Developmental changes in the immunolocalization of the sodium calcium exchanger and the ryanodine receptor of ventricular cardiac myocytes

Excitation of adult mammalian heart plasma membrane (sarcolemma, SL) triggers the opening of L-type voltage-gated ca2+ channels (DHPR), allowing ca2+ to cross the SL and initiate release of ca2+ stores from the sarcoplasmic reticulum (SR) via activation of the ryanodine receptor (RyR), a process termed calcium-induced-calcium release (CICR). The functional proximity of DHPR and RyR, which is essential to CICR, is not exhibited in neonate hearts. It has been proposed that NCX may contribute to ca2+ influx via reverse mode activation during excitation-contraction (EC) coupling in neonate myocytes. To investigate if RyR activation from ca2+entry via reverse mode NCX is possible, we examined the colocalization of these two proteins during development. Changes in colocalization were examined using NCX and RyR immunolabelling and confocal microscopy. Deconvolved images of cardiomyocytes collected from rabbits 3- 56-days old suggests the percentage of NCX colocalized with RyR increases in myocytes of 20- and 56-day old rabbits.