Developmental changes in myocardial mechanical function and subcellular organelles

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Abstract
The developmental changes in myocardial mechanical function and the function of the subcellular organelles were studied. Mechanical function was determined at various Ca2+ concentrations ([Ca2+]0) in the isolated, arterially perfused heart of the fetus (28th day of gestation), newborn (3-5 day old) and adult rabbit. Maximal force of contraction in the fetus (observed at 7.5 mM [Ca2+]0) was significantly less than that in the newborn (observed at 30 mM ([Ca2+]0) and both the fetus and newborn values were significantly less than that in the adult (observed at 15 mM [Ca2+]0). The myofibrillar content in the fetus and ATPase activity in the fetus and newborn were significantly less than in the adult at pCa 4 and 5 where this enzyme was maximally activated. Both the amount of sarcoplasmic reticulum (SR) and SR Ca2+ uptake per g of muscle increased with age. Mitochondrial Ca2+ uptake was not observed at Ca > 10-6 M (physiological range) in all age groups. At Ca concentration < 10-6 M, mitochondrial Ca2+ uptake (per g muscle) in the newborn was significantly greater than in the fetus and adult. Ca2+ uptake by crude homogenate in the newborn was also greater than in the fetus and adult. The age-related change in myocardial contractility is evidently due to the differences in intracellular Ca2+ concentration and myofibrillar content as well as ATPase activity. Intracellular Ca2+ concentration may vary with development depending on the relative capability of Ca2+-releasing system and Ca2+-sequestering system.

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