Effects of Nickel Chloride on Myocardial Contractile Properties in Active Ground Squirrels with Different Responses to Hypothermia

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Abstract
The effects of 1.5 mM nickel chloride on the contraction force, the force–frequency relation, and post-rest potentiation in the myocardium were studied in active Urocitellus undulates ground squirrels with different responses to hypothermia. In a group of ground squirrels that retained a negative trend of the force–frequency relation upon cooling, the contraction forces were 52 ± 15 and 4 ± 3% of the contraction force in a control upon stimulation at frequencies of 0.1 and 1 Hz, respectively, at 20°C. A reduction of myocardial contractility was far greater in a group where the force–frequency relation changed from a biphasic pattern to a positive trend. Contractions were almost completely suppressed; the contraction forces were 6 ± 3 and 1 ± 0.5% at 0.1 and 1.0 Hz, respectively. When stimulation at a frequency of 0.3 Hz was performed in the presence of 1.5 mM Ni2+, post-rest potentiation decreased, on average, by 45–55% in the group with a negative force–frequency relation and by 90–98% in the group with a positive force–frequency relation. At a stimulation frequency of 1.0 Hz, there were 45–55 and 65–85% reductions, respectively. The results showed that the myocardium was significantly less dependent on extracellular Ca2+ input in ground squirrels that retained a negative force–frequency relation as compared with active ground squirrels that displayed a transition to a positive force–frequency relation upon cooling.

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