Agonist-induced force enhancement: the role of isoforms and phosphorylation of the myosin-targeting subunit of myosin light chain phosphatase.
The magnitude of agonist-induced Ca(2+) sensitization of force is tissue-dependent, but an explanation for this diversity is unknown. Ca(2+) sensitization is thought to involve a G-protein-mediated inhibition of myosin light chain phosphatase activity by phosphorylation of the myosin-targeting subunit (MYPT). The MYPT has two isoforms that differ by a central insert, which lies near this phosphorylation site. Expression of MYPT isoforms is both developmentally regulated and tissue-specific. We hypothesized that the presence or absence of the central insert determines the magnitude of agonist-induced Ca(2+) sensitization. Throughout development, the chicken aorta exclusively expresses the splice-in MYPT isoform, and guanosine 5'-O-(thiotriphosphate) (GTPgammaS) produces a significant force enhancement. Early during development, the chicken gizzard expresses the splice-in MYPT isoform, and GTPgammaS produced a Ca(2+) sensitization. In the gizzard coincident with the shift in expression from the splice-in to splice-out MYPT isoform, GTPgammaS no longer produced force enhancement. In addition, adenosine 5'-O-(thiotriphosphate) (ATPgammaS) phosphorylated only adult gizzard tissue, the only tissue that did not demonstrate a Ca(2+) sensitization. These results suggest that the relative expression of splice-in/splice-out MYPT isoforms determines the magnitude of agonist-induced force enhancement and that MYPT phosphorylation is not required for Ca(2+) sensitization.
Richards
Christopher T
CT
Department of Medicine, Case Western Reserve University, Cleveland, Ohio 44106, USA.
Ogut
Ozgur
O
Brozovich
Frank V
FV
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002642:Chick Embryo; D005895:Gizzard, Avian / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism*; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D043263:Myosin-Light-Chain Phosphatase; D009218:Myosins / Q000378:metabolism*; D010749:Phosphoprotein Phosphatases / Q000737:chemistry / Q000378:metabolism*; D010766:Phosphorylation; D013379:Substrate Specificity; D013438:Sulfhydryl Compounds / Q000378:metabolism
D007527:Isoenzymes; D013438:Sulfhydryl Compounds; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D010749:Phosphoprotein Phosphatases; D043263:Myosin-Light-Chain Phosphatase; D009218:Myosins; D002118:Calcium