Na+/Ca2+ exchange activity in neonatal rabbit ventricular myocytes

Area of Science:

• Cardiovascular Physiology
• Neonatal Cardiology
• Cellular Electrophysiology

Background:

• The roles of the Na(+)/Ca(2+) exchanger (NCX) and sarcoplasmic reticulum (SR) Ca(2+) pump in neonatal ventricular myocyte relaxation are less understood than in adults.
• Evidence suggests a higher NCX density at birth that declines during early development.
• This developmental change hypothesizes a greater NCX contribution to cytosolic Ca(2+) decline and potentially reverse mode in neonates.

Purpose of the Study:

• To investigate the developmental changes in NCX function and its role in cellular relaxation in neonatal rabbit ventricular myocytes.
• To compare the relative contributions of NCX and SR Ca(2+) handling during development.

Main Methods:

• Isolation of neonatal rabbit ventricular myocytes from four age groups (3, 6, 10, 20 days).
• Utilized the amphotericin-perforated patch-clamp technique.
• Measured caffeine-induced inward NCX current (I(NCX)) and Ca(2+) transients using fluo-3 fluorescence.

Main Results:

• SR Ca(2+) content, indicated by the integral of I(NCX), was highest in younger myocytes (normalized by cell surface area) and decreased with age.
• The velocity of Ca(2+) extrusion by NCX (V(NCX)) showed linear kinetics with [Ca(2+)] across all age groups, without saturation until 1-3 microM.
• A significant delay between I(NCX) and Ca(2+) transient peaks was observed in the youngest myocytes, suggesting age-related structural differences.

Conclusions:

• NCX plays a more significant role in Ca(2+) handling and relaxation in neonatal ventricular myocytes compared to adult ones.
• Developmental changes in NCX expression and potentially subsarcolemmal structure impact cardiac function.
• These findings highlight the dynamic nature of ion handling mechanisms during early cardiac development.