Introduction

After graduating in 1968 from Faculty of Medicine,  Pontificia Universidad Católica in Chile, I trained in Nephrology and continued in the Research Division of the Cleveland Clinic, Ohio, USA, between 1970-1972. My mentors in Cleveland, Harriet P. Dustan and Robert B. Tarazi had characterized the hemodynamic patterns of the different stages of essential hypertension and of secondary hypertension, which permitted the individualized management of hypertensives. On returning to my alma mater I incorporated these criteria to patient care, but when treating pregnant hypertensives. Due to the lack of information, this approach was unviable to simultaneously manage two patients, one of whom might suffer from the reduction of maternal blood pressure.

Thinking that vasodilators were pivotal in maternal hemodynamics and that tissue kallikrein – by its capacity to generate kinins – could counteract the pressor effect of the renin system (1), I decided to investigate this system, thus starting my research line. I invited my former professor of physiology, Héctor Croxatto, a strong advocate of this vasodilator system, to study normal pregnant women in search of associations between plasma renin activity and urinary kallikrein.

Our work showed that urinary kallikrein, which reflected renal synthesis, preceded the rise of renin, suggesting a primary role for the vasodilatory kallikrein-kinin system in pregnancy (2, 3). If this was so, kallikrein and its main receptor, the B2R, should be expressed in the primary site of pregnancy, the implantation site.

Laboratory Research

To test this hypothesis I had to regain the skills I had as a medical student to study pregnant rats. In them we found kallikrein and the main receptor of bradykinin, the B2R, in the epithelium of the implantation site and in the sinusoids of the mesometrial triangle; in addition, the B2R was present in the placenta (4-8). Returning to women, we confirmed the presence of the kallikrein-kinin system, eNOS, angiotensin-(1-7) [Ang-1-7] and ACE2 (9-10) in the placenta. The two last factors were studied with the help of Bridget Brosnihan, Wake Forest University, USA, and showed a progressive increase of urinary Ang-(1-7) in the same urine samples already tested for kallikrein (11). Since the association between different vasodilators increased with time, and it was vital to study the early gestational stages, we pursued their temporo-spatial patterns in a surrogate of human pregnancy, pregnant guinea-pigs, thus following the suggestion of Fiona Broughton-Pipkin, Nottingham University, UK. With the invaluable advice of Professor Peter Kaufmann, Aachen University, Germany,  we were able to manage this sensitive animal, dissect the different components of the utero-placental site along gestation and demonstrate that kallikrein, B2R, eNOS, Ang-(1-7), ACE2, VEGF, Flt-1, KDR localized in the same cell types [syncytiotrophoblast, fetal endothelium, invasive/intramural and endoluminal trophoblasts (12-14)]. Recent work, with immunogold electron microscopy in human placentas of normal and preeclamptic pregnancies, located the B2R in cell membrane, reticulum and nucleus in decidua, syncytium, extravillous trophoblast (EVT) and fetal endothelium (15). The role of intranuclear B2R – more abundant in normal than preeclamptic EVT – deserves to be pursued. The B2R promotes in vitro EVT migration and invasion (16), and tinkering in midgestation with a B2R antagonist in guinea-pigs (17) we observed decreased placental sufficiency and endoluminal trophoblasts; unfortunately we could not generate a preeclampsia-like syndrome.

Adding the information from our mainly mechanistic studies in humans, rats and guinea-pigs with their functional interactions of vasodilatory and pleitropic factors, we proposed a network that surrounds NOS and NO to modulate arterial remodelling, placental development and an anticoagulated intricate surface of feto-maternal interface (18).

Findings

In parallel to the laboratory research, the relevance of vasodilation in pregnancy prompted the obstetrician Alfredo Germain and myself to use L-Arginine in women with previous recurrent abortions or severe preeclampsia. Due to the positive results of a pilot test we were not brave enough to use a randomized placebo controlled arm (19).  Nine years later a Mexican group confirmed the beneficial effect of increasing the substrate of NOS (20).

In addition to the studies on vasodilators, we found that endothelial flow-mediated dilatation in preeclamptic pregnancy and recurrent abortion was reduced a year after the previous pregnancy in both conditions (21). This permitted us to attribute the endothelium dysfunction in the genesis of both placentation-related disorders rather than to a persistent effect of the preeclampsia. Also interested in the long term association between hypertensive pregnancies and remote cardiovascular disease, we contacted 217 parous women submitted to coronary angiography and obtained their reproductive histories. In those with hypertensive pregnancies, chronic hypertension, hyperlipidemia and premature familial cardiovascular disease prevailed and they presented an increase in the number of stenotic arteries along time (22). These two studies stress the fact that pregnancy constitutes a window to differentiate women at high risk of cardiovascular disease, which has to be routinely incorporated to clinical practice.

In summary, going back and forth between humans and animals I progressed from urinary kallikrein to electron microscopy intracellular expression of the B2R and expanded the range of vasoactive factor expressed at the utero-placental unit. Having recently closed my laboratory, I consider myself very fortunate to have counted with the example of my mentors, with much more than “a little help from my friends” in the field, with the skill of my collaborators and the interaction of ISSP meetings. In addition, in a small scale I have translated my basic research into clinical benefits for my patients.

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References:

1. Urinary kallikrein and plasma renin activity in normal human pregnancy. Valdés G, Espinoza P, Moore R, Croxatto HR. 1981;3:II-55-8.
2. The neurogenic response to 55-degree head-up tilt in normal pregnant women. Valdés G, Salas SP, Foradori A, Gormaz G, Croxatto HR. Hypertension 1986; 8:I-161–4.
3. Urinary kallikrein excretion l cycle, normal pregnancy and lactation. Prenat Neonat Med 1998;3:474-81.
4. Uterine kallikrein in the early pregnant rat. Valdés G, Corthorn J, Scicli AG, Gaete V, Soto J, Ortiz ME et al. Biol Reprod. 1993;49:802-8.
5. Temporospatial changes of kinin B2 receptors during the estrous cycle and pregnancy in the rat uterus. Figueroa CD, Chacón C, Corthorn J, Ehrenfeld P, Müller-Esterl W, Valdés G. Biol Reprod. 2001;64:1590-9.
6. Temporospatial changes of kallikrein-like enzymes during the estrous cycle and pregnancy in the rat uterus. Valdés G, Figueroa CD, Corthorn J.  Biol Reprod. 1996;55:236-45.
7. Variations in uterine kallikrein during cycle and early pregnancy in the rat. Corthorn J, Valdés G. Biol Reprod. 1994;50:1261-4.
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9. Distribution of angiotensin-(1-7) and ACE2 in human placenta of early and term normotensive pregnancy and preeclampsia. G. Valdés, L. Anton, LAA Neves, J. Corthorm, C. Chacón, AM Germain et al. Placenta 2006: 27: 200-207.
10. Expression of kallikrein, bradykinin B2 receptor and endothelial nitric oxide synthase in placentation in normal gestation, preeclampsia and placenta accreta. Corthorn J, Germain AM, Chacón C, Rey S, Soto GX, Figueroa CD et al. Endocrine 2006; 29:491-499.
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12. Spatio-temporal expression of MMP-2, MMP-9 and tissue kallikrein in uteroplacental units of the pregnant guinea-pig (Cavia porcellus). Corthorn J, Rey S, Chacón C, Valdés G. Reprod Biol Endocrinol. 2007 Jul 2;5:27.
13. Angiogenic, hyperpermeability and vasodilator network in utero-placental units along pregnancy in the guinea-pig (Cavia porcellus). Valdés G, Erices R, Chacón C, Corthorn J. Reprod Biol Endocrinol. 2008; 27;6:13
14. Utero-placental expression of angiotensin-(1-7) and ACE2 in the pregnant guinea-pig. Valdés G, Corthorn J, Bharadwaj MS, Joyner J, Schneider D, Brosnihan KB. Reprod Biol Endocrinol.2013;11:5.
15. Utero-placental cellular and nuclear expression of bradykinin B2 receptors in normal and preeclamptic pregnancies. Valdés G, Acuña S, Munizaga A, Soto GX, Figueroa CD. Pregnancy Hypertension: An International Journal on Women´s Cardiovascular Health  2016; 6:30-37
16. Bradykinin promotes migration and invasion of human immortalized trophoblasts. Rafaela Erices R, Corthorn J, Lisboa F, Valdés G. Reprod Biol Endocrinol. 2011; 9: 97.
17. Administration of angiotensin II and a bradykinin B2 receptor blocker in midpregnancy impairs gestational outcome in guinea pigs. Valdés G, Schneider D, Corthorn J, Ortiz R, Acuña S, Padilla O. Reprod Biol Endocrinol 2014;12:49.
18. Vasodilator factors in the systemic and local adaptations of pregnancy. Valdés G, Kaufmann P, Corthorn J, Erices R, Brosnihan KB, Joyner-Grantham J. Reprod Biol Endocrinol. 2009;7:79.
19. Evidence supporting a beneficial role for long-term L-arginine supplementation in high-risk Germain AM, Valdés G, Romanik MC, Reyes MS. Hypertension.2004 Jul;44:e1.
20. Effect of supplementation during pregnancy with L-arginine and antioxidant vitamins in medical food on pre-eclampsia in high risk population: randomised controlled trial. Vadillo-Ortega F, Perichart-Perera O, Espino S, Avila-Vergara MA, Ibarra I, Ahued R et al. BMJ 2011;342:d2901.
21. Endothelial dysfunction: a link among preeclampsia, recurrent pregnancy loss, and future cardiovascular events? Germain AM, Romanik MC, Guerra I, Solari S, Reyes MS, Johnson RJ et al. Hypertension 2007; 49: 90-5.
22. Association of coronary artery disease and remote hypertensive pregnancies. Valdés G, Quezada F, Marchant E, Schultzendorff A, Moran S, Padilla O, Martínez A. Hypertension 2009; 53;733-8.