Biomedical subjects

Kirk P Conrad

Publications and source records attributed to Kirk P Conrad.

At least 19 recordsLinked to original sources

PubMed2007

Renal function during normal pregnancy and preeclampsia.

Glomerular filtration rate and renal plasma flow increase by 40 to 65 and 50 to 85%, respectively, during normal pregnancy in women. Studies using the gravid rat as a model have greatly enhanced our understanding of mechanisms underlying these remarkable changes in the renal circulation during gestation. Hyperfiltration is largely due to increased renal plasma flow, the latter attributable to profound reductions in both the renal afferent and efferent arteriolar resistances. The ovarian hormone, relaxin, mediates renal vasodilation during pregnancy. Relaxin increases vascular gelatinase activity, thereby converting big ET to ET(1-32), which leads to renal vasodilation, hyperfiltration and reduced myogenic reactivity of small renal arteries via the endothelial ET(B) receptor and nitric oxide. Serum concentration of uric acid falls during normal pregnancy as a consequence of increased GFR and/or reduced proximal tubular reabsorption. The elevated urinary excretion of protein during pregnancy is secondary to increased GFR, reduced proximal tubular reabsorption, and perhaps alteration in the electrostatic charge of the glomerular filter. Whether the tubular secretion of Tamm-Horsfall protein increases during normal pregnancy is uncertain. In most women with preeclampsia, renal plasma flow and glomerular filtration rate are at most only modestly decreased as a consequence of increased afferent arteriolar resistance and/or reduced ultrafiltration coefficient. Serum uric acid concentrations are increased mainly as a consequence of reduced renal clearance. Reduced GFR leads to decreased filtered load of uric acid, and plasma volume contraction contributes to increased proximal tubular reabsorption coupled to sodium. The increase in urinary protein excretion in preeclampsia occurs secondary to alterations in the size and/or charge selectivity of the glomerular filter, possible increases in glomerular capillary pressure, and compromise of proximal tubular reabsorption. The renal histologic lesion characteristic of preeclampsia is termed "glomerular endotheliosis". Recent evidence suggests that anti-angiogenic factors emanating from the placenta in preeclampsia contribute to glomerular endotheliosis, proteinuria, and hypertension during disease.

Angiogenic Proteins↗

PubMed2006

Vascular matrix metalloproteinase-9 mediates the inhibition of myogenic reactivity in small arteries isolated from rats after short-term administration of relaxin.

UNLABELLED: During pregnancy and chronic relaxin administration to nonpregnant rats (for days), vascular MMP (matrix metalloproteinase)-2 is increased and mediates renal vasodilation, hyperfiltration, and inhibition of myogenic reactivity of small renal arteries. However, the renal vasodilatory actions of relaxin also occur after only several hours of hormone administration to nonpregnant rats, and we hypothesized a pivotal role for vascular MMP-2. Accordingly, we used gelatin zymography, which reveals not only vascular MMP-2, but also MMP-9 activity in small renal arteries isolated from rats administered recombinant human relaxin (rhRLX) or vehicle for 4-6 h. Furthermore, we tested whether myogenic reactivity is inhibited, and if so, whether the inhibition is mediated by increased vascular MMP-2. Surprisingly, we detected no significant difference in either pro or active MMP-2 in small renal arteries isolated from rhRLX and vehicle control treatment groups. In contrast, vascular MMP-9 was up-regulated by 70% (P < 0.0005 vs. vehicle). These results were completely unexpected and novel. MMP-9 protein expression was confined to the vascular smooth muscle. MMP-9, but not MMP-2 activity, was also increased in mesenteric arteries after short-term rhRLX administration (P < 0.005 and >0.05 vs. vehicle, respectively). Myogenic reactivity was inhibited in small renal arteries isolated from nonpregnant rats treated with rhRLX for 4-6 h (P < 0.01 vs. vehicle) and was completely restored by incubation with MMP-9, but not MMP-2 neutralizing antibodies in vitro. CONCLUSION: In contrast to chronic rhRLX administration, MMP-9 rather than MMP-2 plays a central role in the vasodilatory effect of short-term relaxin administration.

Animals↗

PubMed2006

Influence of recombinant human relaxin on renal hemodynamics in healthy volunteers.

Maternal renal hemodynamic adaptation to human pregnancy is one of the most dramatic of all physiologic changes, but the factors that are responsible have remained elusive. In rat pregnancy, there are comparable renal hemodynamic changes, and in this species there is comprehensive evidence that the ovarian hormone relaxin (RLX) is responsible. This study investigated the renal effects of recombinant human RLX (rhRLX) in humans. Eleven volunteers (six male, five female) received intravenous infusions of rhRLX over 5 h at an infusion rate that was chosen to sustain serum concentrations that are comparable to early pregnancy. The renal clearances of inulin and para-aminohippurate were used to measure GFR and renal plasma flow, respectively. Irrespective of gender, renal plasma flow was increased by 47% compared with baseline levels (P < 0.0001), but no significant change was observed in GFR. There were no side effects or adverse reactions of rhRLX given as an intravenous infusion, and the data suggest that RLX indeed may be one of the elusive renal vasodilatory factors in human pregnancy. Further work is necessary to elucidate the complimentary factors that permit the concomitant increase in GFR during pregnancy.

Adolescent↗

PubMed2006

Relaxin is essential for systemic vasodilation and increased global arterial compliance during early pregnancy in conscious rats.

During early pregnancy, there are marked increases in cardiac output (CO) and global arterial compliance (AC), as well as decreases in systemic vascular resistance (SVR). We recently reported that administration of recombinant human relaxin to nonpregnant female rats elicits changes in systemic hemodynamics and arterial mechanical properties similar to those observed during normal pregnancy. In the present study, we directly tested whether endogenous relaxin mediates the cardiovascular adaptations of pregnancy by neutralizing circulating relaxin with monoclonal antibodies during early gestation. Relaxin neutralizing antibodies were administered daily, beginning on d 8 of rat gestation, to block the functional effects of circulating relaxin. Systemic hemodynamics and arterial properties were assessed between gestational d 11 and 15 using techniques we have previously reported. Pregnant rats administered the neutralizing antibodies failed to exhibit the gestational increases in stroke volume, CO, and global AC or decreases in SVR that were observed in control pregnant rats administered an irrelevant antibody against fluorescein or PBS. In fact, in the pregnant rats administered the relaxin neutralizing antibodies, cardiovascular parameters were not statistically different from those in virgin rats. Interestingly, small renal and first-order mesenteric arteries isolated from midterm pregnant rats administered either relaxin-neutralizing or control antibodies did not exhibit any changes in passive mechanical properties compared with virgin rats. These findings indicate that circulating relaxin mediates the transition of the systemic circulation from the virgin to the pregnant state in the gravid rat model, suggesting a potential role for aberrant relaxin regulation in abnormal pregnancies wherein these cardiovascular adaptations are inadequate or excessive.

Animals↗

PubMed2006

Relaxin has a role in establishing a renal response in pregnancy.

Women with normal ovarian function (n = 9) and women who conceived with ovum donation (no circulating relaxin; n = 9) had serial measurements of renal function made during the first trimester of pregnancy by using 24-hour creatinine clearance (CrCl) and plasma osmolality (P(osm)). All pregnancies were associated with increasing CrCl and reduced P(osm), but the change from baseline was significantly greater in the women with normal ovarian function, indicating that in contrast to the rodent model, other factors in addition to circulating relaxin contribute to gestational renal adaptation to human pregnancy.

Adult↗

PubMed2006

Matrix metalloproteinase-2 activity, protein, mRNA, and tissue inhibitors in small arteries from pregnant and relaxin-treated nonpregnant rats.

Vascular gelatinase activity is essential for pregnancy- and relaxin (Rlx)-induced renal vasodilation and hyperfiltration in rats. The objective of this study was to further elucidate the mechanisms for the increase in vascular matrix metalloproteinase (MMP)-2 activity caused by pregnancy and Rlx. We first corroborated our earlier work by showing that pro- and active forms of MMP-2 were increased in small renal arteries from pregnant compared with virgin rats and Rlx-treated compared with vehicle-treated nonpregnant rats. We next investigated other artery types and showed that MMP-2 activity was upregulated in mesenteric arteries from pregnant rats (pro-MMP-2 by 50% and active MMP-2 by 40%, both P<0.05) and from Rlx-treated nonpregnant rats (pro-MMP-2 by 50% and active MMP-2 by 90%, both P<0.005) compared with their respective controls. To corroborate these results obtained by gelatin zymography, pro-MMP-2 protein was determined by Western analysis in the same small arteries. Pro-MMP-2 protein was increased in small renal arteries from pregnant compared with virgin rats and from Rlx- compared with vehicle-treated nonpregnant rats: pro-MMP-2-to-beta-actin ratio=0.29 vs. 0.21 (P<0.01) and 0.43 vs. 0.32 (P<0.005). Findings were similar for mesenteric arteries. MMP-2 mRNA as measured by real-time PCR was increased in small renal arteries from pregnant and Rlx-treated nonpregnant rats compared with their respective controls. There were no significant differences in tissue inhibitor of metalloproteinase (TIMP-1 or TIMP-2) activity by reverse zymography in small renal arteries. Thus increases in MMP-2 mRNA and protein expression are major factors contributing to increased MMP-2 activity in small arteries from pregnant and Rlx-treated nonpregnant rats.

Animals↗

PubMed2006

Evidence for local relaxin ligand-receptor expression and function in arteries.

Relaxin is a 6 kDa protein hormone produced by the corpus luteum and secreted into the blood during pregnancy in rodents and humans. Growing evidence indicates that circulating relaxin causes vasodilatation and increases in arterial compliance, which may be among its most important actions during pregnancy. Here we investigated whether there is local expression and function of relaxin and relaxin receptor in arteries of nonpregnant females and males. Relaxin-1 and its major receptor, Lgr7, mRNA are expressed in thoracic aortas, small renal and mesenteric arteries from mice and rats of both sexes, as well as in small renal arteries from female tammar wallabies (an Australian marsupial). Using available antibodies for rat and mouse Lgr7 receptor and rat relaxin, we also identified protein expression in arteries. Small renal arteries isolated from relaxin-1 gene-deficient mice demonstrate enhanced myogenic reactivity and decreased passive compliance relative to wild-type (WT) and heterozygous mice. Taken together, these findings reveal an arterial-derived, relaxin ligand-receptor system that acts locally to regulate arterial function.

Animals↗

PubMed2005

Relaxin increases cardiac output and reduces systemic arterial load in hypertensive rats.

Chronic administration of recombinant human relaxin (rhRLX) to conscious, normotensive rats (male and female) increases cardiac output (CO) and global arterial compliance (ACg) and reduces systemic vascular resistance (SVR) with no change in mean arterial pressure (MAP). Effects (magnitude and temporal pattern) of relaxin on systemic hemodynamics and arterial properties in hypertensive animal models are not known. Accordingly, the major goal of the present study was to determine the cardiovascular effects of rhRLX in hypertensive rats using 2 models: Long-Evans rats chronically administered angiotensin II (AII) and spontaneously hypertensive rats (SHR). CO and systemic arterial load, as quantified by SVR and ACg, were obtained using methods reported previously by us. In rats with AII-induced hypertension, acute rhRLX administration (up to 6 hours) significantly increased CO and ACg (24.9+/-3.9 and 34.3+/-12.6% above baseline, respectively) and significantly decreased SVR (17.2+/-3.5%) without changing MAP. In contrast, acute rhRLX administration to SHR and normotensive rats for up to 6 hours failed to produce any significant changes in CO, ACg, SVR, or MAP. However, chronic rhRLX administration (1 to 7 days) to SHR yielded significant changes (24.0+/-8.1 and 22.3+/-6.6% increases in CO and ACg, respectively, and a 13.3+/-5.3% decrease in SVR, with no change in MAP). In conclusion, rhRLX increases CO and reduces arterial load in hypertensive rats without reducing MAP. However, the time course of response to rhRLX treatment is dependent on the model of hypertension such that rats characterized by AII-mediated hypertension responded more rapidly to rhRLX administration than SHR.

Angiotensin II↗

PubMed2005

Evidence against the hypothesis that endothelial endothelin B receptor expression is regulated by relaxin and pregnancy.

The endothelial endothelin B (ET(B)) receptor subtype is critical for renal vasodilation induced by relaxin in nonpregnant rats and during pregnancy (the latter via endogenous circulating relaxin). Here we tested whether expression of vascular ET(B) receptor protein is regulated by relaxin. Small renal arteries were harvested from virgin and midterm pregnant rats as well as nonpregnant rats that were administered recombinant human relaxin (rhRLX) at 4 mug/h or vehicle for 5 d or 4-6 h. Small renal arteries dissected from additional virgin rats were incubated in vitro with rhRLX or vehicle for 3 h at 37 C. ET(B) expression was also evaluated in cultured human endothelial cells: aortic, coronary, umbilical vein, and dermal microvascular endothelial cells. Cells were incubated for 4, 8, or 24 h with rhRLX (5, 1, or 0.1 ng/ml) or vehicle. ET(B) protein expression in arteries and cells was evaluated by Western analysis. No regulation of ET(B) expression was observed in small renal arteries in any of the experimental protocols, nor was there an increase in the vasorelaxation response to ET-3 in small renal arteries incubated in vitro with rhRLX. rhRLX only sporadically altered ET(B) expression in human coronary artery endothelial cells and human umbilical vein endothelial cells at certain time points or doses, and no regulation was observed in human aortic endothelial cells or human dermal microvascular endothelial cells. These results suggest that regulation of ET(B) receptor protein has little or no role in relaxin stimulation of the endothelial ET(B)/nitric oxide vasodilatory pathway.

Animals↗

PubMed2005

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.

Animals↗

PubMed2005

Recombinant human relaxin (rhRLX) modifies systemic arterial properties in conscious rats irrespective of gender, but in a biphasic fashion.

Chronic administration of recombinant human relaxin (rhRLX) to conscious female nonpregnant rats that reaches serum concentrations of 10-30 ng/mL increases cardiac output and reduces systemic arterial load comparable to levels observed in midterm pregnancy. Chronic administration of the hormone to male rats increases cardiac output and reduces systemic arterial load to a similar extent. Short-term or chronic administration of rhRLX to conscious female rats that reaches serum concentrations of approximately 80 ng/mL results in minimal and insignificant changes. We conclude that: (1) rhRLX increases cardiac output and reduces arterial load irrespective of gender, and (2)the rhRLX dose response is biphasic.

Animals↗

PubMed2004

Effects of relaxin on systemic arterial hemodynamics and mechanical properties in conscious rats: sex dependency and dose response.

We previously showed that chronic administration of recombinant human relaxin (rhRLX; 4 microg/h) to conscious female, nonpregnant rats to reach serum levels corresponding to early to midgestation (approximately 20 ng/ml) increases cardiac output (CO) and global arterial compliance (AC) and decreases systemic vascular resistance (SVR), comparable to changes observed in midterm pregnancy. The goals of this study were to test whether chronic administration of rhRLX (4 microg/h) to conscious male rats will yield similar changes in CO and systemic arterial load and to determine whether higher infusion rates of rhRLX (50 microg/h) administered to nonpregnant female rats yielding serum concentrations corresponding to late pregnancy ( approximately 80 ng/ml) will further modify CO and SVR and global AC comparable to late gestation. CO and systemic arterial load, as quantified by SVR and AC, were obtained by using the same methods as in our previous studies. With respect to baseline, chronic rhRLX administration to male rats over 10 days at 4 mug/h increased both CO (20.5 +/- 4.2%) and AC (19.4 +/- 6.9%) and reduced SVR (12.7 +/- 3.9%). These results were comparable to those elicited by the hormone in nonpregnant female rats. In contrast, neither acute (over 4 h) nor chronic (over 6 days) infusion of the higher dose of rhRLX administered to conscious female rats resulted in significant changes in CO, AC, or SVR from baseline. We conclude that 1) rhRLX increases CO and AC and reduces SVR irrespective of sex, and 2) the rhRLX dose response is biphasic insofar as significant alterations in CO and systemic arterial load fail to occur at high serum concentrations.

Animals↗

PubMed2004

Renal handling of homocysteine during normal pregnancy and preeclampsia.

OBJECTIVE: Maternal plasma homocysteine decreases in normal pregnancy and is significantly increased in preeclampsia. The goal of this study was to investigate the role of the maternal kidney in the changes of plasma homocysteine during normal pregnancy and preeclampsia. METHODS: Plasma and 24-hour urine samples were collected in the same women before, during (first, second, and third trimesters), and after normal pregnancy; and in a separate cross-sectional study of normal pregnant, preeclamptic and nonpregnant women and homocysteine concentrations were measured. RESULTS: Longitudinally, maternal plasma homocysteine decreased significantly by the first trimester compared with prepregnancy and postpartum levels (5.6 +/- 1.8 versus 6.8 +/- 0.5 and 7.4 +/- 0.4 microM, respectively, P<.05 by analysis of variance) and paralleled a significant increase in the renal clearance of homocysteine (2.9 +/- 0.4 versus 1.8 +/- 0.2 and 1.6 +/- 0.2 L/24 hours, respectively, P<.001). In addition, plasma homocysteine was significantly elevated in preeclampsia compared with normal pregnancy (4.4 +/- 0.6 versus 3.2 +/- 0.2 microM, P<.04); however, renal clearance was not different (1.2 +/- 0.1 versus 1.0 +/- 0.1 L/24 hours, P=.55). CONCLUSION: Increases in renal clearance contribute to the decrease in plasma homocysteine during normal pregnancy. However, changes in renal handling do not appear to contribute to the increase in plasma homocysteine in preeclampsia.

Cross-Sectional Studies↗

PubMed2004

Nitric oxide synthase isoforms in the rat kidney during pregnancy.

OBJECTIVE: Nitric oxide mediates renal vasodilation and hyperfiltration during pregnancy in conscious rats through the endothelin B (ETB) receptor subtype. We tested the hypothesis that immunoreactive levels of endothelial nitric oxide synthase (eNOS) would be greater in the kidneys of midterm pregnant rats compared with virgin rats. METHODS: We studied midterm pregnancy because renal plasma flow and glomerular filtration rate are maximal at this gestational stage. Western analysis was used to determine the level of eNOS in the three major zones of the kidney-inner medulla, outer medulla, and cortex-and in isolated small renal arteries, and in purified renal microvessels from the cortex. RESULTS: There were no significant differences in eNOS expression between virgin and midterm pregnant rats in any of those renal tissues, regardless of whether immunoreactivity was expressed as arbitrary densitometry units, as "microg placental equivalents" interpolated from the linear portion of a dose-response curve of placental villous protein (2.5-30 microg, positive control) run concurrently on each gel, or normalized for beta-actin. We also investigated other NOS isoforms. In particular, immunoreactive neuronal NOS (nNOS) was detectable in the inner and outer medulla, but it was not significantly different between groups. nNOS immunoreactivity was below the level of detection in the cortex, but mRNA expression was not significantly different between pregnant and virgin rats by reverse transcriptase polymerase chain reaction. CONCLUSION: Our results suggest that an increase in eNOS isoform mass does not contribute to the endothelin and NO-dependent renal vasodilation in rat gestation.

Animals↗

PubMed2004

Mechanisms of renal vasodilation and hyperfiltration during pregnancy.

Glomerular filtration rate (GFR) and renal plasma flow (RPF) increase by 40-65% and 50-85%, respectively, during normal pregnancy in women. Studies using the gravid rat as a model have greatly enhanced our understanding of mechanisms underlying these remarkable changes in the renal circulation during gestation. Hyperfiltration appears to be almost completely due to the increase in RPF, the latter attributable to profound reductions in both the renal afferent and efferent arteriolar resistances. The major pregnancy hormone involved is relaxin. The mediators downstream from relaxin include endothelin (ET) and nitric oxide (NO). New evidence indicates that relaxin increases vascular gelatinase activity during pregnancy, thereby converting big ET to ET(1-32), which leads to renal vasodilation, hyperfiltration, and reduced myogenic reactivity of small renal arteries via the endothelial ET(B) receptor and NO. Whether the chronic volume expansion characteristic of pregnancy contributes to the maintenance of gestational renal changes requires further investigation. Additional studies are also needed to further delineate the molecular basis of these mechanisms and, importantly, to investigate whether they apply to women.

Endothelins↗

PubMed2004

New aspects in the pathophysiology of preeclampsia.

Preeclampsia, the de novo occurrence of hypertension and proteinuria after the 20th week of gestation, continues to exert an inordinate toll on mothers and children alike. Recent clinical trials, new physiologic insights, and novel observations on pathogenesis have altered the thinking about preeclampsia. The mechanisms surrounding relaxin and its effects on the circulation and on matrix metalloproteinases have been elucidated. The growth factor's receptor, fms-like tyrosine kinase 1, has been shown to exist in a soluble form that is able to inactivate vascular endothelial-derived growth factor and human placental growth factor. Compelling evidence has been brought forth suggesting that fms-like tyrosine kinase 1 is a circulating factor that can cause preeclampsia. Preeclamptic women have high circulating levels of asymmetric dimethyl arginine that could account for the generalized endothelial dysfunction observed in preeclampsia. Preeclamptic women also produce novel autoantibodies that may serve to activate angiotensin receptors. These new observations raise the possibility that the treatment of preeclamptic women will soon be improved.

Angiogenic Proteins↗

PubMed2004

Emerging role of relaxin in renal and cardiovascular function.

Although traditionally associated with reproductive processes, relaxin is emerging as an important player in renal and cardiovascular function. Much of our recently acquired understanding of relaxin in this new context has arisen from studies of maternal renal and cardiovascular adaptations to pregnancy in rats where the hormone is turning out to be an important mediator. First, we highlight the influence of relaxin on renal hemodynamics and glomerular filtration rate, as well as on other peripheral circulations. Second, we discuss the effect of relaxin on both the steady and pulsatile systemic arterial load, as well as on the heart, in particular, coronary blood flow. Third, we consider the impact of the hormone on cultured endothelial and vascular smooth muscle cells. Fourth, we address the interaction of relaxin with renal and cardiac disease, as well as its role in angiogenesis. Finally, in Perspectives, we point out several key research questions in need of investigation that relate to a potential autocrine/paracrine role of relaxin in renal and cardiovascular tissues. Furthermore, on the basis of its potent vasodilatory and matrix-degrading attributes, we speculate about the therapeutic potential of relaxin in renal and cardiovascular diseases.

Animals↗

PubMed2004

Relaxin modifies systemic arterial resistance and compliance in conscious, nonpregnant rats.

Relaxin emanates from the corpus luteum of the ovary and circulates during pregnancy. Because the hormone is a potent renal vasodilator and mediates the renal vasodilation and hyperfiltration of pregnancy in conscious rats, we reasoned that it might also contribute to the broader cardiovascular changes of pregnancy. We began investigating this concept by testing whether relaxin can modify systemic arterial hemodynamics and load when chronically administered to nonpregnant rats. The major objectives of the present work were to determine whether relaxin administration to nonpregnant rats 1) modifies cardiac output (CO), systemic vascular resistance, and global arterial compliance (AC), and 2) regulates the passive mechanics of isolated arteries. To accomplish the first objective, we developed a conscious rat model for assessment of global AC. Passive mechanics of small renal arteries were assessed using a pressure arteriograph. Chronic administration of recombinant human relaxin by sc osmotic minipump to conscious, female, nonpregnant rats reduced the steady arterial load by decreasing systemic vascular resistance, increased CO, and reduced the pulsatile arterial load by increasing global AC as quantified by two indices-AC estimated from the diastolic decay of aortic pressure and CO and AC estimated by the ratio of stroke volume-to-pulse pressure. In another group of rats, relaxin administration also regulated the passive mechanics of small renal arteries, indicating that, in addition to reduction in vascular smooth muscle tone, modification of the vascular structure (e.g. extracellular matrix) contributes to the increase in global AC. These findings suggest a role for relaxin in the systemic hemodynamic changes of pregnancy, as well as novel therapeutic potential for relaxin in modifying arterial stiffness and cardiac afterload.

Animals↗