In-vitro Untersuchungen zur Amphotericin B-induzierten Hypokaliämischen Periodischen Paralyse

Familial hypokalemic periodic paralysis is a rare inherited muscle disease characterised by episodic attacks of flaccid muscle paralysis. These episodes go along with an increased shift of K+ ions into muscle, what results in hypokalemia. There is also a depolarization of muscle fibers to membrane potentials at which voltage-gated channels are inactivated. In the end, this results in inexcitability and the characteristic paralysis. It is caused by mutations in the voltage-sensing domain of voltage-gated sodium or calcium channels of skeletal muscle. It is supposed that these mutations create a cation leak, which might be the primary pathogenetic cause of the disease. Acetazolamide (AZ), a carboanhydrase-inhibitor, seems to be an effective therapy. In reference to this guess, we conducted an in-vitro study on rat skeletal muscle preparations that we made leaky for cations by Amphothericin B (AMB) and measured the membrane potential with glass microelectrodes. AMB is an ionophore clinically used as an antifungal drug and is well known for its serious side effects. The study showed that there are two populations of fibers with regard to the resting membrane potential. One is polarized close to the Nernst potential of K+, the other is depolarized to about - 60 mV. In low [K+], the fraction of depolarized fibers is larger. The leak induced by AMB leads to the result that there are more depolarized fibers at intermediate [K+] compared to control. Studies with AZ showed that in low [K+] the fraction of polarized fibers is increased about thirty percent. The conclusion is that a depolarizing leak might shift the region of bistability of resting membrane potentials to near physiologic [K+]. Our study shows that AMB increases the fraction of depolarized muscle fibers at physiologic and reduced [K+]. Thus, AMB might be a suitable pharmacological model for hypokalemic periodic paralysis and AZ a stabilizer for the membrane potential

K⁺-dependent paradoxical membrane depolarization and Na+ overload, major and reversible contributors to weakness by ion channel leaks

Normal resting potential (P1) of myofibers follows the Nernst equation, exhibiting about −85 mV at a normal extracellular K(+) concentration ([K(+)](o)) of 4 mM. Hyperpolarization occurs with decreased [K(+)](o), although at [K(+)](o) < 1.0 mM, myofibers paradoxically depolarize to a second stable potential of −60 mV (P2). In rat myofiber bundles, P2 also was found at more physiological [K(+)](o) and was associated with inexcitability. To increase the relative frequency of P2 to 50%, [K(+)](o) needed to be lowered to 1.5 mM. In the presence of the ionophore gramicidin, [K(+)](o) reduction to only 2.5 mM yielded the same effect. Acetazolamide normalized this increased frequency of P2 fibers. The findings mimic hypokalemic periodic paralysis (HypoPP), a channelopathy characterized by hypokalemia-induced weakness. Of myofibers from 7 HypoPP patients, up to 25% were in P2 at a [K(+)](o) of 4 mM, in accordance with their permanent weakness, and up to 99% were in P2 at a [K(+)](o) of 1.5 mM, in accordance with their paralytic attacks. Of 36 HypoPP patients, 25 had permanent weakness and myoplasmic intracellular Na(+) ([Na(+)](i)) overload (up to 24 mM) as shown by in vivo (23)Na-MRI. Acetazolamide normalized [Na(+)](i) and increased muscle strength. HypoPP myofibers showed a nonselective cation leak of 12–19.5 μS/cm(2), which may explain the Na(+) overload. The leak sensitizes myofibers to reduced serum K(+), and the resulting membrane depolarization causes the weakness. We postulate that the principle of paradoxical depolarization and loss of function upon [K(+)](o) reduction may apply to other tissues, such as heart or brain, when they become leaky (e.g., because of ischemia)