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1 Cardiac Membrane Research Lab, Simon Fraser University, Burnaby, BC, Canada; Cardiovascular Sciences, BC Res Inst for Children's and Women's Health, Vancouver, BC, Canada
2 Laboratorio de Fisiologia Celular, Hospital de Sant Pau, Barcelona, Spain
3 ; Cardiovascular Sciences, BC Res Inst for Children's and Women's Health, Vancouver, BC, Canada
* To whom correspondence should be addressed. E-mail: tibbits{at}sfu.ca.
Much less is known about the contributions of the Na+-Ca2+ exchanger (NCX) and SR Ca2+-pump to cell relaxation in neonatal compared to adult mammalian ventricular myocytes. Based on both biochemical and molecular studies, there is evidence of a much higher density of NCX at birth which subsequently decreases during the next two weeks of development. It has been hypothesized, therefore, that NCX plays a relatively more important role for cytosolic Ca2+ decline in neonates as well as, perhaps, a role in excitation-contraction (E-C) 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 INCX and Ca2+ transient. It was found that the integral of INCX, an indicator of SR Ca2+ content, was greatest in younger age groups when normalized by cell surface area, and decreased with age. The velocity of Ca2+ extrusion of NCX (VNCX) was linear with [Ca2+] and did not indicate saturation kinetics until 1-3 µM for each age group. There was a significantly greater time delay between the peaks of INCX and the Ca2+ transient in the youngest age groups. This observation could be related to structural differences in the subsarcolemmal microdomains as a function of age.
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