Abstract
The ability of isolated cardiac muscle preparations to maintain force of contraction was assessed at different extracellular Ca 2+ concentrations and at various stimulation frequencies. Preparations were incubated in Krebs-Henseleit buffer at 32°C and electrically stimulated at frequencies between 0.25 and 3.0 Hz. Extracellular Ca 2+ was maintained at given concentrations between 0.8 and 2.4 mM, a range that included the plasma-ionized Ca 2+ levels determined for rat and guinea pig. Developed tension of atrial or papillary muscle preparations of guinea pig heart was more stable at higher stimulation frequencies. Rat atria and ventricular strips were more stable at 0.25 than at 0.5 or 1.0 Hz, and the stability increased at 2.0 and 3.0 Hz. Guinea pig atrial muscle was maintained for a longer time at higher media Ca 2+ concentrations; however, no such effect was observed with rat atrial tissue. Thus, the decrementing of developed tension in isolated cardiac muscle preparations is dependent on stimulation frequency and Ca 2+ concentrations and can be retarded by selection of appropriate incubation conditions.
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