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

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.