Electrical stimulation of adult rat cardiomyocytes in culture improves contractile properties and is associated with altered calcium handling.

A major limitation in long-term studies of quiescent adult cadiomyocytes in culture has been the decline in contractile properties of the cells over time. Regular contracting cardiomyocyte cultures may represent a more physiological model. The aim of the present study was to investigate the mechanical properties and calcium handling of myocytes after 24 hours of electrical stimulation at 1 Hz. In a random and blind design, stimulated (S) and unstimulated (U) myocytes were examined using an inverted microscope which allows continuous length recordings and measurements of intracellular Ca2+. Fractional shortening examined at 0.25 Hz was 14.67 +/- 0.51% in S cells and was not significantly different from U cells. However, at higher frequencies we found a significant difference in mechanical properties between the two groups. At 2 Hz fractional shortening was 12.03 +/- 0.67% in S cells, but only 8.07 +/- 0.94% U cells (P < 0.05). We were able to abolish the difference between the two groups by stimulating with the beta-adrenergic agonist isoproterenol. Measurements of Ca2+ transients were made at 1 Hz after loading with fura 2-AM. Peak fura 2 ratio was 25.4% greater in S cell compared to U cells. Resting fura ratios were not significantly different. Caffeine-induced transients were greater in S than in U cells. [3H]-ryanodine-binding and Ca(2+)-ATPase contents were not significantly different. In conclusion, we have found that regular electrical stimulation of adult ventricular myocytes in culture, so that they contract rhythmically, enhances both mechanical properties and calcium transients when compared to quiescent myocytes. These results suggest that regular electrical stimulation is important when studying the function of adult ventricular myocytes in culture.