Hypokalemic periodic paralysis: an omega pore mutation affects inactivation.

Among the human diseases caused by ion channel mutations hypokalemic periodic paralysis (HypoPP) has thrown up more than its fair share of puzzles. Patients have attacks of skeletal muscle paralysis associated with low serum potassium, and harbor dominant mutations affecting the muscle calcium (CaV1.1) or sodium (NaV1.4) channels respectively. Why do mutations in either channel converge on a common phenotype, while other mutations in NaV1.4 lead to a quite different form of periodic paralysis associated with high or normal serum potassium, or muscle hyperexcitability manifesting as myotonia?