Sarcoplasmic reticular Ca²⁺-ATPase inhibition paradoxically upregulates murine skeletal muscle Na_v1.4 function

Skeletal muscle Na⁺ channels possess Ca²⁺- and calmodulin-binding sites implicated in Nav1.4 current (I_Na) downregulation following ryanodine receptor (RyR1) activation produced by exchange protein directly activated by cyclic AMP or caffeine challenge, effects abrogated by the RyR1-antagonist dantrolene which itself increased I_Na. These findings were attributed to actions of consequently altered cytosolic Ca²⁺, [Ca²⁺]_i, on Na_v1.4. We extend the latter hypothesis employing cyclopiazonic acid (CPA) challenge, which similarly increases [Ca²⁺]_i, but through contrastingly inhibiting sarcoplasmic reticular (SR) Ca²⁺-ATPase. Loose patch clamping determined Na⁺ current (I_Na) families in intact native murine gastrocnemius skeletal myocytes, minimising artefactual [Ca²⁺]_i perturbations. A bespoke flow system permitted continuous I_Na comparisons through graded depolarizing steps in identical stable membrane patches before and following solution change. In contrast to the previous studies modifying RyR1 activity, and imposing control solution changes, CPA (0.1 and 1 µM) produced persistent increases in I_Na within 1-4 min of introduction. CPA pre-treatment additionally abrogated previously reported reductions in I_Na produced by 0.5 mM caffeine. Plots of peak current against voltage excursion demonstrated that 1 µM CPA increased maximum I_Na by ~ 30%. It only slightly decreased half-maximal activating voltages (V_0.5) and steepness factors (k), by 2 mV and 0.7, in contrast to the V_0.5 and k shifts reported with direct RyR1 modification. These paradoxical findings complement previously reported downregulatory effects on Nav1.4 of RyR1-agonist mediated increases in bulk cytosolic [Ca²⁺]. They implicate possible local tubule-sarcoplasmic triadic domains containing reduced [Ca²⁺]_TSR in the observed upregulation of Nav1.4 function following CPA-induced SR Ca²⁺ depletion.