of angiotensin II and relaxin in vascular adaptation to pregnancy

In brief

There is a pregnancy-induced vasodilation of blood vessels, which is known to have a protective effect on cardiovascular function and can be maintained postpartum. This review outlines the cardiovascular changes that occur in a healthy human and rodent pregnancy, as well as different pathways that are activated by angiotensin II and relaxin that result in blood vessel dilation.

Abstract

During pregnancy, systemic and uteroplacental blood flow increase to ensure an adequate blood supply that carries oxygen and nutrients from the mother to the fetus. This results in changes to the function of the maternal cardiovascular system. There is also a pregnancy-induced vasodilation of blood vessels, which is known to have a protective effect on cardiovascular health/function. Additionally, there is evidence that the effects of maternal vascular vasodilation are maintained post-partum, which may reduce the risk of developing high blood pressure in the next pregnancy and reduce cardiovascular risk later in life. At both non-pregnant and pregnant stages, vascular endothelial cells produce a number of vasodilators and vasoconstrictors, which transduce signals to the contractile vascular smooth muscle cells to control the dilation and constriction of blood vessels. These vascular cells are also targets of other vasoactive factors, including angiotensin II (Ang II) and relaxin. The binding of Ang II to its receptors activates different pathways to regulate the blood vessel vasoconstriction/vasodilation, and relaxin can interact with some of these pathways to induce vasodilation. Based on the available literature, this review outlines the cardiovascular changes that occur in a healthy human pregnancy, supplemented by studies in rodents. A specific focus is placed on vasodilation of blood vessels during pregnancy; the role of endothelial cells and endothelium-derived vasodilators will also be discussed. Additionally, different pathways that are activated by Ang II and relaxin that result in blood vessel dilation will also be reviewed.

Renal tubular function in the gravid rat

Pregnancy in the rat is accompanied by enhanced reabsorption of salt and water throughout most, if not all, of the gestational period. Many mechanisms have been suggested but definitive answers are still awaited. The major area of controversy centres around the detection of changes at term. There is general agreement that, at least in mid-gestation, the increase in reabsorption can be attributed to increases in the proximal tubules, the loop of Henle and the collecting duct. The contribution of the proximal tubule to the increased reabsorption at term is still uncertain. Enhanced salt and water reabsorption is demonstrated in distal nephron segments irrespective of the stage of gestation. Micropuncture and microperfusion experiments have identified increased reabsorption of water, sodium and chloride in the loop of Henle, but it appears that there is net addition of glucose, urea and potassium to the tubular fluid in this segment which, at least for potassium and glucose, offsets to some extent increased reabsorption by the proximal tubule. Altered renal handling of other solutes (uric acid, calcium and magnesium) also occurs throughout pregnancy but the mechanisms responsible and nephron sites involved remain to be investigated. Attempts to attribute altered reabsorption to direct renal effects of changes in maternal hormones are inconclusive. Prolactin mimics some of the pregnancy-associated increases in reabsorption following chronic administration to male and non-pregnant female rats. These effects might be due to a direct renal action of the hormone or even to the volume expansion following its dipsogenic action.

Glomerular filtration and volume regulation in gravid animal models

The gestational increase in glomerular filtration rate that occurs in the normal rat is exclusively the result of an increase in renal plasma flow and there is no sustained increase in glomerular capillary blood pressure during a normal pregnancy. The factor or factors that initiate the gestational renal vasodilatation (and plasma volume expansion) are maternal, not fetoplacental in origin. The precise nature of the initiating factors has not yet been defined, although it is unlikely that the gestational plasma volume expansion can be the sole cause of the increased glomerular filtration rate seen in pregnancy. A number of vasoactive hormones are activated in pregnancy but as yet no clear candidate has emerged as 'the renal vasodilator'. Preliminary evidence suggests that nitric oxide may play an important role in gestational vasodilatation. The normal kidney in pregnancy exhibits substantial renal reserve to amino acid infusion and unimpaired autoregulatory ability despite being already vasodilated by the gestational stimulus. There are marked and sometimes contradictory changes in the various volume sensing and control systems in pregnancy. In general, the sensors perceiving and controlling intravascular volume are reset during a normal pregnancy to enable to mother to accommodate the increased plasma volume without provoking a natriuretic response. Whether the expanded plasma volume of pregnancy is perceived as normal or underfilled is not clear at this time and may vary according to the volume regulatory system. Repetitive pregnancies do not have any cumulative, long-term deleterious effects on renal function, when the underlying function is normal, when it has been compromised by removal of renal mass or during chronic systemic hypertension in the spontaneously hypertensive rat. In the short term, pregnancy does not worsen kidney function when underlying glomerular damage is due to immune stimuli, ablation of renal mass or gentamicin, or in the spontaneously hypertensive rat. Therefore, the chronic renal vasodilatation of pregnancy does not appear to be a damaging entity, unlike other states of low preglomerular arteriolar resistance, studied in the male rat. When pregnancy is superimposed on Adriamycin nephrosis or chronic blockade of nitric oxide, hypertension occurs and renal function declines. In both situations endothelial damage/dysfunction occurs, as is also seen in pre-eclampsia. Further study of the effects of pregnancy in animal models of endothelial dysfunction will prove rewarding.

Renal tubular function in the gravid rat

Pregnancy in the rat is accompanied by enhanced reabsorption of salt and water throughout most, if not all, of the gestational period. Many mechanisms have been suggested but definitive answers are still awaited. The major area of controversy centres around the detection of changes at term. There is general agreement that, at least in mid-gestation, the increase in reabsorption can be attributed to increases in the proximal tubules, the loop of Henle and collecting duct. The contribution of the proximal tubule to the increased reabsorption at term is still uncertain. Enhanced salt and water reabsorption is demonstrated in distal nephron segments irrespective of the stage of gestation. Micropuncture and microperfusion experiments have identified increased reabsorption of water, sodium and chloride in the loop of Henle, but it appears that there is net addition of glucose, urea and potassium to the tubular fluid in this segment which, at least for potassium and glucose, offsets to some extent increased reabsorption by the proximal tubule. Altered renal handling of other solutes (uric acid, calcium and magnesium) also occurs throughout pregnancy but the mechanisms responsible and nephron sites involved remain to be investigated. Attempts to attribute altered reabsorption to direct renal effects of changes in maternal hormones are inconclusive. Prolactin mimics some of the pregnancy-associated increases in reabsorption following chronic administration to male and non-pregnant female rats. These effects might be due to a direct renal action of the hormone or even to the volume expansion following its dipsogenic action.

Glomerular filtration and volume regulation in gravid animal models

The gestational increase in glomerular filtration rate (GFR) that occurs in the normal rat is the result exclusively of an increase in plasma flow rate, and there is no sustained increase in glomerular capillary blood pressure during a normal pregnancy. The factor or factors that initiate the gestational renal vasodilatation (and plasma volume expansion) are maternal, not fetoplacental in origin. Apart from ruling out prostaglandins as an initiating agent, animal studies have not yet defined the precise nature of the initiating factors; it is unlikely that the gestational plasma volume expansion can be the sole cause of the increased GFR seen in pregnancy. The normal kidney in pregnancy exhibits substantial renal reserve to amino acid infusion, despite being already vasodilated by the gestational stimulus. The renal volume-sensing and control system of tubuloglomerular feedback is fully operative in pregnancy, and appears to be 'reset' to perceive the expanded plasma volume of pregnancy as normal. This observation agrees with many other indications that the sensors perceiving and controlling intravascular volume are reset during a normal pregnancy to enable the mother to accommodate the increased plasma volume without provoking a natriuretic response. Multiple pregnancies do not have any cumulative, long-term deleterious effects on renal function, either when the underlying function is normal or when it has been compromised by removal of renal mass plus high-protein feeding. In the short-term, pregnancy does not worsen kidney function when underlying glomerulonephritis is present. Therefore, the hyperfiltration of pregnancy does not appear to be a damaging entity, unlike other hyperfiltration states studied in the male rat. Still unknown is the mechanism by which pregnancy does worsen underlying glomerular disease in some women. The preliminary data in the rat, presented above, suggest that the exacerbating influence may be something other than the glomerular haemodynamic changes of pregnancy.

Renal hemodynamics during pregnancy in chronically catheterized, conscious rats

Two types of experiments were performed, cross-sectional and longitudinal. In the cross-sectional studies, rats were mated, later prepared surgically, and then 5 or more days after surgery, each examined twice during days 11 to 15 or days 18 to 20 of gestation. Nonpregnant rats matched for age and prepregnant weight served as controls. In the longitudinal studies, rats were catheterized and, starting 6 days later, examined twice; then the same rats were mated and each was studied on days 5, 8, 12, 16, and 20 of gestation, as well as on day 5 postpartum. In the cross-sectional studies, glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were elevated by approximately 26% and 20%, respectively, above nonpregnant controls at 11 to 15 days of gestation (GFR, 2739 +/- 94 vs. 2181 +/- 134 microliters . min-1, P less than 0.005; ERPF, 9367 +/- 295 vs. 7785 +/- 422 microliters . min-1, P less than 0.01). By 18 to 20 days of gestation, GFR and ERPF had returned to levels that were not significantly different from nonpregnant values. The longitudinal studies confirmed these findings in every respect and further revealed that GFR and ERPF were elevated above nonpregnant values as early as day 5 of gestation (P less than 0.005). Thereafter, they rose to peak values, at 12 and 16 days of gestation, of 3122 +/- 144 and 10,584 +/- 541 microliters . min-1, and then returned to nonpregnant levels by day 20 of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal function during lactation in the rat

1. Glomerular filtration rate (g.f.r.), renal plasma flow (r.p.f.), salt and water reabsorption and plasma concentrations of progresterone and prolactin were measured in virgins, 19 day pregnant and lactating rats. 2. G.f.r. was raised by about 40% in animals at 6-7 days of lactation when compared with virgins but towards the end of lactation returned to virgin levels. Salt and water reabsorption followed a similar pattern. 3. At the time of maximum change of g.f.r. during lactation r.p.f. was raised by about 30% and so filtration fraction rose by about 10% when compared with virgins. 4. Plasma prolactin and progesterone concentrations rose early in lactation but remained elevated when the renal changes had returned to normal. 5. Factors other than changes in prolactin and progesterone concentrations must be implicated in the changes in renal function that occur during lactation.

Effects of pregnancy on glomerular dynamics: micropuncture study in the rat

Glomerular dynamics were investigated by micropuncture in 15-day pregnant Munich-Wistar rats. Female non-pregnant rats were used as control. In hydropenia, single nephron glomerular filtration rate (SNGFR) averaged 37.2 nl/min/g kidney weight in pregnant rats, and 26.2 nl/min/g kidney weight in controls (P less than 0.0005). During pregnancy, mean glomerular plasma flow (GPF) rose from 63.0 to 104.5 nl/min (P less than 0.0005), and glomerular capillary hydrostatic pressure (PG) from 44.8 to 50.6 mm Hg (P less than 0.0005). This rise in PG accounted for an increase in effective filtration pressure (EFP). Mean EFP at the efferent end of the glomerulus rose from near zero to 9.7 mm Hg, indicating filtration pressure disequilibrium. After extracellular fluid volume expansion with saline, GPF and EFP were still greater during pregnancy. A filtration pressure disequilibrium occurred also in non-pregnant rats, allowing calculation of definite values of the ultrafiltration coefficient (Kf). During pregnancy, mean Kf was reduced from 0.0332 to 0.0285 nl/(sec X mm Hg) (P less than 0.005). SNGFR was moderately, but not significantly, increased. These results show that during pregnancy glomerular dynamics is characterized by a rise in GPF and EFP, and by a reduction in Kf. Following these changes, a plasma-flow dependent rise in SNGFR occurs in hydropenia. After expansion, instead, SNGFR is unmodified because the dependence of ultrafiltration on plasma-flow declines while the influence of Kf increases at high values of GPF.

The effect of pseudopregnancy on glomerular filtration rate and salt and water reabsorption in the rat

1. Glomerular filtration rate (G.F.R.) and salt and water reabsorption were measured in age-matched virgin rats and rats at different stages of pseudopregnancy and post-pseudopregnancy. 2. Tubular reabsorption and G.F.R. were significantly higher in later pseudopregnancy. Values at mid-pseudopregnancy were intermediate between virgin controls and late-pseudopregnancy. In post-pseudopregnancy G.F.R. and reabsorption had returned to values not different from the virgins. 3. Expansion of extracellular fluid volume (e.c.f.v.) and elongation of proximal tubules were observed during pseudopregnancy. In post-pseudopregnancy increased tubular length was still apparent but e.c.f.v. was not. 4. The remarkable similarity in the changes in e.c.f.v. and renal functions and structure during pseudopregnancy to those in early pregnancy suggests that the feto-placental unit is not necessary for the pregnancy changes. 5. The differences between the time course of change in plasma progesterone and the time courses of changes in e.c.f.v., renal functions and tubular morphology in late pseudopregnancy suggest that progesterone is not directly involved.

The effect of pregnancy on glomerular filtration rate and salt and water reabsorption in the rat

1. Glomerular filtration rate (G.F.R.) and salt and water reabsorption were measured in age-matched (12- to 13-week-old) virgin rats and rats at different stages of pregnancy and infused with saline at 37.5 and 200 microliters/min. 2. G.F.R. and salt and water reabsorption were significantly higher in early pregnancy and remained at steady, high values until at least mid-pregnancy. G.F.R. was slightly lower close to term, but sodium and water reabsorption remained significantly enhanced. 3. Significant expansion of extracellular fluid volume (e.c.f.v) was first apparent during mid-pregnancy, indicating that e.c.f.v. expansion could not be directly responsible for the early increase in G.F.R. 4. Proximal tubules were significantly elongated 5- days after mating until term, indicating that an increased reabsorptive area in the proximal tubule may be at least partly responsible for the increased reabsorption in pregnancy.

Effects of pregnancy on glucose reabsorption by the proximal convoluted tubule in the rat

1. Free-flow micropuncture techniques were used to investigate glucose, sodium and water reabsorption along the proximal convoluted tubule in 7-8 day pregnant rats undergoing saline or glucose infusion and in virgin controls. 2. Absolute proximal tubular reabsorption of glucose was greater in pregnant animals than virgins during both saline and glucose infusion; and fractional reabsorption of glucose was higher in pregnant than in virgin animals during glucose infusion. 3. During glucose loading, less glucose escaped reabsorption in the proximal tubule in pregnant than in virgin animals. It is concluded that the increased excretion of glucose observed in pregnant rats cannot be due to failure to proximal tubular mechanisms. 4. During saline infusion pregnant animals had a higher single nephron glomerular filtration rate (S.N.G.F.R.) when compared with virgin animals and a corresponding increase in reabsorption of sodium and water. The small increases in S.N.G.F.R. and sodium reabsorption in pregnant animals (compared with virgin animals) during glucose infusion were not statistically significant. 5. Fractional reabsorption of sodium (and water) along the proximal convoluted tubule was not significantly different in the four series. It is concluded that glomerulo-tubular balance operates during pregnancy.

The mechanism of the increase in glomerular filtration rate in the twelve-day pregnant rat

1. Whole kidney and micropuncture techniques were employed to investigate the determinants of glomerular ultrafiltration in virgin and 12-day pregnant rats. 2. A significant increase in whole kidney glomerular filtration rate (g.f.r.) and superficial cortical single nephron g.f.r. was noted in pregnant rats compared to virgins. 3. Increases in whole kidney and glomerular plasma flow rate also occurred in pregnancy which were in proportion to the increase in rate of filtration. No differences were noted in the hydrostatic and oncotic pressures which influence formation of glomerular ultrafiltrate in the superficial nephron population. 4. Reduction in arterial haematocrit and no change in mean red cell volume indicate that a plasma volume expansion has occurred by day 12 of pregnancy in the rat. 5. It is concluded that the increased g.f.r. seen in 12-day pregnant rats is exclusively the result of an increase in renal plasma flow rate (r.p.f.) since the other determinants of glomerular ultrafiltration are unaffected by pregnancy. The plasma volume expansion which also occurs must be, at least in part, responsible for the increase in r.p.f.

Renal hemodynamics and volume homeostasis during pregnancy in the rat

We believe that in pregnant rats, GFR is elevated at term, whereas renal plasma flow appears not to be increased. The mechanism for this increment is unclear, as is the specific time during gestation when renal hemodynamics increase. Rats increase their salt appetite and significantly positive sodium balance occurs, which is most pronounced during the last week of gestation. The retained sodium is distributed in part to the products of conception and in part to the mother, particularly in increasing her extracellular fluid volume. Although chronic volume expansion is present, absolute tubular sodium reabsorption remains elevated. The mechanism for this elevated tubular reabsorption appears no to be related to aldosterone because reduction or inhibition of this mineralocorticoid does not affect sodium balance significantly.

Changes in renal haemodynamics and kidney weight during pregnancy in the unanaesthetized rat

1. Glomerular filtration rate ([(3)H]inulin clearance) and renal plasma flow (PAH clearance) were measured in unanaesthetized Sprague-Dawley rats early in pregnancy (8-10 days) and near term (18-20 days); the results were compared with studies in virgin litter-mates. Evidence of kidney growth was sought by analysing renal dry weights as well as [(14)C]choline incorporation into membrane phospholipids of renal cortical slices.2. Inulin clearances were similar early in pregnancy (pregnant 2.36 +/- 0.12 (S.E. of mean) vs. non-pregnant 2.33 +/- 0.10 ml./min) but near term values were significantly increased (pregnant 2.80 +/- 0.05 vs. non-pregnant 2.39 +/- 0.05 ml./min, P < 0.001). PAH clearances were similar in pregnant and litter-mate control animals at each stage of the study.3. Total renal weight increased significantly both early and late in pregnancy, but renal dry weights as well as [(14)C]choline incorporation were similar in pregnant and non-pregnant animals both at 8-10 days gestation and near term.4. Data confirm our previous findings on renal haemodynamics in anaesthetized rats. Observations that renal dry weight and [(14)C]choline incorporation into phospholipids are similar in litter-mate pregnant and control animals suggests that renal enlargement in this species is due to increments in water content and not to accelerated growth.