Contractile response of human anterior vaginal muscularis in women with and without pelvic organ prolapse

Abstract

The aim of this study was to compare the contractility of the anterior vaginal muscularis (AVM) from women with and without pelvic organ prolapse (POP). In vitro experiments were performed to measure the peak force generated in response to potassium chloride (KCl; 125 mmol/L) and phenylephrine by AVM tissue from women with and without POP. Cross-sectional areas and co-localization of α(1A) adrenergic receptor protein with smooth muscle α-actin in AVM strips were determined by histology and immunofluorescence, respectively. There were no differences in the mean amplitude of force generated in response to KCl normalized to either wet weight or muscle cross-sectional area (mN/mm(2)) between women with and without POP (P > .30). However, AVM from women with prolapse produced a significantly higher mean force to KCl normalized to total cross-sectional area compared to controls (P = .007). While the control samples demonstrated a consistent response to phenylephrine, there was no response to this stimulant generated by AVM tissue from women with POP. The proportion of co-localized α(1A) adrenergic receptors with smooth muscle α actin in AVM tissue was significantly less in women with POP compared to normal controls (P < .0001). Although there was significantly greater tissue stress generated by AVM from women with prolapse compared to controls, there were no differences in muscle stress. Absent response to phenylephrine by AVM from women with prolapse may be related to a lower expression of α(1A) adrenergic receptors in vaginal smooth muscle.

Figure 1.

Representative Masson trichome staining of 5 μm sections (magnification ×40) of a (A) full thickness vaginal biopsy of the anterior vaginal wall from a control patient used to measure vaginal wall thickness, (B) a cross-section of an anterior vaginal muscularis (AVM) strip from a control patient, and (C) a cross-section of an AVM strip from a woman with prolapse. (B) and (C) AVM strips were used to measure total and smooth muscle cross-sectional areas (mm²). Red arrows: blood vessel; black arrows: vaginal smooth muscle; white bracket: vaginal muscularis layer.

Figure 2.

Representative anterior vaginal muscularis (AVM) force tracing. (A) In an AVM strip from a control patient, there was a consistent response to phenylephrine, an α₁ adrenergic agonist. (B) There was no response generated by phenylephrine by a representative AVM strip from a patient with prolapse. For each strip in the prolapse group, the response to KCl (125 mmol/L) stimulation after phenylephrine (arrow in B) indicated that the prolapse smooth muscle remained viable and able to generate force in response to a membrane-independent agonist.

Figure 3.

Representative anterior vaginal muscularis (AVM) strip 5 μm sections confirming the presence of α_1A adrenergic receptor in vaginal smooth muscle from women without (A-C) and with (D-F) pelvic organ prolapse (magnification ×600). In the control AVM, smooth muscle α actin-FITC is shown in green (A), α_1A adrenergic receptor-CY3 is shown in red (B), and co-localization of the α_1A adrenergic receptor to smooth muscle α actin is shown in orange, and nuclear staining with DAPI shown in blue (C). In an AVM from a patient with prolapse, smooth muscle α-actin (D) and α_1A adrenergic receptor (E) were identified. Although, the intensity of staining of α_1A adrenergic receptor was diminished in the prolapse AVM compared to controls, α_1A adrenergic receptor was similarly co-localized with smooth muscle α actin in each strip (nuclear staining with DAPI in blue) (F). FITC indicates fluorescein isothiocyanate; DAPI, 4’,6-diamidino-2-phenylindole.