Colocalization of dihydropyridine and ryanodine receptors in neonate rabbit heart using confocal microscopy

Because of undeveloped T tubules and sparse sarcoplasmic reticulum, Ca2+-in duced Ca2+ release (CICR) may not be the major mechanism providing contract ile Ca2+ in the neonatal heart. Spatial association of dihydropyridine rece ptors (DHPRs) and ryanodine receptors (RyRs), a key factor for CICR, was ex amined in isolated neonatal rabbit ventricular myocytes aged 3-20 days by d ouble-labeling immunofluorescence and confocal microscopy. We found a signi ficant increase (P< 0.0005) in the degree of colocalization of DHPR and RyR during development. The number of voxels containing DHPR that also contain ed RyR in the 3-day-old group (62 +/- 1.8%) was significantly lower than in the other age groups (76 +/- 1.3 in 6-day old, 75 +/- 1.2 in 10-day old, a nd 79 +/- 0.9% in 20-day old). The number of voxels containing RyR that als o contained DHPR was significantly higher in the 20-day-old group (17 +/- 0 .5%) compared with the other age groups (10 +/- 0.7 in 3-day old, 11 +/- 0. 6 in 6-day old, and 11 +/- 0.5% in 10-day old). During this period, the pat tern of colocalization changed from mostly peripheral to mostly internal co uplings. Our results provide a structural basis for the diminished prominen ce of CICR in neonatal heart.