The Force-Frequency Dependence in the Heart Papillary Muscle of Ground Squirrel as a Reflection of Changes in the Functional State of Animals during the Annual Cycle

Abstract—A study was conducted to explore the characteristics of the dependence of the contraction force (isometric mode at 30°C) on the frequency of stimulation (0.1–4.0 Hz) in the right ventricle papillary muscles of the heart of the ground squirrel Spermophilus undulatus in groups of animals in the summer (June–July) and autumn (October–November) periods and the period of hibernation, as well as of animals that were active in the interbout euthermic period, entering into torpor (T_heart ~ 30°C), in torpor, and in interbout awakenings (T_heart ~ 30°C). It has been shown that the dependence of the contraction force on the frequency of stimulation (rhythm–inotropic relationship) in papillary muscles of the ground squirrel heart is of the negative type in its major manifestations regardless of the functional state of the animals; that is, the force of contraction at high frequencies (from 1.0 to 4.0 Hz) is always less than at low frequencies (in the region close to 0.1 Hz). At the same time, two types of the rhythm–inotropic relationship could be distinguished in each group, namely, one with a relatively weak (type 1) and another with a pronounced (type 2) frequency dependence. A weak positive component may occur in both types of rhythm–inotropic relationship (due to increased force of contraction); it was observed at the frequency of stimulation from 0.2 to 0.5 Hz in type 1 (it occurred only in the animals at the period of hibernation, namely, when they are active between bouts, in torpid state or when they entered into a state of torpor, that is, tending to reduce activity); it was observed at a high frequency of stimulation (above 1.0 Hz) in type 2 and mostly expressed in animals at the periods of resumption of functional activity, that is, upon awakening in the period of hibernation and in the final spring awakening. These data indicate that the process of formation of a particular type of rhythm–inotropic relationship in the heart of hibernating animals occurs before changes in the functional state of animals during the annual cycle.