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

Much less is known about the contributions of the Na(+)/Ca(2+) exchanger (NCX) and sarcoplasmic reticulum (SR) Ca(2+) pump to cell relaxation in neonatal compared with adult mammalian ventricular myocytes. Based on both biochemical and molecular studies, there is evidence of a much higher density of NCX at birth that subsequently decreases during the next 2 wk of development. It has been hypothesized, therefore, that NCX plays a relatively more important role for cytosolic Ca(2+) decline in neonates as well as, perhaps, a role in excitation-contraction coupling in reverse mode. We isolated neonatal ventricular myocytes from rabbits in four different age groups: 3, 6, 10, and 20 days of age. Using an amphotericin-perforated patch-clamp technique in fluo-3-loaded myocytes, we measured the caffeine-induced inward NCX current (I(NCX)) and the Ca(2+) transient. We found that the integral of I(NCX), an indicator of SR Ca(2+) content, was greatest in myocytes from younger age groups when normalized by cell surface area and that it decreased with age. The velocity of Ca(2+) extrusion by NCX (V(NCX)) was linear with Ca(2+) and did not indicate saturation kinetics until Ca(2+) reached 1-3 microM for each age group. There was a significantly greater time delay between the peaks of I(NCX) and the Ca(2+) transient in myocytes from the youngest age groups. This observation could be related to structural differences in the subsarcolemmal microdomains as a function of age.