Angiogenic growth factors are new and essential players in the sustained relaxin vasodilatory pathway in rodents and humans

Relaxin is emerging as an important vasodilator of pregnancy and is being tested for afterload reduction in acute heart failure. However, the mechanisms underlying relaxin-induced vasodilation are incompletely understood. The aims of this study were to establish a new in vitro model for relaxin-induced vasodilation and to use this approach, as well as chronically instrumented, conscious rats, to investigate the role of angiogenic growth factors in the relaxin vasodilatory pathway. Incubation of rat and mouse small renal arteries with recombinant human H2 relaxin for 3 hours in vitro attenuated myogenic constriction, which was blocked by inhibitors of gelatinases, the endothelin B receptor, and NO synthase. These findings corroborate ex vivo observations in arteries isolated from relaxin-infused nonpregnant and midterm pregnant rats, thereby validating the new experimental approach and enabling the study of human arteries. Incubation of small human subcutaneous arteries with relaxin for 3 hours in vitro also attenuated myogenic constriction through the same molecular intermediates. Vascular endothelial growth factor receptor inhibitor SU5416, 3 different vascular endothelial growth factor, and 2 different placental growth factor neutralizing antibodies prevented relaxin from attenuating myogenic constriction in rat and mouse small renal and human subcutaneous arteries. SU5416 administration also prevented relaxin-induced renal vasodilation and hyperfiltration in chronically instrumented, conscious rats. Small renal arteries isolated from these rats demonstrated increased matrix metalloproteinase 2 activity in the relaxin-infused group, which was not prevented by SU5416. We conclude that there is concordance of relaxin vasodilatory mechanisms in rats, mice, and humans, and angiogenic growth factors are novel and essential intermediates.

Comparative effects of aprotinin and human recombinant R24K KD1 on temporal renal function in Long-Evans rats

Bovine aprotinin, a reversible inhibitor of plasmin and kallikrein, has been clinically approved for over two decades to prevent perioperative blood loss during cardiac surgery. However, because of postoperative renal dysfunction in thousands of these patients, aprotinin was voluntarily withdrawn from the market. Our earlier studies indicated that a R24K mutant of the first Kunitz-type domain of human tissue factor pathway inhibitor-2 (R24K KD1) exhibited plasmin inhibitory activity equivalent to aprotinin in vitro. In this study, we compared the effects on renal function after infusion of aprotinin and recombinant R24K KD1 in chronically instrumented, conscious rats. Aprotinin-infused rats exhibited statistically significant decreases in glomerular filtration rate and effective renal plasma flow relative to rats infused with phosphate-buffered saline (PBS) or R24K KD1 dissolved in PBS. In addition, aprotinin-treated rats exhibited marked increases in serum creatinine, blood urea nitrogen, urinary protein, and effective renal vascular resistance, whereas these renal parameters remained essentially unchanged in vehicle and R24K KD1-treated rats for a one-week period. Moreover, with use of a highly sensitive apoptosis detection assay, a significant increase in the rate of early and late apoptotic events in renal tubule cells occurred in aprotinin-treated rats relative to R24K KD1-treated rats. In addition, histological examination of the rat kidney revealed markedly higher levels of protein reabsorption droplets in the aprotinin-infused rats. Our data collectively provide suggestive evidence that R24K KD1 does not induce the renal dysfunction associated with aprotinin, and may be an effective clinical alternative to aprotinin as an antifibrinolytic agent in cardiac surgery.

Role of relaxin in maternal renal vasodilation of pregnancy

The remarkable hemodynamic changes of normal pregnancy are briefly reviewed. In addition, new findings and current concepts related to the underlying hormonal and molecular mechanisms are presented. Finally, work that is in progress as well as future directions is briefly discussed.

Mechanisms of renal vasodilation and hyperfiltration during pregnancy: current perspectives and potential implications for preeclampsia

A thorough understanding of the renal and cardiovascular adaptations to normal gestation is essential for proper diagnosis and management of hypertensive disorders and renal diseases during pregnancy. Here, we briefly review the renal hemodynamic changes of normal pregnancy. In addition, we present new findings and current concepts related to the underlying hormonal and molecular mechanisms. Finally, we speculate on the potential contribution of these insights from normal pregnancy to the pathogenesis of preeclampsia.

Implantation and Pregnancy Following In Vitro Fertilization and the Effect of Recombinant Human Relaxin Administration in Macaca fascicularis1

Implantation and early pregnancy, and the potential effects of the reproductive-hormone relaxin, were examined in the cynomolgus macaque (Macaca fascicularis) following in vitro fertilization and embryo transfer. Mature oocytes were collected from regularly cycling, female cynomolgus monkeys subjected to ovarian superovulation using recombinant human FSH and hCG. Oocytes fertilized in vitro were cultured to the 4- to 8-cell stage, slow-cooled, and stored in liquid nitrogen before thawing and embryo transfer. Regularly cycling recipients were administered recombinant human relaxin or vehicle for 21 days through the peri-implantation period (Day 0 = pump implantation), during which time the thawed embryos were transferred (Day 7). Endometrial thickness and the number of gestational sacs were monitored by ultrasound at three time points (Days 7, 21, and 28). The number of days of placental sign (implantation bleeding) in pregnant females and menses in nonpregnant females were also recorded. Implantation (gestational sacs/embryo transferred) and multiple pregnancy (multiple gestations/ pregnant recipient) rates were slightly higher in relaxin-treated recipients compared to vehicle-treated recipients. Administration of relaxin was associated with increased implantation bleeding in pregnant females. Endometrial thickness was increased in relaxin-treated recipients at Days 7 and 28 compared to Day 0, but these differences were not observed at the same time points in vehicle-treated females. Systemic administration of recombinant human relaxin in an in vitro fertilization/embryo transfer setting was associated with effects consistent with a role for this hormone in endometrial physiology in primates.

Relaxin is essential for renal vasodilation during pregnancy in conscious rats

Marked vasodilation in the kidney and other nonreproductive organs is one of the earliest maternal adaptations to occur during pregnancy. Despite the recognition of this extraordinary physiology for over four decades, the gestational hormone responsible has remained elusive. Here we demonstrate a key role for relaxin, a member of the IGF family that is secreted by the corpus luteum in humans and rodents. Using a gravid rodent model, we employ two approaches to eliminate relaxin or its biological activity from the circulation: ovariectomy and administration of neutralizing antibodies. Both abrogate the gestational elevation in renal perfusion and glomerular filtration, as well as preventing the reduction in myogenic reactivity of isolated, small renal arteries. Osmoregulatory changes, another pregnancy adaptation, are also abolished. Our results indicate that relaxin mediates the renal vasodilatory responses to pregnancy and thus may be important for maternal and fetal health. They also raise the likelihood of a role for relaxin in other cardiovascular changes of pregnancy, and they suggest that, like estrogen, relaxin should be considered a regulator of cardiovascular function.

Impact of gender and endothelin on renal vasodilation and hyperfiltration induced by relaxin in conscious rats

Chronic administration of the hormone relaxin elicits renal vasodilation that is dependent on nitric oxide (NO) in both conscious intact and ovariectomized female rats. Our first objective was to test whether the hormone, when administered to approximate serum concentrations found in midterm pregnant rats, induces renal vasodilation in males. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) increased significantly, on average, by 33 and 49% over baseline, respectively, after 5 days of recombinant human relaxin (rhRLX) administration to 12 conscious male rats by subcutaneous osmotic minipump. There were also significant decreases in hematocrit, plasma osmolality, and sodium concentration. Another objective was to determine whether endogenous endothelin (ET; via the endothelial ET(B) receptor) mediates the NO-dependent renal vasodilation produced by relaxin. rhRLX or vehicle was administered to conscious female rats (n = 9 and 8 rats, respectively). On the fifth day, baseline GFR and ERPF were both increased, on average, by 20-30% in the rats administered rhRLX (P < 0.05 vs. vehicle). Next, the specific ET(B)-receptor antagonist RES-701-1 was infused intravenously over 4 h in both groups of rats. In response to RES-701-1, there was a significant decline in both GFR and ERPF in the rats receiving rhRLX such that renal function converged in the two groups of animals. We conclude 1) relaxin induces marked changes in the renal circulation and in osmoregulation regardless of gender and 2) relaxin-induced renal vasodilation and hyperfiltration are mediated by endothelin through the endothelial ET(B) receptor subtype and NO.

Endothelin mediates renal vasodilation and hyperfiltration during pregnancy in chronically instrumented conscious rats

Profound vasodilation of the kidneys and other nonreproductive organs transpires during early pregnancy. Because nitric oxide (NO) was found to mediate renal vasodilation and hyperfiltration in conscious pregnant rats, and endogenous endothelin (ET) was suggested to be vasodilatory in the renal circulation of nonpregnant rats, we tested whether endothelin mediates the NO-dependent changes in the renal circulation during pregnancy. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured in conscious pregnant and virgin rats before and during infusion of 30 micrograms/min RES-701-1 (a selective ETB receptor subtype antagonist). Baseline GFR and ERPF were significantly increased by 35% in gravid rats relative to virgin controls. During infusion of RES-701-1, the pregnant rats responded more robustly, showing a greater decline in both GFR and ERPF such that renal function converged in the two groups of rats. ERPF also converged in pregnant and virgin rats during infusion of SB-209760, a nonselective ETA/B receptor subtype antagonist. Combined infusion of Nomega-nitro-L-arginine methyl ester [L-NAME, an NO synthase (NOS) inhibitor] and RES-701-1 reduced GFR and ERPF to levels comparable to those reached with either agent given alone, suggesting inhibition of a common vasodilatory pathway. RES-701-1 and SB-209670 significantly lowered the cGMP content of small renal arteries from gravid and virgin rats in vitro, strengthening the link between the renal endothelial ETB receptor subtype and NO. Importantly, we showed that RES-701-1 is not a direct inhibitor of NOS. We conclude that endothelin mediates the NO-dependent changes in the renal circulation of conscious rats during pregnancy.

Relaxin is a potent renal vasodilator in conscious rats

The kidneys and other nonreproductive organs vasodilate during early gestation; however, the "pregnancy hormones" responsible for the profound vasodilation of the renal circulation during pregnancy are unknown. We hypothesized that the ovarian hormone relaxin (RLX) contributes. Therefore, we tested whether the administration of RLX elicits renal vasodilation and hyperfiltration in conscious adult, intact female rats. After several days of treatment with either purified porcine RLX or recombinant human RLX 2 (rhRLX), effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) increased by 20%-40%. Comparable renal vasodilation and hyperfiltration was also observed in ovariectomized rats, suggesting that estrogen and progesterone are unnecessary for the renal response to rhRLX. The nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester completely abrogated the increase in ERPF and GFR elicited by chronic administration of purified porcine RLX. In contrast, the renal vasoconstrictory response to angiotensin II was attenuated by the RLX treatment. Short-term infusion of purified porcine RLX to conscious rats over several hours failed to increase ERPF and GFR. Plasma osmolality was consistently reduced by the chronic administration of both RLX preparations. In conclusion, the renal and osmoregulatory effects of chronic RLX administration to conscious rats resemble the physiological changes of pregnancy in several respects: (a) marked increases in ERPF and GFR with a mediatory role for nitric oxide; (b) attenuation of the renal circulatory response to angiotensin II; and (c) reduction in plasma osmolality.

Prostaglandins maintain renal vasodilation and hyperfiltration during chronic nitric oxide synthase blockade in conscious pregnant rats

Rats demonstrate renal vasodilation and hyperfiltration in pregnancy. Because both NO and cGMP biosynthesis are increased in gravid rats and because acute administration of NO synthase inhibitors abrogates renal vasodilation and hyperfiltration, NO most likely mediates the renal circulatory changes of gestation. In the present study, we tested the effect of chronic inhibition of NO synthase on effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) in chronically instrumented, conscious, gravid rats. Because gestation is a relatively long-term condition, we postulated that chronic withdrawal of NO would result in sustained inhibition of renal vasodilation and hyperfiltration. Contrary to our hypothesis, the renal circulatory changes of pregnancy were maintained during chronic blockade of NO synthase. That is, subcutaneous administration of 10 micrograms/min N omega-nitro-L-arginine methyl ester (NAME) for 48 hours did not significantly reduce GFR in either virgin or pregnant rats; thus, hyperfiltration persisted in the latter despite chronic NO synthase blockade. In contrast, ERPF was reduced and effective renal vascular resistance (ERVR) increased in both groups of rats during NAME administration but in a parallel fashion, such that renal vasodilation persisted in the gravid animals despite chronic inhibition of NO synthase. However, with superimposition of acute prostaglandin synthesis inhibition (meclofenamate, 10 mg/kg IV), renal vasodilation and hyperfiltration were abolished; ie, the combined treatments of chronic NO synthase blockade and acute prostaglandin synthesis inhibition led to the equalization of GFR, ERPF, and ERVR in conscious virgin and pregnant rats. Inhibition of prostaglandin synthesis alone had little affect on the renal circulation, as previously reported. In summary, prostaglandins are recruited to maintain renal vasodilation and hyperfiltration during chronic NO synthase blockade in conscious pregnant rats.

Acute blockade of nitric oxide synthase inhibits renal vasodilation and hyperfiltration during pregnancy in chronically instrumented conscious rats

Because the kidneys are vasodilated and the endogenous production of nitric oxide is increased in gravid rats, we tested whether nitric oxide mediates the renal vasodilatory response to pregnancy. Chronically instrumented, conscious rats of gestational days 12-14 were studied concurrently with age-matched virgin control animals. GFR and effective renal plasma flow (ERPF) were determined by the renal clearances of inulin and para-aminohippurate before and during acute infusion of N omega-nitro-L-arginine methyl ester (NAME; 2, 20, and 50 micrograms/min) or NG-monomethyl-L-arginine (100 micrograms/min). Baseline GFR and ERPF were significantly increased, and effective renal vascular resistance was decreased by 30-40% in gravid rats compared with virgin controls. During infusion of all three dosages of NAME and NG-monomethyl-L-arginine, effective renal vascular resistance, GFR, and ERPF were equalized in the pregnant and virgin rats (the only exception being GFR during the 20 micrograms/min NAME infusion). When compared with virgin rats, the gravid animals were more responsive to nitric oxide synthase inhibition, showing a significantly greater decline in GFR and ERPF and rise in effective renal vascular resistance at each timepoint during the infusion of inhibitor. To exclude the possibility that nonspecific renal vasoconstriction per se led to equalization of renal function in the two groups of rats, we investigated angiotensin II. In contrast to the results observed with nitric oxide synthase inhibitors, pregnant rats were less responsive to the renal vasoconstrictory effects of angiotensin II, such that the baseline differences in renal parameters measured before infusion of the hormone were increased during the infusion. To determine whether nitric oxide synthase was inhibited to a similar extent in gravid and virgin rats, aortic and renal cortical cGMP content was assayed ex vivo at the end of inhibitor infusion. The lower 2-micrograms/min dose of NAME consistently reduced cGMP content of these tissues to comparable levels in the two groups of rats. In conclusion, we suggest that nitric oxide mediates reduced renal vascular resistance and hyperfiltration during pregnancy in conscious rats.