Slowing effects of Mg2+ on contractile kinetics of skinned preparations of rat hearts depending on myosin heavy chain isoform content.
The effects of changes in Mg2+ concentration on the kinetics of stretch activation were investigated on skinned rat heart preparations under maximal Ca2+ activation. Muscle strips of hyper- and hypothyroid rat hearts were investigated at 0.5 and 1 mM free Mg2+; the total ATP concentration was 8 mM which resulted in saturating MgATP2- concentrations above 5 mM. Preparations containing exclusively the cardiac alpha-myosin heavy chain (hyper- and hypothyroid atria, hyperthyroid ventricles) showed an acceleration of the kinetics of stretch activation by a factor of about 1.5 (P<0.01, paired t-test) when free Mg2+ was decreased from 1 to 0.5 mM. Conversely, preparations containing exclusively the beta-myosin heavy chain isoform showed only a small acceleration by a factor of 1.05 (P<0.05, paired t-test) under the same conditions. The fact that the Mg2+ sensitivity was dependent on the myosin heavy chain isoform excludes the possibility that Mg2+ exhibits only unspecific effects on contractile proteins. Several hypotheses for explaining the observed Mg2+ effects are discussed. The conditions used in our experiments might be close to the physiological situation and, thus, changes of Mg2+ concentration must be considered as possible factors modulating the contractile kinetics especially of atrial muscle tissue.
D000244:Adenosine Diphosphate / Q000502:physiology; D000818:Animals; D003402:Creatine Kinase / Q000378:metabolism; D006321:Heart / Q000187:drug effects* / Q000502:physiology*; D006652:Histological Techniques; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000378:metabolism*; D018995:Myosin Heavy Chains / Q000378:metabolism*; D020033:Protein Isoforms / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors; D014018:Tissue Distribution
D020033:Protein Isoforms; D000244:Adenosine Diphosphate; D003402:Creatine Kinase; D018995:Myosin Heavy Chains; D008274:Magnesium