Comparison of the digitalis receptor in erythrocytes from preterm infants and adults

We compared #6rubidium by erythrocytes of preterm infants and adults as a measurement of their Na⁺, K⁺, ATPase enzyme system. In neonates, total uptake (0.92 ± 0.13 Mg/10⁶ cells) and specific uptake (0.64 ± 0.076 /xg/10⁶ cells) were significantly higher than in adults (0.52 ± 0.1 and 0.29 ± 0.06 Mg/10⁶ cells, respectively; p < 0.025). The percentage of specific uptake from total uptake was higher in infants (73.3 ± 2.3%) than in adults (57.9 ± 4.6%) (p < 0.005). No differences were found in the affinity constant of⁸⁶Rb uptake between infants (4.35 ± 0.48 ng/ ml) and adults (4.85 ± 0.48 ng/ml). Stratification of infants according to their serum K⁺ concentrations revealed that levels above 5.4 mEq/Iiter were associated with a higher specific uptake (0.79 ± 0.107 Mg/10⁶ cells) than in nor-mokalemic infants (0.54 ± 0.09 Mg/10⁶ cells) or adults (0.304 ± 0.061 Mg/10⁶ cells) (p < 0.05). The difference between hyperkalemic and normokalemic infants persisted after excluding those who received adult packed cells (0.88 ± 0.1 and 0.6 ± 0.12 Mg/10⁶ cells, respectively) (p < 0.05). Infants with serum K⁺ > 5.8 mEq/liter received on average significantly more K⁺ in previous days (2.46 ± 0.49 versus 1.13 + 0.34 mEq/kgday; p < 0.025). The different K⁺level could not be attributed to different creatinine clearance in the two groups.