REGIONAL ALTERATIONS IN THE EXPRESSION OF SMOOTH MUSCLE MYOSIN ISOFORMS IN RESPONSE TO PARTIAL BLADDER OUTLET OBSTRUCTION

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Abstract
Smooth muscle (SM) myosin (SMM) isoform composition is altered in response to partial bladder outlet obstruction (PBOO). A recent study showed that during PBOO the upper dome region of the bladder is subjected to greater expansion pressure than the base and regional differences in contractility exist in the detrusor of PBOO rabbits. We hypothesized that alteration in SMM isoform composition in response to PBOO may show regional heterogeneity. Detrusor samples were obtained from 9 defined regions of the bladders from dysfunctional PBOO rabbits (greater than 30 voids per 24 hours) and sham operated adult New Zealand White rabbits. Reverse transcriptase-polymerase chain reaction, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting were used to determine the relative levels of SMM isoform expression at the mRNA and protein levels. Contractile responses to bethanechol and KCl were also determined. Myosin isoform expression was uniform throughout the detrusor from sham operated subjects with all regions expressing SM-B almost completely. However, in response to PBOO the dome region showed approximately 70% SM-B and 30% SM-A isoforms, whereas the base region expressed only 35% SM-B and, thus, 65% SM-A. This change also correlated with an approximately 2-fold higher protein level expression of SM-B in the dome region of PBOO rabbit bladders. Expression of the SMemb SMM isoform was significantly increased in PBOO rabbits at the mRNA and protein levels but only in the dome region. Regional differences in SMM isoform expression in the PBOO rabbit bladders correlated with altered contractility. Alteration in SMM isoform composition in response to PBOO shows regional heterogeneity and may be involved in the mechanism responsible for regional localized differences in detrusor contractility in PBOO rabbits.