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 cadiomyocyt es in culture has been the decline in contractile properties of the ce lls over time. Regular contracting cardiomyocyte cultures may represen t a more physiological model. The aim of the present study was to inve stigate the mechanical properties and calcium handling of myocytes aft er 24 hours of electrical stimulation at 1 Hz. In a random and blind d esign, stimulated (S) and unstimulated (U) myocytes were examined usin g 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 dif ferent from U cells. However, at higher frequencies we found a signifi cant difference in mechanical properties between the two groups. At 2 Hz fractional shortening was 12.03 +/- 0.67 % in S cells, but only 8.0 7 +/- 0.94 % U cells (P < 0.05). We were able to abolish the differenc e between the two groups by stimulating with the beta-adrenergic agoni st isoproterenol. Measurements of Ca2+ transients were made at 1 Hz af ter loading with fura 2-AM. Peak fura 2 ratio was 25.4% greater in S c ell compared to U cells. Resting fura, ratios were not significantly d ifferent. Caffeine-induced transients were greater in S than in U cell s. [H-3]-ryanodine-binding and Ca2+-ATPase contents were not significa ntly different. In conclusion, we have found that regular electrical s timulation of adult ventricular myocytes in culture, so that they cont ract rhythmically, enhances both mechanical properties and calcium tra nsients when compared to quiescent myocytes. These results suggest tha t regular electrical stimulation is important when studying the functi on of adult ventricular myocytes in culture.