['In smooth muscle (SM), myosin heavy chain (MHC) is expressed predominantly as two isoforms, SM1 and SM2, which are encoded by a single gene and expressed by alternative splicing mechanisms. Although functional differences of these isoforms are unknown, changes in SM1/SM2 ratio have been reported in various pathophysiologic conditions. We analyzed MHC composition of bladder detrusor SM from rabbits of different ages to determine whether SM1 and SM2 isoform expressions are developmentally regulated.', 'Rabbit bladders on the -11, -4, 1, 7, 14, 21, and 90th days of life were analyzed for SM MHC isoform expression at protein and mRNA levels. Porous sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), S1 protection assay, and histological analysis were employed.', 'The predominant MHC isoform in fetal and neonatal bladders was SM1. In the third postnatal week, the SM1/SM2 ratio decreased from 2.3 to 1.0. A stable SM1/SM2 ratio of 0.6 was observed in the adult animal. Although expression of SM1 mRNA was 2.6-fold greater than that of SM2 in the fetus, the relative amount of SM2 mRNA increased rapidly after birth and remained the predominant isoform throughout adult life. Developmental changes in relative amounts of SM1 and SM2 protein in bladder tissues were virtually identical to those of SM1 and SM2 mRNA. SM cell growth and disappearance of primitive mesenchyme from the bladder occurred concomitantly with the MHC isoform shift.'] Are smooth muscle myosin heavy chains developmentally regulated in the rabbit bladder?