Strain-dependent urethral response

Abstract

Aims: The Sprague-Dawley (SD) rat, an out-bred, all-purpose strain, has served well for lower urinary tract research. However, to test new cellular therapies for conditions such as stress urinary incontinence, an in-bred rat strain with immune tolerance, such as the Lewis rat, may be more useful. The objective of this study was to reveal any differences in lower urinary tract continence mechanisms between the Lewis and SD rat.

Methods: The contribution of (1) the striated and smooth muscle to the mechanical and functional properties of the urethra in vitro, and (2) the striated sphincter to leak point pressure (LPP) and reflex continence mechanisms in vivo were assessed in normal (control) Lewis and SD rats and in a model of stress urinary incontinence produced by bilateral pudendal nerve transection.

Results: Control, Lewis rats had significantly lower LPP, significantly less fast-twitch skeletal muscle and relied less on the striated sphincter for continence than control, SD rats, as indicated by the failure of neuromuscular blockade with alpha-bungarotoxin to reduce LPP. Nerve transection significantly decreased LPP in the SD rat, but not in the Lewis rat. Although the Lewis urethra contained more smooth muscle than the SD rat, it was less active in vitro as indicated by a low urethral baseline pressure and lack of response to phenylephrine.

Conclusions: We have observed distinct differences in functional and mechanical properties of the SD and Lewis urethra and have shown that the Lewis rat may not be suitable as a chronic model of SUI via nerve transection.

Fig. 1

A: Vertical leak point pressure (V-LPP) for both Lewis (n = 3) and Sprague–Dawley (n = 3) rats in continent and SUI states. B: Supine leak point pressure (Su-LPP) for both Lewis (n = 7) and Sprague–Dawley (n = 4) rats before and after injection with alpha-bungarotoxin. Data shown as average ± SEM, ^*P < 0.05.

Fig. 2

Change in urethral pressure response (increase from baseline; A-URS) during sneeze, urethral baseline pressure (UBP) and change in abdominal pressure (ΔP_abd) for both Lewis (n = 8) and Sprague–Dawley (n = 10) rats. Data shown as average ± SEM, ^*P < 0.05.

Fig. 3

A,C: Baseline and (B,D) passive pressure–diameter curves from the proximal and middle portions of continent Lewis (n = 4) and Sprague–Dawley (n = 6) rats. Data shown as average ± SEM, no significant differences were noted.

Fig. 4

Responses of the middle urethra of continent Lewis (n = 5) and Sprague-Dawley (n = 6) rats to various agonists and antagonists (100 μM Nω-nitro-L-arginine, 40 μM phenylephrine, 5 mM acetylcholine and 3 mM EDTA). Responses are represented as percentage change in outer diameter from the outer diameter just prior to a change in pressure or administration of each drug. Data were digitized at rate of 1 Hz with 30 minute equilibrations at each condition. Values reported as average ± SEM, ^*P < 0.05.

Fig. 5

Percent of the total cross-sectional area staining positive for (A) smooth muscle actin. B: Fast twitch skeletal muscle, or (C) total cross-sectional area of both the Lewis (n = 7) and Sprague–Dawley (n = 4–7) urethra. Data shown as average ± SEM, ^*P < 0.05.