Molecular Properties of Excitation-Contraction Coupling Proteins in Infant and Adult Human Heart Tissues

Excitation-contraction coupling (ECC) proteins in the human heart were characterized using human atrial tissues from different age groups. The samples were classified into one infant group (Group A: 0.2-7 years old) and three adult groups (Group B: 21-30; Group C: 41-49; Group D: 60-66). Whole homogenates (WH) of atrial tissues were assayed for ligand binding, $^{45}Ca^{2+}$ uptake and content of ECC proteins by Western blotting. Equilibrium [$^3H$]ryanodine binding to characterize the ryanodine receptor (RyR) of the sarcoplasmic reticulum (SR) showed that the maximal [$^3H$]ryanodine binding ($B_{max}$) to RyR was similar in all the age groups, but the dissociation constant ($k_d$) of ryanodine was higher in the infant group than the adult groups. Oxalate-supported $^{45}Ca^{2+}$ uptake into the SR, a function of the SR SERCA2a activity, was lower in the infant group than in the adult groups. Similarly, [$^3H$]PN200-110 binding, an index of dihydropyridine receptor (DHPR) density, was lower in the infant group. Expression of calsequestrin and triadin assessed by Western blotting was similar in the infant and adult groups, but junctin expression was considerably higher in the adult groups. These differences in key ECC proteins could underlie the different $Ca^{2+}$ handling properties and contractility of infant hearts.