1994 Volume 85 Issue 10 Pages 1534-1542
We studied the difference in the proportion of fast and slow myosin subunits in axially subdivided external urethral sphincter of male rabbit using myosin light chain and heavy chain analyses. The whole urethras from 6 adult male Japanese White rabbits were sagittally bisected and one halves from all animals were processed for myosin light chain analysis and another halves from all animals for myosin heavy chain analysis. Each half urethra containing the external urethral sphincter was subdivided into 4 parts, namely prostatic (P), prostatic apical (PA), infraprostatic (IP) and bullbourethral glandular (BUG) portions, from cranial to caudal direction. The electrophoretic samples from 4 different parts were separately processed. The external urethral sphincter muscle in each part from all animals were processed together for a sample because of the minute amount of the muscle. Two-dimensional electrophoresis according to the procedure of O'Farrell for myosin light chains and SDS-polyacrylamide gel electrophoresis containing 40% glycerol for myosin heavy chains were carried out. Molar ratio of myosin subunits were estimated by the dye elution procedure. The relative proportions of slow myosin molecules in respective parts were 33.4% (P), 26.3% (PA), 18.5% (IP) and 11.0% (BUG) from myosin light chain analysis and 20.3% (P), 16.1% (PA), 7.2% (IP) and 5.0% (BUG) from myosin heavy chain analysis. The urethral striated musculature in male rabbit was predominantly composed of fast myosin subunits as a whole. But the proportion of slow myosin subunits occupied a relatively high percentage in the proximal region and tended to decrease toward the distal end. From these study we infer that slow twitch fibers in the proximal region are likely responsible for continence at rest and that fast twitch fibers of the whole of external urethral sphincter are recruited in stress conditions, for example, during coughing or sneezing.