Relaxin induces rapid dilation of rodent small renal and human subcutaneous arteries via PI3 kinase and nitric oxide

The peptide hormone relaxin is a potent vasodilator with therapeutic potential in diseases complicated by vasoconstriction, including heart failure. However, the molecular mediators and magnitude of vasodilation may vary according to duration of exposure and artery type. The objective of these studies was to determine mechanisms of rapid (within minutes) relaxin-induced vasodilation and to examine whether relaxin dilates arteries from different animal species and vascular beds. Rat and mouse small renal, rat mesenteric, and human sc arteries were isolated, mounted in a pressure arteriograph, and treated with recombinant human relaxin (rhRLX; 1-100 ng/ml) after preconstriction with phenylephrine. Rat and mouse small renal as well as human sc arteries dilated in response to rhRLX, whereas rat mesenteric arteries did not. Endothelial removal or pretreatment with l-N(G)-monomethyl arginine (L-NMMA) abolished rapid relaxin-induced vasodilation; phosphatidylinositol-3-kinase (PI3K) inhibitors also prevented it. In cultured human endothelial cells, rhRLX stimulated nitric oxide (assessed using 4-amino-5-methylamino-2'7'-difluorofluorescein) as well as Akt and endothelial NO synthase (eNOS) phosphorylation by Western blotting but not increases in intracellular calcium (evaluated by fura-2). NO production was attenuated by inhibition of Gα(i/o) and Akt (using pertussis toxin and the allosteric inhibitor MK-2206, respectively), PI3K, and NOS. Finally, the dilatory effect of rhRLX in rat small renal arteries was unexpectedly potentiated, rather than inhibited, by pretreatment with the vascular endothelial growth factor receptor inhibitor SU5416. We conclude that relaxin rapidly dilates select arteries across a range of species. The mechanism appears to involve endothelial Gα(i/o) protein coupling to PI3K, Akt, and eNOS but not vascular endothelial growth factor receptor transactivation or increased calcium.

Relaxin regulates vascular wall remodeling and passive mechanical properties in mice

Administration of recombinant human relaxin (rhRLX) to conscious rats increases global arterial compliance, and small renal arteries (SRA) isolated from these rats demonstrate increased passive compliance. Here we characterize relaxin-induced vascular remodeling and examine its functional relevance. SRA and external iliac arteries (EIA) were examined in rhRLX-treated (1.0 μg/h for 5 days) and relaxin knockout mice. Arterial geometric remodeling and compositional remodeling were quantified using immunohistochemical and biochemical techniques. Vascular mechanical properties were quantified using an ex vivo preparation wherein pressure-diameter data were obtained at various axial lengths. Compared with vehicle-treated mice, SRA from rhRLX-treated mice showed outward geometric remodeling (increased unstressed wall area and wall-to-lumen area ratio), increased smooth muscle cell (SMC) density, reduction in collagen-to-total protein ratio, and unchanged elastin-to-tissue dry weight ratio. Compared with wild-type mice, relaxin knockout mice exhibited the opposite pattern: decreased unstressed wall area and wall-to-lumen area ratio, decreased SMC density, and increased collagen-to-total protein ratio. Although tissue biaxial strain energy of SRA was not different between rhRLX- and vehicle-treated groups at low-to-physiological circumferential and axial strains, it was lower for the rhRLX-treated group at the highest circumferential strain. In contrast to SRA, relaxin administration was not associated with any vascular remodeling or changes in passive mechanics of EIA. Thus relaxin induces both geometric and compositional remodeling in vessel-specific manner. Relaxin-induced geometric remodeling of SRA is responsible for the increase in passive compliance under low-to-physiological levels of circumferential and axial strains, and compositional remodeling becomes functionally relevant only under high circumferential strain.

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.