Regional variation in myosin isoforms and phosphorylation at the resting tone in urinary bladder smooth muscle

Urinary bladder filling and emptying requires coordinated control of bladder body and urethral smooth muscles. Bladder dome, midbladder, base, and urethra showed significant differences in the percentage of 20-kDa myosin light chain (LC₂₀) phosphorylation (35.45 ± 4.6, 24.7 ± 2.2, 13.6 ± 2.1, and 12.8 ± 2.7%, respectively) in resting muscle. Agonist-mediated force was associated with a rise in LC₂₀ phosphorylation, but the extent of phosphorylation at all levels of force was less for urethral than for bladder body smooth muscle. RT-PCR and quantitative competitive RT-PCR analyses of total RNA from bladder body and urethral smooth muscles revealed only a slight difference in myosin heavy chain mRNA copy number per total RNA, whereas mRNA copy numbers for NH₂-terminal isoforms SM-B (inserted) and SM-A (noninserted) in these muscles showed a significant difference (2.28 × 10⁸ vs. 1.68 × 10⁸ for SM-B and 0.12 × 10⁸ vs. 0.42 × 10⁸ for SM-A, respectively), which was also evident at the protein level. The ratio of COOH-terminal isoforms SM2:SM1 in the urethra was moderately but significantly lower than that in other regions of the bladder body. A high degree of LC₂₀ phosphorylation and SM-B in the bladder body may help to facilitate fast cress-bridge cycling and force generation required for rapid emptying, whereas a lower level of LC₂₀ phosphorylation and the presence of a higher amount of SM-A in urethral smooth muscle may help to maintain the high basal tone of urethra, required for urinary continence.