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.

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.

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.