The effects of nickel chloride on papillary muscle contractility under normothermic and hypothermic conditions: Comparison of active and hibernating ground squirrels (Urocitellus undulatus) with Wistar rats.

Extracellular Ca plays a pivotal role in the regulation of cardiac contractility under normal and extreme conditions. Here, by using nickel chloride (NiCl), a non-specific blocker of extracellular Ca influx, we studied the input of extracellular Ca on the regulation of papillary muscle (PM) contractility under normal and hypothermic conditions in ground squirrels (GS), and rats. By measuring isometric force of contraction, we studied how NiCl affects force-frequency relationship and the rest effect in PM of these species at 30 °C and 10 °C. We found that at 30 °C 1.5 mM NiCl significantly reduced force of contraction across entire frequency range in active GS and rats, whereas in hibernating GS force of contraction was reduced at low and high frequency range. Additionally, NiCl evoked spontaneous contractility in rats but not GS PM. The rest effect was significantly reduced by NiCl for active GS and rats but not hibernating GS. At 10 °C, NiCl fully reduced contractility in active GS and, to a lesser extent, in rats, whereas in hibernating GS it was significant only at 0.3 Hz. The rest effect was significantly reduced by NiCl in both active and hibernating GS, whereas it was unmasked in rats that had high contractility under hypothermic conditions in control. Our results show a significant contribution of extracellular Ca to myocardial contractility in GS not only in active but also in hibernating states, especially under hypothermic conditions, whereas limitation of extracellular Ca influx in rats under hypothermia can play protective role for myocardial contractility.