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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

SR Ca²⁺ refilling upon depletion and SR Ca²⁺ uptake rates during development in rabbit ventricular myocytes

¹Cardiac Membrane Research Lab, Simon Fraser University, Burnaby and ²Cardiovascular Sciences, Child and Family Research Institute, Vancouver, British Columbia, Canada; and ³Laboratorio de Fisiología Celular, Cardiología, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain

Submitted 7 June 2007 ; accepted in final form 3 October 2007

While it has been reported that a sparse sarcoplasmic reticulum (SR) and a low SR Ca²⁺ pump density exist at birth, we and others have recently shown that significant amounts of Ca²⁺ are stored in the neonatal rabbit heart SR. Here we try to determine developmental changes in SR Ca²⁺ loading mechanisms and Ca²⁺ pump efficacy in rabbit ventricular myocytes. SR Ca²⁺ loading (load_SR) and k_0.5 (Ca²⁺ concentration at half-maximal SR Ca²⁺ uptake) were higher and lower, respectively, in younger age groups. Inhibition of the L-type calcium current (I_Ca) with 15 µM nifedipine dramatically reduced load_SR in older but not in younger age groups. In contrast, subsequent inhibition of the Na⁺/Ca²⁺ exchanger (NCX) with 10 µM KB-R7943 strongly reduced load_SR in the younger but not the older age groups. Accordingly, the time integral of the inward NCX current (tail I_NCX) elicited on repolarization was highly sensitive to nifedipine in the older groups and sensitive to KB-R7943 in the younger groups. Interestingly, slow SR loading took place in the presence of both nifedipine and KB-R7943 in all age groups, although it was less prominent in the older groups. We conclude that the SR loading capacity at the earliest postnatal stages is at least as large as that of adult myocytes. However, reverse-mode NCX plays a prominent role in SR Ca²⁺ loading at early postnatal stages while I_Ca is the main source of SR Ca²⁺ loading at late postnatal and adult stages.

sarco(endo)plasmic reticulum Ca²⁺ pump; sarcoplasmic reticulum Ca²⁺ loading; cytosolic Ca²⁺ concentration; L-type Ca²⁺ channel; Na⁺/Ca²⁺ exchanger; neonate cardiomyocytes; store-operated Ca²⁺ channel