The hormone relaxin (RLX) has a molecular weight of approximately 5,000-6,000 and is a member of the insulin-growth factor family. In humans and rats, the ovary is the major, if not sole, source of circulating RLX. In pregnant rats, RLX promotes growth and softening of the cervix and vagina, promotes growth of the mammary nipples, and inhibits spontaneous uterine contractility (reviewed in ref. 1).During early pregnancy in humans, rats, and other mammals, vasodilation of nonreproductive organs leads to a marked decline in total peripheral vascular resistance (2). The kidneys contribute to this fall in peripheral resistance; a nadir in renal vascular resistance and a reciprocal peak in renal blood flow and glomerular filtration rate of 40%-80% above nonpregnant levels are reached by the end of the first trimester in women (reviewed in ref.3). Maximal changes, albeit of a lesser magnitude (20%-40% from nonpregnant levels), are observed in the renal circulation of conscious rats by gestational day 12 (3).The "pregnancy hormones" that cause renal vasodilation and hyperfiltration have not been identified. For several reasons, we hypothesized that RLX contributes. First, the plasma concentration of RLX increases rapidly after conception in women (reviewed in ref. 1), corresponding with the marked increase in both glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) during the first trimester (3). The stimulus for secretion of RLX by the corpus luteum is human chorionic gonadotrophin (hCG), which rises in parallel (1). Second, circulating RLX also increases during the luteal phase of the menstrual cycle (1, 4-6), coinciding with a transient 20% increase in GFR and ERPF (7-12). Third, the gestational rise in RLX corresponds with another early physiological adaptation in human pregnancy, namely, osmoregulatory changes (13). Chronic administration of recombinant human RLX 2 (rhRLX) to ovariectomized rats reduced plasma osmolality and the osmotic threshold for arginine vasopressin release to levels comparable to the gravid condition (14). Fourth, the osmoregulatory changes of pregnancy were mimicked by administering hCG to women in the luteal phase of the menstrual cycle and intact female rats, but not to men or ovariectomized rats, suggesting the intermediary role of an ovarian hormone (15-17). Associated with the osmoregulatory changes observed in women after administration of hCG was a 15%-20% rise in GFR, which again was not observed in men administered the hormone (Davison, J.M., personal communication). Fifth, although renal vasodilation and hyperfiltration are observed in gravid rats as early as day 5 of gestation, before detectable increases in ovarian and plasma RLX occur, there is a marked jump in GFR and ERPF between gestational days 8 and 12 (18) Received for publication October 27, 1998, and accepted in revised form January 4, 1999. Relaxin is a potent renal vasodilator in conscious ratsThe kidneys and other nonreproductive organs vasodilate during early gestation; however, the "pregnancy h...

Abstract-During pregnancy, relaxin stimulates nitric oxide (NO)-dependent renal vasodilation, hyperfiltration and reduced myogenic reactivity of small renal arteries via the endothelial ET B receptor subtype. Our objective in this study was to elucidate the mechanisms by which relaxin stimulates the endothelial ET B receptor/NO vasodilatory pathway. Using chronically instrumented conscious rats, we demonstrated that a specific peptide inhibitor of the gelatinases MMP-2 and -9, cyclic CTTHWGFTLC (cyclic CTT), but not the control peptide, STTHWGFTLS (STT), completely reversed renal vasodilation and hyperfiltration in relaxin-treated rats. Comparable findings were observed with a structurally different and well-established, general antagonist of MMPs, GM6001. In contrast, phosphoramidon, an inhibitor of endothelin-converting enzyme, did not significantly change the renal vasodilatory response to relaxin administration. When small renal arteries were incubated with either of the general MMP inhibitors, GM6001 or TIMP-2 (tissue inhibitor of MMP), or with the specific gelatinase inhibitor, cyclic CTT, the reduced myogenic reactivity of these blood vessels from relaxin-treated nonpregnant and midterm pregnant rats was totally abolished. Moreover, a neutralizing antibody specific for MMP-2 completely abrogated the reduced myogenic reactivity of small renal arteries from relaxin-treated nonpregnant and midterm pregnant rats. In contrast, phosphoramidon did not significantly affect the reduction in myogenic reactivity. Using gelatin zymography, we showed increased pro and active MMP-2 activity in small renal arteries from relaxin-treated nonpregnant and midterm pregnant rats relative to the control animals. Thus, inhibitors of MMPs in general and of gelatinases in particular reverse the renal vascular changes induced by pregnancy or relaxin administration to nonpregnant rats. Finally, the typical reduction in myogenic reactivity of small renal arteries from relaxin-treated nonpregnant rats was absent in ET B receptor-deficient rats, despite an increase in vascular MMP-2 activity. These results indicate an essential role for vascular gelatinase, which is in series with, and upstream of, the endothelial ET B receptor/NO signaling pathway in the renal vasodilatory response to relaxin and pregnancy. (Circ Res. 2003;93:1249-1257.)Key Words: nitric oxide Ⅲ endothelin B receptor Ⅲ pregnancy Ⅲ renal circulation Ⅲ matrix metalloproteinases V asodilation of nonreproductive organs is one of the earliest and most dramatic of maternal adaptations to human pregnancy. The gravid rat manifests comparable circulatory changes including renal vasodilation and hyperfiltration. 1,2 Consequently, this animal model has been extensively investigated in order to elucidate the mechanisms underlying the renal vasodilatory response to pregnancy. Endothelin (ET) has been shown to mediate nitric oxide (NO)-dependent renal vasodilation and hyperfiltration during pregnancy, as well as reduced myogenic reactivity of small renal arteries via the ET B rec...

IntroductionAmong the most striking changes seen in biology are those associated with pregnancy. In particular, profound vasodilation of nonreproductive organs including the kidney epitomizes the maternal cardiovascular adaptation to early gestation in women. Cardiac output, global arterial compliance, effective renal plasma flow (ERPF), and GFR rise from 30% to 80%, while vascular resistances plummet and blood pressure declines modestly (refs. 1, 2; and reviewed in ref. 3). These alterations begin immediately after conception, peak by the end of the first or beginning of the second trimester, and persist throughout gestation. It is likely that the circulations of nonreproductive organs such as the kidney serve as arteriovenous shunts during early gestation. Thus, ventricular afterload falls, which initiates the enormous increase in cardiac output and, subsequently, the expansion of plasma volume -maternal adaptations associated with healthy pregnancies. Furthermore, pressor response to administration of angiotensin II and vascular reactivity to infusion of norepinephrine become attenuated. Insight into the mechanisms responsible for these vasodilatory phenomena may be particularly critical, since in preeclampsia, the attenuation of pressor responsiveness to angiotensin II, the reduced vascular reactivity to norepinephrine, and the systemic and renal vasodilation are compromised (3).Although 17β-estradiol has been traditionally viewed as the uterine and systemic vasodilator of pregnancy (4, 5), this hormone has little, if any, effect on the renal circulation (5-8), which markedly vasodilates so early in pregnancy. Progesterone may have limited capacity to vasodilate the renal circulation (6, 9, 10); however, an alternative possibility is that the pregnancy protein hormones are involved. Relaxin is an approximately 6-kDa protein secreted by the corpus luteum during human gestation (reviewed in ref. 11). Stimulated by the luteotrophic hormone human chorionic gonadotrophin, serum levels of relaxin increase immediately after conception (11) corresponding to the gestational rise in ERPF and GFR (3). Relaxin also circulates, albeit at lower levels, in the luteal phase of the menstrual cycle (11) and is associated with a 20% increase in ERPF and GFR at that time (e.g., see ref. 12). The hormone is temporally linked to another early pregnancy adaptationosmoregulatory changes (13). In gravid rats, relaxin is secreted by the corpora lutea with serum levels first detectable on gestational day 8 (11). 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:...