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Protein kinase C and A sites on troponin I regulate myofilament Ca 2+ sensitivity and ATPase activity in the mouse myocardium
Author(s) -
Pi YeQing,
Zhang Dahua,
Kemnitz Kara R.,
Wang Hao,
Walker Jeffery W.
Publication year - 2003
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2003.045260
Subject(s) - myofilament , troponin i , phosphorylation , myosin light chain kinase , protein kinase c , protein kinase a , phospholamban , chemistry , medicine , phorbol , myosin , biochemistry , endocrinology , biology , microbiology and biotechnology , myocardial infarction
Cardiac troponin I (cTnI) is a phosphoprotein subunit of the troponin‐tropomyosin complex that is thought to inhibit cardiac muscle contraction during diastole. To investigate the contributions of cTnI phosphorylation to cardiac regulation, transgenic mice were created with the phosphorylation sites of cTnI mutated to alanine. Activation of protein kinase C (PKC) by perfusion of hearts with phorbol‐12‐myristate‐13‐acetate (PMA) or endothelin‐1 (ET‐1) inhibited the maximum ATPase rate by up to 25 % and increased the Ca 2+ sensitivity of ATPase activity and of isometric tension by up to 0.15 pCa units. PKC activation no longer altered cTnI phosphorylation, depressed ATPase rates or enhanced myofilament Ca 2+ sensitivity in transgenic mice expressing cTnI that could not be phosphorylated on serines 43/45 and threonine 144 (PKC sites). Modest changes in myosin regulatory light chain phosphorylation occurred in all mouse lines, but increases in myofilament Ca 2+ sensitivity required the presence of phosphorylatable cTnI. For comparison, the β‐adrenergic agonist isoproterenol caused a 38 % increase in maximum ATPase rate and a 0.12 pCa unit decrease in myofilament Ca 2+ sensitivity. These β‐adrenergic effects were absent in transgenic mice expressing cTnI that could not be phosphorylated on serines 23/24 (protein kinase A, PKA, sites). Overall, the results indicate that PKC and PKA exert opposing effects on actomyosin function by phosphorylating cTnI on distinct sites. A primary role of PKC phosphorylation of cTnI may be to reduce the requirements of the contractile apparatus for both Ca 2+ and ATP, thereby promoting efficient ATP utilisation during contraction.