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Unlabelled: Previous reports indicate that H+/K+-adenosine triphosphatase (ATPase) might be expressed in the heart., Aims: The objectives of the present study were to explore the presence of H+/K+-ATPase protein and gene expression in the rat heart and to investigate whether the enzyme could contribute to potassium transport across the sarcolemma., Methods and Results: We performed reverse transcription-polymerase chain reaction (RT-PCR) on mRNA from myocardium and isolated cardiomyocytes using primers specific for the gastric H+/K+-ATPase alpha-subunit. The PCR products were sequenced and the predicted gastric H+/K+-ATPase sequence was verified. Western blots from myocardium detected a 34-kDa band and a 94-kDa band, indicating the beta-subunit and alpha-subunit of the gastric H+/K+-ATPase, respectively. Immunocytochemistry detected significant immunoreactivity of the beta-subunit in cardiomyocytes. H+/K+-ATPase-dependent potassium transport was assessed by 86Rb+-uptake in isolated cardiomyocytes. Both ouabain and the selective H+/K+-ATPase inhibitor Schering 28080 reduced 86Rb+-uptake at maximum specific inhibition, by 70 and 25%, respectively; the effects were additive. Competitive RT-PCR analysis indicated a significant upregulation of the myocardial H+/K+-ATPase in heart failure after myocardial infarction., Conclusion: The gastric isoform of H+/K+-ATPase is expressed in rat cardiac myocytes, both at transcript and protein levels. Functional studies indicate that the enzyme could contribute to potassium and pHi regulation in cardiomyocytes.

The aim of the study was to determine whether progression of heart failure is associated with deterioration of cardiomyocyte function. Cell dimensions, contractility and calcium transients were measured in cardiomyocytes isolated from the left ventricle of female Wistar rats 1, 4, and 13 weeks after coronary artery ligation or sham-operation. Relative cardiomyocyte shortening decreased from 26% in controls to 11% 1 week after myocardial infarction and recovered to 18 and 20% after 4 and 13 weeks, respectively. Diastolic and systolic calcium concentrations increased markedly 1 week after myocardial infarction with subsequent reduction after 4 and 13 weeks. Time to 50% relaxation was prolonged by 31% after 1 week and 20% after 4 and 13 weeks with corresponding changes in diastolic calcium clearance. Cardiomyocyte length increased by 6, 24, and 26% after 1, 4 and 13 weeks, respectively, whereas myocyte width increased by 4, 11 and 27%. Cardiomyocytes adjacent to the infarct hypertrophied more and initially had more markedly impaired function than in the remote area. Left ventricular diastolic diameter assessed by echocardiography increased by 47, 66 and 84% after 1, 4 and 13 weeks, respectively, and systolic diameter increased by 120, 162 and 194%. Left ventricular end-diastolic pressure increased from 6 mmHg to 24, 25 and 36 mmHg. Whereas initial deterioration of cardiac function is associated with reduced cardiomyocyte contractile function, chronic heart failure progression is not accompanied by further impairment of intrinsic cardiomyocyte contractility in this model. Cardiomyocyte hypertrophy and dysfunction are more marked adjacent to the infarction.