P-321 Uterine fibroids secrete vasodilatory factors that induce their response via the opioid pathway
Authors: Hill C , Yasin S , Whitmore J , Tempest N , Fakhreldin M , Oktaba M , Rainbow R , Hapangama D 
Published: July 2024

Study question

Do uterine fibroids produce vasoactive factors that could affect vasculature and induce fibroid associated symptoms?

Summary answer

Ex vivo secretions from uterine fibroids induce a vasodilatory response in rat mesenteric arteries that is mediated via the opioid receptor pathway.

What is known already

Uterine fibroids (leiomyoma) are a common but clinically neglected gynaecological condition, where benign smooth muscle tumours arise from the myometrium and cause heavy menstrual bleeding (HMB), pain and infertility. Fibroids affect up to one in two women of reproductive age and are the main reason for failure of both medical and surgical treatments in women with HMB. Fibroids are proposed to affect the systemic vasculature system, with angiogenic growth factors and abnormal vasculature implicated in fibroid pathophysiology. However, the precise mechanism of fibroid induced HMB has yet to be fully elucidated.

Study design, size, duration

Fibroid and myometrium biopsies were prospectively collected from 13 patients undergoing MyoSure, myomectomy or hysterectomy for the treatment of HMB associated with fibroids at Liverpool Women’s Hospital, a tertiary referral centre. The collection and use of human tissue was ethically approved, and all participants gave informed written consent. Tissue biopsies were incubated in a basal medium for 24 hours. Conditioned medium was collected and used for wire myography assays to examine vascular reactivity.

Participants/materials, setting, methods

Second order mesenteric arteries from female Wistar rats were mounted on a wire myograph system and toned with uridine triphosphate (UTP) before being exposed to control (non-conditioned), normal myometrium (n = 4) or fibroid (n = 9) conditioned medium. Vascular activity was measured relative to UTP induced vasoconstriction. To determine the mediatory factors of observed vasoreactivity, inhibitors of well characterised pathways were systematically tested in the presence of conditioned medium.

Main results and the role of chance

Fibroid conditioned medium caused a marked dilation of UTP induced vasoconstriction in rat mesenteric arteries, which was significantly greater than that induced by non-conditioned, or control myometrium conditioned medium. Blockage of ATP-sensitive potassium (KATP) channels with PNU37883A completely inhibited fibroid induced vasodilation. However, blockage of other potassium channels did not inhibit dilation, suggesting that a KATP-mediated PKA-dependent pathway is being stimulated. To test the potential involvement of endothelial-derived nitric oxide synthase in vasodilation, vessels were treated with the inhibitor L-NAME, which partially blocked fibroid induced vasodilation. The specificity of nitric oxide synthase inhibition was confirmed by treating vessels with acetylcholine or carbachol, which induced vasodilation via activation of nitric oxide synthase. In the presence of L-NAME, the vasodilatory effects of acetylcholine and carbachol were blocked, thus confirming inhibitor specificity. An adenosine receptor inhibitor, theophylline, had no effect on fibroid induced vasodilation, thus ruling out the adenosine pathway. Blockage of bradykinin and adrenergic receptors also failed to prevent fibroid induced dilation in mesenteric arteries. However, blocking opioid receptors with naloxone completely inhibited vasodilation in the presence of fibroid conditioned medium. These findings suggest that fibroid induced vasodilation is mediated via endothelial nitric oxide synthase and an unknown opioid compound.

Limitations, reasons for caution

This study was performed using excised tissue in an ex vivo environment, which may not fully simulate the fibroid secretome in vivo. Although rat mesenteric arteries are a well-established model, human uterine arteries may be more appropriate to study fibroid induced vasoreactivity.

Wider implications of the findings

Fibroids are responsible for up to half of all hysterectomies and healthcare costs run into the billions globally. This study demonstrates that fibroid secreted factors can impact vasoreactivity, which has important connotations for disease pathophysiology regarding HMB. Targeting vasoactive compounds produced by fibroids may be a future novel treatment option.

Trial registration number

NA