SKELETAL-MUSCLE DHP RECEPTOR MUTATIONS ALTER CALCIUM CURRENTS IN HUMAN HYPOKALEMIC PERIODIC PARALYSIS MYOTUBES

1. Mutations in the gene encoding the al-subunit of the skeletal muscl e dihydropyridine (DHP) receptor are responsible for familial hypokala emic periodic paralysis (HypoPP), an autosomal dominant muscle disease . We investigated myotubes cultured from muscle of patients with argin ine-to-histidine substitutions in putative voltage sensors, IIS4 (R528 H) and IVS4 (R1239H), of the DHP receptor alpha 1-subunit. 2. Analysis of the messenger ribonucleic acid (mRNA) in the myotubes from such pa tients indicated transcription from both the normal and mutant genes. 3. In control myotubes, the existence of the slow L-type current and o f two rapidly activating and inactivating calcium current components ( T-type with a maximum at about -20 mV and 'third type' with a maximum at + 10 to + 20 mV) was confirmed. In the myotubes from patients with either mutation, the third-type current component was seen more freque ntly and, on average, with larger amplitude. 4. In myotubes with the I VS4 mutation (R1239H) the maximum L-type current density was smaller t han control (-.053 +/- 0.31 vs. -1.41 +/- 0.71 pA pF(-1)). The voltage dependence of activation was normal, and hyperpolarizing prepulses to -120 mV for 20 s did not increase the reduced current amplitude durin g test pulses. 5. In myotubes with the IIS4 mutation (R528H) the L-typ e current-voltage relation, determined at a holding potential of -90 m V, was normal. However, the voltage dependence of inactivation was shi fted by about 40 mV to more negative potentials (voltage at half-maxim um inactivation, V-1/2 = -41.5 +/- 8.2 vs. -4.9 +/- 4.3 mV in normal c ontrols). 6. We conclude that the arginine-to-histidine exchanges enha nce inactivation of the L-type calcium channel and do not alter its vo ltage-dependent activation.