Cloning and expression of cardiac Kir2.1 and Kir2.2 channels in thermally acclimated rainbow trout

Potassium currents are plastic entities that modify electrical activity of the heart in various physiological conditions including chronic thermal stress. We examined the molecular basis of the inward rectifier K+ current (Y-K1) in rainbow trout acclimated to cold (4 degrees C, CA) and warm (18 degrees C, WA) temperature. Inward rectifier K+ channel (K-ir)2.1 and K(ir)2.2 transcripts were expressed in atrium and ventricle of the trout heart, K(ir)2.1 being the major component in both cardiac chambers. The relative expression of K(ir)2.2 was, however, higher (P < 0.05) in atrium than ventricle. The density of ventricular I-K1 was similar to 25% larger (P < 0.05) in WA than CA trout. Furthermore, the I-K1 of the WA trout was 10 times more sensitive to Ba2+ (IC50 0.18 +/- 0.42 uM) than the I-K1 of the CA trout (1.17 +/- 0.44 mu M) (P < 0.05), and opening kinetics of single K(ir)2 channels was slower in WA than CA trout (P < 0.05). When expressed in COS-1 cells, the homomeric K(ir)2.2 channels demonstrated higher Ba2+ sensitivity (2.88 +/- 0.42 mu M) than K(ir)2.1 channels (24.99 +/- 7.40 mu M) (P < 0.05). In light of the different Ba2(1+) sensitivities of rainbow trout (om)K(ir)2.1 and omK(ir)2.2 channels, it is concluded that warm acclimation increases either number or activity of the omK(ir)2.2 channels in trout ventricular myocytes. The functional changes in I-K1 are independent of omK(ir)2 transcript levels, which remained unaltered by thermal acclimation. Collectively, these findings suggest that thermal acclimation modifies functional properties and subunit composition of the trout K(ir)2 channels, which may be needed for regulation of cardiac excitability at variable temperatures.