Heterogeneity of bladder myocytes in vitro: modulation of myosin isoform expression

We studied the expression of myosin heavy chain isoforms differing at the N -terminal (SMA, SM-B) and the C-terminal (SM1, SM2) regions and non-muscle myosin heavy chain II-A and II-B (NMMHC II-A and B) in newborn and adult ra bbit bladder smooth muscle cells (SMCs) and in cultures of enzymatically di ssociated neonatal detrusor, RT-PCR analyses revealed that 94.5 +/-3.27% of MHC transcripts of the adult bladder SMCs contained the 21-nucleotide inse rt (SM-B) compared with 83.8 +/-3.2% in the newborn bladder, with the remai nder of the mRNA being non-inserted (SM-A), In 3, 7, and 10 days of primary culture (proliferating, confluent, and post-confluent, respectively) and u p to 4 subculture passages, bladder myocytes expressed predominantly SM-A. Immunofluorescence microscopy revealed heterogeneity in cultured myocytes, i.e. SM-B positive cells coexisting with negatively stained cells. In adult bladder, the C-terminal isoforms SM1 and SM2 represented, 43.1 +/-4.3% and 56.89 +/-4.3% of the mRNA, respectively, while newborn bladders expressed 72.5 +/-7% SM1 and 27.5 +/-7% SM2, Upon culturing, cells predominantly expr essed SM1 at both the mRNA and protein levels. NMMHC Ii-A was expressed by both adult and newborn bladders and in culture, whereas NMMHC II-B was expr essed at low levels only in newborn bladders, but upregulated in culture. T hese data indicate that bladder myocytes in vitro undergo modulation with r elative overexpression of SM-A and SM1 and upregulation of NMMHC II-B, Info rmation on the mechanisms responsible for this modulation in vitro might pr ovide an understanding of the nature of altered myosin isoform expression a ssociated with smooth muscle dysfunction in certain bladder diseases. (C) 2 001 Harcourt Publishers Ltd.