Drug-induced cardiac abnormalities in premature infants and neonates

Age-dependent changes in electrophysiology and calcium handling: implications for pediatric cardiac research

Rodent models are frequently employed in cardiovascular research, yet our understanding of pediatric cardiac physiology has largely been deduced from more simplified two-dimensional cell studies. Previous studies have shown that postnatal development includes an alteration in the expression of genes and proteins involved in cell coupling, ion channels, and intracellular calcium handling. Accordingly, we hypothesized that postnatal cell maturation is likely to lead to dynamic alterations in whole heart electrophysiology and calcium handling. To test this hypothesis, we employed multiparametric imaging and electrophysiological techniques to quantify developmental changes from neonate to adult. In vivo electrocardiograms were collected to assess changes in heart rate, variability, and atrioventricular conduction (Sprague-Dawley rats). Intact, whole hearts were transferred to a Langendorff-perfusion system for multiparametric imaging (voltage, calcium). Optical mapping was performed in conjunction with an electrophysiology study to assess cardiac dynamics throughout development. Postnatal age was associated with an increase in the heart rate (181 ± 34 vs. 429 ± 13 beats/min), faster atrioventricular conduction (94 ± 13 vs. 46 ± 3 ms), shortened action potentials (APD₈₀: 113 ± 18 vs. 60 ± 17 ms), and decreased ventricular refractoriness (VERP: 157 ± 45 vs. 57 ± 14 ms; neonatal vs. adults, means ± SD, P < 0.05). Calcium handling matured with development, resulting in shortened calcium transient durations (168 ± 18 vs. 117 ± 14 ms) and decreased propensity for calcium transient alternans (160 ± 18- vs. 99 ± 11-ms cycle length threshold; neonatal vs. adults, mean ± SD, P < 0.05). Results of this study can serve as a comprehensive baseline for future studies focused on pediatric disease modeling and/or preclinical testing.NEW & NOTEWORTHY This is the first study to assess cardiac electrophysiology and calcium handling throughout postnatal development, using both in vivo and whole heart models.

Expression of calcium channel transcripts in the zebrafish heart: dominance of T-type channels

Calcium channels are necessary for cardiac excitation-contraction (E-C) coupling, but Ca²⁺ channel composition of fish hearts is still largely unknown. To this end, we determined transcript expression of Ca²⁺ channels in the heart of zebrafish (Danio rerio), a popular model species. Altogether, 18 Ca²⁺ channel α-subunit genes were expressed in both atrium and ventricle. Transcripts for 7 L-type (Ca_v1.1a, Ca_v1.1b, Ca_v1.2, Ca_v1.3a, Ca_v1.3b, Ca_v1.4a, Ca_v1.4b), 5 T-type (Ca_v3.1, Ca_v3.2a, Ca_v3.2b, Ca_v3.3a, Ca_v3.3b) and 6 P/Q-, N- and R-type (Ca_v2.1a, Ca_v2.1b, Ca_v2.2a, Ca_v2.2b, Ca_v2.3a, Ca_v2.3b) Ca²⁺ channels were expressed. In the ventricle, T-type channels formed 54.9%, L-type channels 41.1% and P/Q-, N- and R-type channels 4.0% of the Ca²⁺ channel transcripts. In the atrium, the relative expression of T-type and L-type Ca²⁺ channel transcripts was 64.1% and 33.8%, respectively (others accounted for 2.1%). Thus, at the transcript level, T-type Ca²⁺ channels are prevalent in zebrafish atrium and ventricle. At the functional level, peak densities of ventricular T-type (I_CaT) and L-type (I_CaL) Ca²⁺ current were 6.3±0.8 and 7.7±0.8 pA pF^-1, respectively. I_CaT mediated a sizeable sarcolemmal Ca²⁺ influx into ventricular myocytes: the increment in total cellular Ca²⁺ content via I_CaT was 41.2±7.3 µmol l^-1, which was 31.7% of the combined Ca²⁺ influx (129 µmol l^-1) via I_CaT and I_CaL (88.5±20.5 µmol l^-1). The diversity of expressed Ca²⁺ channel genes in zebrafish heart is high, but dominated by the members of the T-type subfamily. The large ventricular I_CaT is likely to play a significant role in E-C coupling.