SM2^<+/-> male mice are predisposed to develop urinary tract obstruction and hyper contractility of the bladder smooth muscle upon ageing

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We previously showed that complete loss of smooth muscle myosin heavy chain isoform 2 (SM2) resulted in postnatal lethality, but in het mice a partial loss of SM2 (SM2+/–) was accompanied by down-regulation of SM1 with unaltered SM2:SM1 ratio. To determine whether a normal bladder function would be maintained throughout its lifespan, we aged WT and SM2+/– mice up to 18 months and analyzed a) SM2:SM1 ratio b) bladder smooth muscle structure and c) function in SM2+/– het mice. A notable finding was that ~50% of 15–18 months old male SM2+/– mice exhibited urinary retention in bladder with the distention of upper urethra. In SM2+/– mouse bladder with urinary retention, the SM2:SM1 ratio was decreased but not in SM2+/– mouse bladder that did not develop urinary retention. Interestingly in the distended bladder the expression levels of α-actin and tropomyosin remained unaltered despite a reduction in the number of myosin thick filaments. These distended bladders showed hypersensitivity to submaximal K+ depolarization and M3-receptor stimulation, without a significant increase in myosin light chain phosphorylation. We therefore suggest that a partial loss of SM2 may predispose male mice to develop lower urinary tract obstruction during ageing. In addition our data suggest that bladder obstruction can cause a further reduction in SM2 expression and SM2:SM1 ratio, and a hyper-contractility of the bladder smooth muscle.
2011-08-01