Contractile and cytoskeletal proteins in smooth muscle during hypertrophy and its reversal.

Hypertrophy of rat urinary bladder smooth muscle was induced by partial urethral obstruction. Bladder weight increased from 70 to 240 mg after 10 days and to 700 mg after 7 wk. Removal of the obstruction after 10 days caused a regression of bladder weight to 130 mg. The relative volume of smooth muscle in the bladder wall increased during hypertrophy. The concentration of myosin in the smooth muscle cells decreased in 10-day hypertrophied bladders, whereas the concentration of actin was unchanged. The actin-myosin ratio was 2.3 in controls, 3.3 in 10-day obstructed bladders, and 2.9 in 7-wk obstructed bladders. After removal of obstruction, the ratio was normalized. Two isoforms of myosin heavy chains were identified (SM1 and SM2). The relative amount of SM2 decreased during hypertrophy. The relative proportion of actin isoforms (alpha, beta, and gamma) was altered toward more gamma and less alpha. These changes were reversible upon removal of the obstruction. Desmin was the dominating intermediate filament protein. The concentration of desmin and filamin increased in the hypertrophic bladders. The increased desmin-actin and filamin-actin ratios in obstructed bladders were normalized after removal of the obstruction. The results suggest that the turnover of contractile and cytoskeletal proteins is fast and can be regulated in response to changes in the functional demands in smooth muscle.