DECREASED EXPRESSION OF SMOOTH MUSCLE α-ACTIN RESULTS IN DECREASED CONTRACTILE FUNCTION OF THE MOUSE BLADDER

Abstract

Smooth muscle alpha-actin (SMalphaA) is an important actin isoform for functional contractility in the mouse bladder. Alterations in the expression of SMalphaA have been associated with a variety of bladder pathological conditions. Recently, a SMalphaA-null mouse was generated and differences in vascular tone and contractility were observed between wild-type and SMalphaA-null mice suggesting alterations in function of vascular smooth muscle. We used SMalphaA-null mice to explore the hypothesis that SMalphaA is necessary for normal bladder function. Reverse transcriptase polymerase chain reaction, Western blotting and immunohistochemical staining were used to confirm the absence of SMalphaA transcript and protein in the bladder of SMalphaA-null mice. In vitro bladder contractility compared between bladder rings harvested from wild-type and SMalphaA-null mice was determined by force measurement following electrical field stimulation (EFS), and exposure to chemical agonists and antagonists including KCl, carbachol, atropine and tetrodotoxin. Resulting force generation profiles for each tissue and agent were analyzed. There was no detectable SMalphaA transcript and protein expression in the bladder of SMalphaA-null mice. Nine wild-type and 9 SMalphaA-null mice were used in the contractility study. Bladders from SMalphaA-null mice generated significantly less force than wild-type mice in response to EFS after KCl. Similarly, bladders from SMalphaA-null mice generated less force than wild-type mice in response to pretreatment EFS, and EFS after carbachol and atropine, although the difference was not significant. Surprisingly, the bladders in SMalphaA-null mice appeared to function normally and showed no gross or histological abnormalities. SMalphaA appears to be necessary for the bladder to be able to generate normal levels of contractile force. No functional deficits were observed in the bladders of these animals but no stress was placed on these bladders. To our knowledge this study represents the first report to demonstrate the importance of expression of SMalphaA in force generation in the bladder.