Changes in the expression of ion channels, connexins and Ca²⁺-handling proteins in the sino-atrial node during postnatal development

Abstract

There are important postnatal changes in the sino-atrial node (SAN), the pacemaker of the heart. Compared with the neonate, the adult has a slower intrinsic heart rate and a longer SAN action potential. These changes may be due to differences in ion channel expression. Consequently, we investigated postnatal developmental changes in the expression of ion channels and Ca 2+ -handling proteins in the SAN to see whether this is indeed the case. Using quantitative PCR, in situ hybridization and immunohistochemistry, we investigated the expression of ion channels, Ca 2+ -handling proteins and connexins in the SAN from neonatal (2-7 days of age) and adult (∼6 months of age) New Zealand White rabbits. The spontaneous beating rate of adult SAN preparations was 21% slower than that of neonatal preparations. During postnatal development, quantitative PCR revealed a significant decline in the SAN of the following mRNAs: HCN4 (major isoform responsible for I f ), Na V 1.5 (responsible for I Na ), Ca V 1.3 (in part responsible for I Ca,L ) and NCX1 (responsible for inward I NaCa ). These declines could be responsible for the slowing of the pacemaker during postnatal development. There was a significant decline during development in mRNA for delayed rectifier K + channel subunits (K V 1.5, responsible for I K,ur , K V LQT1 and minK, responsible for I K,s , and ERG, responsible for I K,r ) and this could explain the prolongation of the action potential. In situ hybridization confirmed the changes observed by quantitative PCR. In addition, immunohistochemistry revealed hypertrophy of nodal cells during postnatal development. Moreover, there were complex changes in the expression of Ca 2+ -handling proteins with age. In summary, there are significant postnatal changes in the expression of ion channels and Ca 2+ -handling proteins in the SAN that could explain the established changes in heart rate and action potential duration that occur during normal development.

Ultrastructural Maturation of Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes in a Long-Term Culture

“…The connexin (gap junction proteins) are reported to be more abundant in the neonate than the adult. 34 The significant decrease in GJA1 expression levels in 180-and 360-day hiPSC-CMs compared with 30-day hiPSC-CMs also suggested maturation of hiPSC-CMs.…”

Calcium Transient and Sodium‐Calcium Exchange Current in Human versus Rabbit Sinoatrial Node Pacemaker Cells

“…Specifically, I NaCa was scaled down to 74% of its value in the Courtemanche et al [20] model. Recently, Allah et al [21] published a study in which they used the same experimental approach as Chandler et al [8] to determine the expression of ion channels and Ca 2+ handling proteins in the SAN, right atrium, and left ventricle of the neonate and adult rabbit. From their Figure 6 [21], it appears that the mRNA expression level of the NCX1 gene in the adult rabbit SAN is ≈78% and ≈69% of that in right atrium and left ventricle, respectively.…”

Unique Ca2+-Cycling Protein Abundance and Regulation Sustains Local Ca2+ Releases and Spontaneous Firing of Rabbit Sinoatrial Node Cells

“…For example, Kir2 channels are responsible for the inward rectifier K + current (I K1 ), a key player in generating the resting membrane potential in working myocardium (atrial and ventricular myocytes). Although a similar mRNA expression pattern of Kir2.1 and Kir2.2 was observed in the SA node and atrium of adult rabbits [47], I K1 was present in rabbit atrial myocytes [53], but entirely absent in rabbit SANC [2]. …”

“…Other studies began to compare expression of ionic channels and Ca 2+ handling proteins at the messenger RNA level between SA node and working myocardium of varied species including humans, rabbits and mice [47,48,49,50,51,52]. mRNA expression, however, could be inconsistent with the protein expression or functional data due to messenger instability, ratio of protein synthesis and degradation or post translational modification.…”