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The effects of the NH2-terminal fragments of M130, a 130 kDa regulatory subunit of smooth muscle myosin phosphatase, on contraction and myosin light chain phosphorylation were investigated in Triton X-100-permeabilized porcine renal artery.
Incubation of the permeabilized fibres with M1301-633 (a fragment containing amino acid residues 1-633) or M13044-633 enhanced the Ca2+-induced contraction and shifted the [Ca2+]i-force relationship to the left (EC50 of Ca2+: 330 nM, control, without fragment; 145 nM, M1301-633; 163 nM, M13044-633). Pre-incubation for 1-3 h was needed for these long constructs.
M1301-374, M130304-511 and M130297-374, i.e. relatively short constructs compared with M1301-633 and M13044-633, also induced leftward shifts of the [Ca2+]i-force relationship (EC50 of Ca2+: 65 nM, 72 nM and 180 nM, respectively). However, these required no pre-incubation.
Deletion of residues 304-374 from the most potent construct, M1301-374, abolished the Ca2+-sensitizing effect.
Wortmannin inhibited the enhancement of contraction induced by M130 fragments when added before contraction was initiated and partially inhibited the effects when added after steady-state contraction.
M1301-374 slowed the rate of relaxation in Ca2+-free medium. The time for 50 % relaxation with this fragment was 510 ± 51 s, compared with 274 ± 14 s for control.
The levels of myosin light chain phosphorylation (22·4 %) and force (34·5 %) obtained with 300 nM Ca2+ were increased by 3 µM M1301-374 to 35·7 and 92·2 %, respectively. However, M1301-374 had no effect on the phosphorylation-force relationship.
In conclusion, the NH2-terminal M130 fragments containing residues 304-374 inhibited myosin phosphatase, increased myosin light chain phosphorylation and increased the Ca2+ sensitivity of the contractile apparatus in permeabilized porcine renal artery.
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