INa and IKir are reduced in Type 1 hypokalemic and thyrotoxic periodic paralysis.

We evaluated voltage-gated Na(+) (I(Na)) and inward rectifier K(+) (I(Kir)) currents and Na(+) conductance (G(Na)) in patients with Type 1 hypokalemic (HOPP) and thyrotoxic periodic paralysis (TPP). We studied intercostal muscle fibers from five subjects with HOPP and one with TPP. TPP was studied when the patient was thyrotoxic (T-toxic) and euthyroid. We measured: (1) I(Kir), (2) action potential thresholds, (3) I(Na), (4) G(Na), (5) intracellular [Ca(2+)], and (6) histochemical fiber type. HOPP fibers had lower I(Na), G(Na), and I(Kir) and increased action potential thresholds. Paralytic attack frequency correlated with the action potential threshold, G(Na) and I(Na), but not with I(Kir). G(Na), I(Na), and [Ca(2+)] varied with fiber type. HOPP fibers had increased [Ca(2+)]. The subject with TPP had values for G(Na), I(Na), action potential threshold, I(Kir), and [Ca(2+)] that were similar to HOPP when T-toxic and to controls when euthyroid. HOPP T-toxic TPP fibers had altered G(Na), I(Na), and I(Kir) associated with elevation in [Ca(2+)].