Age‐dependent changes in myosin composition correlate with enhanced economy of contraction in guinea‐pig hearts

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Abstract
The composition of myosin heavy chains (MHCs) was investigated in young (1‐ to 8‐week‐old) and mature (9‐ to 26‐week‐old) guinea‐pigs using two monoclonal antibodies directed specifically against α‐MHC and β‐MHC. In addition, maximum force and the rate of ATP consumption during isometric contraction were measured in chemically skinned trabeculae taken from the same hearts. An age‐dependent shift in the MHC composition was found. The α‐MHC fraction decreased from 0.17 ± 0.02 (mean ± s.e.m.; n= 24) in young to 0.04 ± 0.01 (n= 43) in mature hearts. This shift was correlated with a decrease in tension cost (i.e. ATP consumption per second per trabecula volume/force per cross‐sectional area) from 4.1 ± 0.2 mmol kN−1 m−1 s−1 (n= 23) in young to 2.5 ± 0.1 mmol kN−1 m−1 s−1 (n= 57) in mature hearts. From the results it follows that the slow β‐MHC isoform, which predominates in hearts of mature guinea‐pigs, is about 5 times more economical than the fast α‐MHC isoform. Calcium sensitivity of force and ATP consumption decreased with age, but stabilized within a few weeks after birth. The pronounced dependence of cardiac energetics on MHC composition should be taken into account in long‐term studies of cardiac overload.