ESTROGEN-DEPENDENT EXPRESSION OF THE SM2 SMOOTH MUSCLE-TYPE MYOSIN ISOFORM IN RABBIT MYOMETRIUM

Ovarectomized rabbits displayed a decreased SM1 to SM2 ratio of smooth muscle-type myosin heavy chain isoforms compared to unoperated, virgi n females which was reversed after 17 beta-oestradiol administration t o a value similar to that of control animals. When this steroid was gi ven to sexually immature animals or to adult virgin rabbits, SM2 expre ssion was not induced, as also happened with proliferating myometrial smooth muscle cells grown in vitro. In growing rabbit, the 17 beta-oes tradiol administration induced the formation of the circular and the l ongitudinal muscle layers, characteristics of sexually competent femal es. The SM2 isoform was up-regulated during postnatal development and the SM1 to SM2 ratio changed during pregnancy and post-partum period b ut not with human gonadotropin treatment which increases the level of circulating progesterone. Immunofluorescence staining of adult myometr ium with anti-SM2 antibody indicated that this isoform is localized to the longitudinal layer exclusively and, in contrast to the circular l ayer, its expression was independent of oestrogen level. Difference in oestrogen sensitivity between the two layers was also detected for th e expression of the intermediate filament protein vimentin and the thi n filament protein calponin. Changes of SM2 expression in the myometri um correlated with variations in the oestrogen receptor density as als o confirmed by decreased SM2 content/oestrogen receptor density in the circular layer when ovarectomized females were treated with the oestr ogen antagonist ICI 182,780. Our results indicate that: (1) a specific distribution of myosin heavy chain exists within rabbit myometrium, a nd (2) SM2 myosin expression in this smooth muscle is under oestrogen control.