Myocardial protection during open-heart surgery is largely based on using high-potassium cardioplegic solutions to induce rapid electromechanical arrest of the beating heart (Suleiman et al. 2001). However, hyper-kalaemic solutions may increase intracellular calcium and may cause post-operative ventricular dysfunction (Spinale, 1999). In this study, we used isolated perfused rat myocytes to study the effects of hyperkalaemic solutions on cell contractile function and intracellular calcium concentration ([Ca²⁺]_i). Male Wistar rats were humanely killed by cervical dislocation and ventricular myocytes isolated by collagenase digestion. Isolated cells were loaded with indo-1 AM allowing measurement of the diastolic [Ca²⁺]_i whilst cell contraction was determined using an edge-tracking device (Williams et al. 2000). Cells were field stimulated at 1.0 Hz and superfused with Hepes-based buffer with or without different concentrations of KCl (20, 30, 40 or 50 mM) at 37 °C. Results are expressed as means ± S.E.M., and statistical significance calculated using ANOVA.

After perfusion for 10 min, only the highest concentrations of KCl (40 and 50 mM) produced a significant reduction in cell contraction: contractile activity decreased to 18.2 ± 11.7 % of control (n = 5, P < 0.01) for 50 mM and to 54.0 ± 19.6 % of control for 40 mM (n = 6, P < 0.01). 20 mM KCl caused significant increase in contractile activity to 116 ± 6.86 % of control (n = 4, P = 0.04), whereas 30 mM KCl had no significant effect. The action of high potassium concentration is unlikely to be due to an osmotic effect, as 50 mM sucrose or choline chloride did not induce significant changes in the magnitude of contraction.

The diastolic [Ca²⁺]_i was elevated on perfusion with all KCl solutions, although initially the increase was significantly higher with 40 and 50 mM KCl. This initial increase (after 2 min of perfusion) was approximately 120 % of control for 20 and 30 mM KCl solutions and approximately 150 % of control for 40 mM and 50 mM KCl solutions.

This work shows that single isolated ventricular myocytes require hyperkalaemic solutions with significantly higher KCl concentrations to induce their mechanical arrest compared with whole heart studies (20 mM KCl is used), and that these hyperkalaemic solutions result in calcium loading of the cells. It is likely that the efficacy of hyperkalaemia on different parts of the heart (e.g. pacemaker, the conduction system, etc.) is not the same.

Spinale, F.G. (1999). Ann. Thorac. Surg. 68, 1934-1941.

Suleiman, M.S., Halestrap, A.P. & Griffiths, E.J. (2001). Pharmacol. Ther. 89, 29-46.

Williams, H., Kerr, P.M., Suleiman, M.-S. & Griffiths, E.J. (2000). Exp. Physiol. 85, 505-510.