452 - Effects of bilirubin on the distribution of small conductance calcium and voltage-activated K+ channels (SK) in lipid rafts of the Nucleus Tractus Solitarius

Poster Number: 452
Publication Number: 452.1227

Background: Apnea of prematurity is a common cause of short- and long-term morbidity in preterm infants. There is a growing body of evidence supporting the association of apnea in neonates with hyperbilirubinemia. Our group has previously shown that bilirubin localizes to lipid rafts, microdomains of the plasma membrane which regulate ion channel activity. We are focused on the nucleus tractus solitarius (nTS) as an area implicated in control of respiratory drive. We have shown that the large conductance calcium- and voltage-activated K+ channels is redistributed in lipid rafts of the nTS using the Gunn rat model of preterm hyperbilirubinemia. The homozygous Gunn rat (jj) lacks the ability to conjugate bilirubin. Administration of sulfadimethoxine (SDMX) results in acute elevation of free bilirubin which can cross the blood brain barrier and cause acute bilirubin encephalopathy. The small conductance calcium- and voltage-activated K+ channels (SK) are other lipid raft-associated proteins that are important in regulating the neuronal activity of the NTS.

Objective: To study the effects of bilirubin on the lipid raft distribution of SK channels in the nTS in a rat model of preterm hyperbilirubinemia.

Design/Methods: On postnatal day (P) 5, heterozygous (Nj) and jj Gunn rat pups were treated intraperitoneally with 200 mg/kg SDMX or an equivalent volume of saline (n=3 per group). P5 rat pups are equivalent to 25 – 29 week preterm infants. The nTS was dissected on P6 and lipid rafts were isolated and pooled into a lipid raft and non-lipid raft pools. Both pools were immunoblotted for the SK channel variants 1, 2, and 3. Relative densiometric units were compared with one way ANOVA and Tukey post hoc pairwise comparison for significance.

Results: The percent of SK1 (p=0.028), SK2 (p=0.035), and SK3 (p=0.012) found in LR of P6 jj Gunn rat pup nTS are significantly reduced after treatment with SDMX as compared to jj pups treated with saline. The percent of all SK variants found in LR of P6 Nj Gunn rats were not affected by treatment with SDMX.

Conclusion(s): The acute elevation of free bilirubin in P5 Gunn rats results in a significant reduction of the percent of SK1, SK2, and SK3 in lipid rafts. These results suggest another mechanism through which bilirubin may perturb neuronal activity of the NTS leading to apnea of prematurity in preterm infants with elevated blood bilirubin.