ALTERNATIVE SPLICING AND CYCLING KINETICS OF MYOSIN CHANGE DURING HYPERTROPHY OF HUMAN SMOOTH-MUSCLE CELLS

We investigated in vivo expression of myosin heavy chain (MHC) isoform s, 17 kDa myosin light chain (MLC(17)), and phosphorylation of the 20 kDa MLC (MLC(20)) as well as mechanical performance of chemically skin ned fibers of normal and hypertrophied smooth muscle (SM) of human myo metrium. According to their immunological reactivity, we identified th ree MHC isoenzymes in the human myometrium: two SM-MHC(SM1 with 204 kD a and SM2 with 200 kDa), and one non-muscle specific MHC (NM with 196 kDa). No cross-reactivity was detected with an antibody raised against a peptide corresponding to a seven amino acid insert at the 25K/50Kju nction of the myosin head (a-25K/50K) in both normal and hypertrophied myometrium. In contrast, SM-MHC of human myomatous tissue strongly re acted with a-25 K/50K. Expression of SM1/SM2/NM (%) in normal myometri um was 31.7/34.7/33.6 and 35.1/40.9/24 in hypertrophied myometrium. Th e increased SM2 and decreased NM expression in the hypertrophied state was statistically significant (P < 0.05). MHC isoform distribution in myomatous tissue was similar to normal myometrium (35.3/35.3/29.4). I n vivo expression of MLC(17a) increased from 25.5% in normal to 44.2% in hypertrophied (P < 0.001) myometrium. Phosphorylation levels of MLC (20) upon maximal Ca2+-calmodulin activation of skinned myometrial fib ers were the same in normal and hypertrophied myometrial fibers. Maxim al force of isometric contraction of skinned fibers (pCa 4.5, slack-le ngth) was 2.85 mN/mm(2) and 5.6 mN/mm(2) in the normal and hypertrophi ed stale, respectively (P ( 0.001). Apparent maximal shortening Veloci ty (Vmax(app) extrapolated from the force-velocity relation) of myomet rium rose from 0.13 muscle length s(-1) (MVs) in normal to 0.24 MVs in hypertrophied fibers (P < 0.001).). (C) 1997 Wiley-Liss, Inc.