Partial exposure of frog heart to high-potassium solution: an easily reproducible model mimicking ST segment changes

Abstract

By simply inducing burn injuries on the bullfrog heart, we previously reported a simple model of abnormal ST segment changes observed in human ischemic heart disease. In the present study, instead of inducing burn injuries, we partially exposed the surface of the frog heart to high-potassium (K⁺) solution to create a concentration gradient of the extracellular K⁺ within the myocardium. Dual recordings of ECG and the cardiac action potential demonstrated significant elevation of the ST segment and the resting membrane potential, indicating its usefulness as a simple model of heart injury. Additionally, from our results, Na⁺/K⁺-ATPase activity was thought to be primarily responsible for generating the K⁺ concentration gradient and inducing the ST segment changes in ECG.

Keywords: Na+/K+-ATPase activity; ST segment change; bullfrog heart; ischemic heart disease; partial exposure to high-potassium solution.

Fig. 1.

Partial exposure of high-potassium solution to bullfrog heart and the changes in electrocardiogram (ECG) and the transmembrane action potential. (A) To partially expose the cardiac muscle to high-potassium solution, we gently placed a cotton bar immersed with 1 M KCl solution on the subepicardial myocardium adjacent to the ventricular surface, where the ECG- and the suction- electrodes were placed. (B) The ECG waves (top) and the action potential of ventricular cardiomyocytes (bottom) were simultaneously recorded before (a) and after (b) 1 M KCl exposure. Dashed lines represent the peak of the action potential and the resting membrane potential levels before KCl exposure (baseline levels).

Fig. 2.

Ion transport through Na⁺/K⁺-ATPase and K_ATP-channels in cardiomyocytes. (A) Na⁺/K⁺-ATPase transports potassium (K⁺) ions into the cell but sodium (Na⁺) ions out of the cell. K_ATP-channel facilitates the outward leakage of K⁺ ions. In ischemic conditions, hypoxia diminishes the activity of Na⁺/K⁺-ATPase, but stimulates the activity of K_ATP-channels. Exogenously, ouabain inhibits the activity of Na⁺/K⁺-ATPase, while nicorandil stimulates the opening of K_ATP-channel. (B) Immunohistochemistry using an antibody for Na⁺/K⁺-ATPase α-1 subunit (brown) in ventricular cardiomyocytes of bullfrog heart, counterstained with hematoxylin. Magnification × 20.

Fig. 3.

Effects of ouabain and nicorandil on ECG and the transmembrane action potential. Ventricular surface of frog hearts was partially exposed to 10 mM ouabain (A) or 10 mM nicorandil (B). The ECG waves (top) and the action potential of cardiomyocytes (bottom) were simultaneously recorded before (a) and after (b) the drug exposure. Dashed lines represent the peak of the action potential and the resting membrane potential levels before the drug exposure (baseline levels).

Fig. 4.

Effects of insulin on high-potassium-induced changes in ECG and the transmembrane action potential. After partial exposure to 1M KCl, frog hearts were washed out by external solution alone (A) or the external solution containing 50 U insulin (B). The ECG waves (top) and the action potential of cardiomyocytes (bottom) were simultaneously after 1 M KCl exposure, 3 and 6 min after the washout. Dashed lines represent the peak of the action potential and the resting membrane potential levels after KCl exposure. (C) Numerical changes in the ST segment elevation in the frog hearts washed out by the external solution alone and those by the insulin-containing external solution. ST segment elevation was measured after 0, 1.5, 3, 4.5 and 6 min after the washout. ^#P<0.05 vs. external solution alone. Values are means ± SEM (external solution alone, n=11; insulin-containing external solution, n=9). Differences were analyzed by ANOVA followed by Dunnett’s or Student’s t test.