Characteristics of force-frequency relations in the myocardium of the squirrel Citellus undulatus

The force-frequency relationship (FFR) in papillary muscles of the heart of active ground squirrel in different seasons was studied. For comparison, similar preparations from rat and rabbit were used. It was shown that the FFR of papillary muscles of active ground squirrel undergo significant seasonal changes. In summer and a part of autumn squirrels, a negative staircase (a decrease in the isometric force with increasing stimulation frequency) similar to that in adult rat was revealed. The FFR of the majority of autumn, winter and spring squirrels were polyphasic and contained both positive and negative components. Changes in the force in response to the introduction of pauses at a constant stimulation frequency were recorded. Two types of the post-rest recovery pattern were revealed in the myocardium of ground squirrels. For frequencies range with the negative direction of FFR, a typical pattern of rest-potentiation similar to that in rat papillary muscles was observed. The amplitude of the first post-rest contraction (F1) was usually higher than that of the preceding steady-state contraction. In papillary muscles of autumn animals the F1 value was greater that in summer, which suggests an enhanced release of Ca2+ from the sarcoplasmic reticulum. There was no post-rest potentiation in the range of frequencies with positive direction of FFR, and the post-rest recovery pattern in these cases was principally different from those of rat and rabbit preparations. It was proposed that seasonal differences of the FFR of active ground squirrel heart are associated with changes in the ratio of activities of the calcium-transporting system in the hibernation period.