The myocardium undergoes many functional and structural changes during postnatal development. These changes allow organization of proteins and organelles to adapt to conditions that are best for the efficient function of the growing heart. The age-related functional changes of the cardiac pump are known. However, nothing is known about contractile characteristics of single myocytes and whether such characteristics are also associated with changes in Ca²⁺ mobilization. Measurement of contractility and Ca²⁺ mobilisation can therefore provide a useful insight into the cell function and Ca²⁺ cycling.

Contractile changes were studied in freshly isolated cardiomyocytes from 14, 21, 28 day old and adult male rats. The cardiomyocytes were perfused with HEPES buffer (2ml/min) in a chamber and field stimulated at 34°C. An edge detecting device was used to measure changes in myocyte length upon contraction. Isolated cardiomyocytes were also loaded with the Ca²⁺ fluorescent dye Fura-2 to measure Ca²⁺ transients (excitation at 340 and 370nm and emission at 520nm).

Cardiomyocyte size varied significantly between all age groups in length, width and surface area.

Contractility results showed a significant difference between all age groups, with the maximal percentage cell shortening seen in adult, minimal at 14 days. This agrees with changes in intact heart.

Stimulation-induced Ca²⁺ transients were measured in myocytes of all age groups. The amplitude of Ca²⁺ transients were greatest in the adult myocytes, which decrease with decreasing age. The results also showed that the time to peak for Ca²⁺ transients decreases with age. The diastolic and systolic Ca²⁺ concentrations of the cardiomyocytes also showed significant differences with [Ca²⁺] increasing with age.

These results indicate that in older myocytes Ca²⁺ mobilization is more efficient than in younger cells. It is suggested that the role of L-type Ca²⁺ channels in excitation-contraction coupling becomes more important with age. The opposite is supposed to occur for Na-Ca exchanger. Therefore entry on the exchanger will likely be slower than when the channels open in adult. Indeed, the sarcoplasmic reticulum of the immature rat myocytes, although present and functional is not fully developed. In fact is has been shown that ryanodine receptor inhibitors have very little effect upon neonatal cell contraction. Additionally it has been suggested that the Ca²⁺ sensitivity of cardiac myofilaments increases over development reaching adult levels at approximately 15 days of age. This work shows that there are significant transitional changes in myocyte contractility and Ca²⁺ transients occurring between 2-3 weeks of age.