phenotype of trout heart was induced by 4-wk acclimation at 4°C and was characterized by 32.7% increase in relative heart mass and 49.8% increase in ventricular myocyte size compared with warm-acclimated (WA; 18°C) fish (P Ͻ 0.001). Effect of temperature acclimation on transcriptome of the rainbow trout heart was examined using speciesspecific microarray chips containing 1,380 genes. After 4 wk of temperature acclimation, 8.8% (122) of the genes were differently expressed in CA and WA hearts, and most of them (82%) were upregulated in the cold (P Ͻ 0.01). Transcripts of genes engaged in protein synthesis and intermediary metabolism were most strongly upregulated, whereas genes contributing to the connective tissue matrix were clearly repressed. Extensive upregulation of the genes coding for ribosomal proteins and translation elongation and initiation factors suggest that the protein synthesis machinery of the trout heart is enhanced in the cold and is an essential part of the compensatory mechanism causing and maintaining the hypertrophy of cardiac myocytes. The prominent depression of collagen genes may be indicative of a reduced contribution of extracellular matrix to the remodeling of the CA fish heart. Temperature-related changes in transcripts of metabolic enzymes suggest that at mRNA level, glycolytic energy production from carbohydrates is compensated in the heart of CA rainbow trout, while metabolic compensation is absent in mitochondria. In addition, the analysis revealed three candidate genes: muscle LIM protein, atrial natriuretic peptide B, and myosin light chain 2, which might be central for induction and maintenance of the hypertrophic phenotype of the CA trout heart. These findings indicate that extensive modification of gene expression is needed to maintain the temperature-specific phenotype of the fish heart. gene expression; fish heart; temperature-induced hypertrophy PHYSIOLOGICAL PLASTICITY is needed to adapt body functions to changing environmental conditions and involves activation of proper genetic programs. Ectothermic animals of north-temperate latitudes experience large seasonal changes in temperature, which strongly affect the rate of body functions. To compensate for the effects of temperature changes, ectotherms can respond to chronic temperature changes by increasing the quantity of tissue or enzyme needed for different physiological tasks, or by expressing proteins isoforms that are more appropriate for the new thermal conditions (16,19,21). On the other hand, proteins which are needed in lesser amounts in the new thermal regime could be depressed or downregulated. Although expression of proteins can be changed by multiple mechanisms during synthesis and degradation, temperaturedependent changes in transcription of genes are one of the key events in modifying the proteome of the tissues (16, 37).Circulatory system integrates different body functions through the transport of material and humoral messages and serves the well being of all body cells by providing oxygen and fuels for e...

The zebrafish (Danio rerio) has become a popular model for human cardiac diseases and pharmacology including cardiac arrhythmias and its electrophysiological basis. Notably, the phenotype of zebrafish cardiac action potential is similar to the human cardiac action potential in that both have a long plateau phase. Also the major inward and outward current systems are qualitatively similar in zebrafish and human hearts. However, there are also significant differences in ionic current composition between human and zebrafish hearts, and the molecular basis and pharmacological properties of human and zebrafish cardiac ionic currents differ in several ways. Cardiac ionic currents may be produced by non-orthologous genes in zebrafish and humans, and paralogous gene products of some ion channels are expressed in the zebrafish heart. More research on molecular basis of cardiac ion channels, and regulation and drug sensitivity of the cardiac ionic currents are needed to enable rational use of the zebrafish heart as an electrophysiological model for the human heart.

Zebrafish are increasingly used as a model for human cardiac electrophysiology, arrhythmias, and drug screening. However, K ion channels of the zebrafish heart, which determine the rate of repolarization and duration of cardiac action potential (AP) are still incompletely known and characterized. Here, we provide the first evidence for the presence of the slow component of the delayed rectifier Kchannels in the zebrafish heart and characterize electrophysiological properties of the slow component of the delayed rectifier Kcurrent, I. Zebrafish atrium and ventricle showed strong transcript expression of the kcnq1 gene, which encodes the Kv7.1 α-subunit of the slow delayed rectifier K channel. In contrast, the kcne1 gene, encoding the MinK β-subunit of the delayed rectifier, was expressed at 21 and 17 times lower level in ventricle and atrium, respectively, in comparison to the kcnq1. I was observed in 62% of ventricular myocytes with mean (± SEM) density of 1.23 ± 0.37 pA/pF at + 30 mV. Activation rate of I was 38% faster (τ = 1248 ± 215 ms) than kcnq1:kcne1 channels (1725 ± 792 ms) expressed in 3:1 ratio in Chinese hamster ovary cells. Microelectrode experiments demonstrated the functional relevance of I in the zebrafish heart, since 100 μM chromanol 293B produced a significant prolongation of AP in zebrafish ventricle. We conclude that AP repolarization in zebrafish ventricle is contributed by I, which is mainly generated by homotetrameric Kv7.1 channels not coupled to MinK ancillary β-subunits. This is a clear difference to the human heart, where MinK is an essential component of the slow delayed rectifier Kchannel.

Calcium channels are necessary for cardiac excitation-contraction (E-C) coupling, but Ca channel composition of fish hearts is still largely unknown. To this end, we determined transcript expression of Ca channels in the heart of zebrafish (), a popular model species. Altogether, 18 Ca channel α-subunit genes were expressed in both atrium and ventricle. Transcripts for 7 L-type (Ca1.1a, Ca1.1b, Ca1.2, Ca1.3a, Ca1.3b, Ca1.4a, Ca1.4b), 5 T-type (Ca3.1, Ca3.2a, Ca3.2b, Ca3.3a, Ca3.3b) and 6 P/Q-, N- and R-type (Ca2.1a, Ca2.1b, Ca2.2a, Ca2.2b, Ca2.3a, Ca2.3b) Ca 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 Ca channel transcripts. In the atrium, the relative expression of T-type and L-type Ca channel transcripts was 64.1% and 33.8%, respectively (others accounted for 2.1%). Thus, at the transcript level, T-type Ca channels are prevalent in zebrafish atrium and ventricle. At the functional level, peak densities of ventricular T-type () and L-type () Ca current were 6.3±0.8 and 7.7±0.8 pA pF, respectively. mediated a sizeable sarcolemmal Ca influx into ventricular myocytes: the increment in total cellular Ca content via was 41.2±7.3 µmol l, which was 31.7% of the combined Ca influx (129 µmol l) via and (88.5±20.5 µmol l). The diversity of expressed Ca channel genes in zebrafish heart is high, but dominated by the members of the T-type subfamily. The large ventricular is likely to play a significant role in E-C coupling.

Inward rectifier potassium current (IK1) and Kir2 composition of the zebrafish (Danio rerio) heart drKir2.1a, drKir2.1b, drKir2.2a, drKir2.2b, drKir2.3, and drKir2.4) were expressed in the zebrafish heart. drKir2.4 and drKir2.2a were the dominant isoforms in both the ventricle (92.9 ± 1.5 and 6.3 ± 1.5 %) and the atrium (28.9 ± 2.9 and 64.7 ± 3.0 %). The remaining four channels comprised together less than 1 and 7 % of the total transcripts in ventricle and atrium, respectively. The four main gene products (drKir2.1a, drKir2.2a, drKir2.2b, drKir2.4) were cloned, sequenced, and expressed in HEK cells for electrophysiological characterization. drKir2.1a was the most weakly rectifying (passed more outward current) and drKir2.2b the most strongly rectifying (passed less outward current) channel, while drKir2.2a and drKir2.4were intermediate between the two. In regard to sensitivity to Ba 2+ block, drKir2.4 was the most sensitive (IC50 = 1.8 μM) and drKir2.1a the least sensitive channel (IC50 = 132 μM). These findings indicate that the Kir2 isoform composition of the zebrafish heart markedly differs from that of the mammalian hearts. Furthermore orthologous Kir2 channels (Kir2.1 and Kir2.4) of zebrafish and mammals show striking differences in Ba 2+ -sensitivity. Those structural and functional differences needs to be taken into account when zebrafish is used as a model for human cardiac electrophysiology, cardiac diseases, and in screening cardioactive substances. AUTHOR'S PROOF!49 Keywords separated by ' -'Zebrafish -Heart -Inward rectifier potassium current -Kir2 channel 50 Foot note information ION CHANNELS, RECEPTORS AND TRANSPORTERS