Ca²⁺ store-dependent regulation of contractile function of ground squirrel versus rat PM.

<p>(<b>A</b> and <b>B</b>) Representative recordings of ground squirrel and rat PM contractility under transition from nominal 1.8 mM Ca²⁺ to Ca²⁺-free Tyrode solution at 0.1 Hz stimulation frequency. (<b>C</b>) Same protocol in ground squirrel PM at doubled external Ca²⁺ levels also increase the force of contraction and further enhanced the amplitude of resting tension. Arrows in panels point to an increase in resting tension observed in ground squirrel but not rat PM. (<b>D</b> and <b>E</b>) Comparison of averaged relative force of contraction and relative resting tension (n = at least 3 in each experimental group) measured in the presence of CPA relative to control force of contraction before treatments. *, denotes statistically significant difference (P<0.05) defined by single group t-test comparing values with 1 in panel (<b>D</b>) and with 0 in panel (<b>E</b>). ‡, denotes significant difference (P<0.05) between relative resting tensions at 1.8 mM versus 3.6 mM of external Ca²⁺ in ground squirrel PM. (<b>F</b>) The profile of changes in the force of contraction and resting tension under manipulations with external Ca²⁺ and CPA was unaffected by the blocker of L-type Ca²⁺ channels, nifedipine, in line with moderate dependence of PM contractility on voltage-gated Ca²⁺ entry at low stimulation frequencies (<b>G</b>; *, P<0.05 for effects of nifedipine compared to corresponding controls; n = 4 in each group). (<b>H</b>) In ground squirrel PM, 2-APB reversed the contraction force potentiated by CPA to value below control. The averaged (n = 3) relative force of contraction is shown in panel (<b>D</b>).