The Effect of the Extracellular Ca2+ Concentration on the Force–Frequency Dependence in the Myocardium of the Guinea Pig: Potentiation by a Pause under Pronounced Hypothermia

Abstract—The effects of a four-fold decrease in the concentration of Ca2+ in a perfusing medium under pronounced hypothermia at 20°C on the contractile responses of the papillary muscles of the right ventricle of a guinea pig, that is, the frequency–force dependence in the frequency range of 0.1–1.0 Hz and the effect of potentiation by a pause at stimulation frequencies of 0.3 and 0.8 Hz, were studied. It was shown that with a decrease in the concentration of extracellular Ca2+ there was a significant decrease in the contraction force of papillary muscles from 40 to 70%, reaching a maximum of 72 ± 3% at a frequency of 0.4 Hz; at the same time, the positive frequency–forcer dependence remained. The absolute values of the effect of potentiation by a pause decreased and the decrease was most pronounced at a lower stimulation frequency (79 ± 6% and 40 ± 19% at 0.3 and 0.8 Hz, respectively). Thus, it was shown that the predominant dependence of contraction on extracellular sources of Ca2+ in the myocardium of guinea pigs persisted under pronounced hypothermia; at the same time, an increase in the frequency of stimulation led to an increase in the content of Ca2+ in the sarcoplasmic reticulum.