Relaxin inhibits patellar tendon healing in rats: a histological and biochemical evaluation

Abstract

Background: Female patients are more likely to have tendon injuries than males, especially those who has a higher concentration of relaxin. Previous studies have demonstrated that relaxin attenuates extracellular matrix (ECM) formation. However, the mechanism of relaxin on tendon repair remains unclear. We hypothesize that relaxin inhibits tendon healing by disrupting collagen synthesis.

Methods: A patellar tendon window defect model was established using Sprague-Dawley rats. The center of the patellar tendon was removed from the patella distal apex and inserted to the tibia tuberosity in width of 1 mm. Then, the rats were injected with saline (0.2 μg/kg/day) or relaxin (0.2 μg/kg/day) for two and four weeks, which was followed by biomechanical analysis and histological and histochemical examination.

Results: Mechanical results indicated that relaxin induces a significant decrease in tear resistance, stiffness, and Young's modulus compared to those rats without relaxin treatment. In addition, it was shown that relaxin activates relaxin family peptide receptor 1(RXFP1), disturbs the balance between matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteases (TIMPs), and reduces the deposition of collagen in injury areas.

Conclusions: Relaxin impairs tendon healing in rats. Also, relaxin might lead to tendon injury more commonly for females than males.

Keywords: Extracellular matrix (ECM); Female; Relaxin; Relaxin family peptide receptor 1(RXFP1); Tendon healing.

Fig. 1

A 1 × 4 mm window defect was created in patellar tendon of rat

Fig. 2

Histopathological and immunohistochemical findings of repaired tendons in the vehicle and relaxin groups. a: H&E and Masson staining. b: Histological scores at the 2nd and 4th postoperative weeks. c: Immunohistochemical staining of collagen I and collagen III in the tendon wound areas in the 2nd and 4th postoperative weeks. *p < 0.05 vs. native tendon, #p < 0.05 compared with vehicle group. (Scale bar = 200 μm). d: Relative expression levels collagen I and collagen III in relaxin group compared with vehicle group. *p < 0.05 compared with vehicle group

Fig. 3

Relaxin activates RXFP1 expression. a: Immunohistochemical staining of RXFP1 in the tendon wound areas in the 2nd and 4th postoperative weeks. b: Quantification of RXPF1 positive cells expression in the tendon wound areas. *p < 0.001 compared with vehicle group. (Scale bar = 200 μm)

Fig. 4

Relaxin disrupts the balance between MMPs and TIMP. a: Immunohistochemical staining of TIMP1, MMP1, MMP9, and MMP13 in the tendon wound areas in the 2nd and 4th postoperative weeks. (Scale bar = 200 μm). b: Relative expression of MMP1, MMP9, MMP13 and TIMP1 in tendon wound areas. MMP9 and MMP13 shows a significant difference in the 2nd and 4th postoperative weeks. *p < 0.001 compared with vehicle group