To assess the functional significance of upregulation of the cardiac current (IK1), we have produced and characterized the first transgenic (TG) mouse model of IK1 upregulation. To increase IK1 density, a pore-forming subunit of the Kir2.1 (green fluorescent protein-tagged) channel was expressed in the heart under control of the alpha-myosin heavy chain promoter. Two lines of TG animals were established with a high level of TG expression in all major parts of the heart: line 1 mice were characterized by 14% heart hypertrophy and a normal life span; line 2 mice displayed an increased mortality rate, and in mice < or =1 mo old, heart weight-to-body weight ratio was increased by >100%. In adult ventricular myocytes expressing the Kir2.1-GFP subunit, IK1 conductance at the reversal potential was increased approximately 9- and approximately 10-fold in lines 1 and 2, respectively. Expression of the Kir2.1 transgene in line 2 ventricular myocytes was heterogeneous when assayed by single-cell analysis of GFP fluorescence. Surface ECG recordings in line 2 mice revealed numerous abnormalities of excitability, including slowed heart rate, premature ventricular contractions, atrioventricular block, and atrial fibrillation. Line 1 mice displayed a less severe phenotype. In both TG lines, action potential duration at 90% repolarization and monophasic action potential at 75-90% repolarization were significantly reduced, leading to neuronlike action potentials, and the slow phase of the T wave was abolished, leading to a short Q-T interval. This study provides a new TG model of IK1 upregulation, confirms the significant role of IK1 in cardiac excitability, and is consistent with adverse effects of IK1 upregulation on cardiac electrical activity.

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Previous studies have shown that cardiac inward rectifier potassium current ( IK1) channels are heteromers of distinct Kir2 subunits and suggested that species- and tissue-dependent expression of these subunits may underlie variability of IK1. In this study, we investigated the contribution of the slowly activating Kir2.3 subunit and free intracellular polyamines (PAs) to variability of IK1 in the mouse heart. The kinetics of activation was measured in Kir2 concatemeric tetramers with known subunit stoichiometry. Inclusion of only one Kir2.3 subunit to a Kir2.1 channel led to an approximate threefold slowing of activation kinetics, with greater slowing on subsequent additions of Kir2.3 subunits. Activation kinetics of IK1 in both ventricles and both atria was found to correspond to fast-activating Kir2.1/Kir2.2 channels, suggesting no major contribution of Kir2.3 subunits. In contrast, IK1 displayed significant variation in both the current density and inward rectification, suggesting involvement of intracellular PAs. The total levels of PAs were similar across the mouse heart. Measurements of the free intracellular PAs in isolated myocytes, using transgenically expressed Kir2.1 channels as PA sensors, revealed “microheterogeneity” of IK1 rectification as well as lower levels of free PAs in atrial myocytes compared with ventricular cells. These findings provide a quantitative explanation for the regional heterogeneity of IK1.

Obstructive sleep apnoea (OSA) affects 9–24% of the adult population. OSA is associated with atrial disease, including atrial enlargement, fibrosis and arrhythmias. Despite the link between OSA and cardiac disease, the molecular changes in the heart which occur with OSA remain elusive. To study OSA‐induced cardiac changes, we utilized a recently developed rat model which closely recapitulates the characteristics of OSA. Male Sprague Dawley rats, aged 50–70 days, received surgically implanted tracheal balloons which were inflated to cause transient airway obstructions. Rats were given 60 apnoeas per hour of either 13 sec. (moderate apnoea) or 23 sec. (severe apnoea), 8 hrs per day for 2 weeks. Controls received implants, but no inflations were made. Pulse oximetry measurements were taken at regular intervals, and post‐apnoea ECGs were recorded. Rats had longer P wave durations and increased T wave amplitudes following chronic OSA. Proteomic analysis of the atrial tissue homogenates revealed that three of the nine enzymes in glycolysis, and two proteins related to oxidative phosphorylation, were down regulated in the severe apnoea group. Several sarcomeric and pro‐hypertrophic proteins were also up regulated with OSA. Chronic OSA causes proteins changes in the atria which suggest impairment of energy metabolism and enhancement of hypertrophy.

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