Effects of Isoflurane on Intracellular Calcium and Myocardial Crossbridge Kinetics in Tetanized Papillary Muscles

Abstract

Background Isoflurane depresses the intracellular Ca2+ transient and force development during a twitch, but its effects on crossbridge cycling rates are difficult to predict because of the transient nature of the twitch. Measurements of the effects of isoflurane on crossbridge cycling kinetics during tetanic contractions, which provide a steady state level of activation in intact cardiac muscle, have not been previously reported. Methods Ferret right ventricular papillary muscles were isolated, and superficial cells were microinjected with the bioluminescent photoprotein aequorin to monitor the intracellular Ca2+ concentration. The rate of tension redevelopment (kTR) was measured during steady state isometric activation (tetanic stimulation, frequency 20 Hz, 1 microM ryanodine, temperature = 30 degrees C) in the absence of isoflurane (2, 6, and 12 mM extracellular [Ca2+]) and in the presence of 0.5, 1.0, and 1.5 minimum alveolar concentration isoflurane (12 mM extracellular [Ca2+]). Results Intracellular [Ca2+], isometric force, and kTR all increased when the extracellular [Ca2+] increased. Isoflurane (0.5, 1.0, and 1.5 minimum alveolar concentration) caused intracellular [Ca2+], isometric force, and kTR to decrease in a dose-dependent manner in the presence of 12 mM extracellular [Ca2+]. In the presence of increasing concentrations of isoflurane, the relation between intracellular [Ca2+] and force remained unchanged, whereas the relation between intracellular [Ca2+] and kTR was shifted toward higher [Ca2+]. Conclusions These results indicate that isoflurane depresses myocardial crossbridge cycling rates. It appears that this effect is partially mediated by a decrease in the intracellular [Ca2+]. However, additional mechanisms must be considered to explain the shift of the relation between intracellular [Ca2+] and kTR toward higher [Ca2+].