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DEVELOPMENTAL PHYSIOLOGY AND PREGNANCY

Regional differences in myosin heavy chain isoform expression and maximal shortening velocity of the rat vaginal wall smooth muscle

¹Division of Urology, University of Pennsylvania, ²Department of Pharmacology and Physiology, Drexel University College of Medicine, and ³Department of Pathobiology, University of Pennsylvania, Philadelphia, Pennsylvania

Submitted 15 February 2006 ; accepted in final form 1 May 2006

Contractility of the proximal and distal vaginal wall smooth muscle may play distinct roles in the female sexual response and pelvic support. The goal of this study was to determine whether differences in contractile characteristics of smooth muscle from these regions reside in differences in the expression of isoforms of myosin, the molecular motor for muscle contraction. Adult female Sprague-Dawley rats were killed on the day of estrus, and the vagina was dissected into proximal and distal segments. The V_max at peak force was greater for tissue strips of the proximal vagina compared with that of distal (P < 0.01), although, at steady state, the V_max for the muscle strips from the two regions was not different. Furthermore, at steady state, muscle stress was higher (P < 0.001) for distal vaginal strips (n = 5). Consistent with the high V_max for the proximal vaginal strips, RT-PCR results revealed a higher %SM-B (P < 0.001) in the proximal vagina. A greater expression of SM-B protein (P < 0.001) was also detected by Western blotting (n = 4). Interestingly, there was no regional difference noted in SM-1/SM-2 isoforms (n = 6). The proximal vagina had a higher expression of myosin heavy chain protein (P < 0.01) and a greater percentage of smooth muscle bundles (P < 0.001). The results of this study are the first demonstration of a regional heterogeneity in V_max and myosin isoform distribution in the vagina wall smooth muscle and confirm that the proximal vaginal smooth muscle exhibits phasic contractile characteristics compared with the distal vaginal smooth muscle, which is tonic.

vagina; smooth muscle; female sexual response