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Effects of mutations causing hypokalaemic periodic paralysis on the skeletal muscle L-type Ca²⁺ channel expressed in Xenopus laevis oocytes

James A. Morrill * and Stephen C. Cannon *¹²

1. A truncated form of the rabbit _1S Ca²⁺ channel subunit (_1SC) was expressed with the _1b, ₂ and auxiliary subunits in Xenopus laevis oocytes. After 5-7 days, skeletal muscle L-type currents were measured (469 ± 48 nA in 10 mM Ba²⁺). All three of the auxiliary subunits were necessary to record significant L-type current. A rapidly inactivating, dihydropyridine-insensitive endogenous Ba²⁺ current was observed in oocytes expressing the auxiliary subunits without an exogenous subunit. Expression of full-length _1S gave 10-fold smaller currents than the truncated form.

2. Three missense mutations causing hypokalaemic periodic paralysis (R528H in domain II S4 of the _1S subunit; R1239H and R1239G in domain IV S4) were introduced into _1SC and expressed in oocytes. L-type current was separated from the endogenous current by nimodipine subtraction. All three of the mutations reduced L-type current amplitude (~40 % for R528H, ~60-70 % for R1239H and R1239G).

3. The disease mutations altered the activation properties of L-type current. R528H shifted the G(V) curve ~5 mV to the left and modestly reduced the voltage dependence of the activation time constant, _act. R1239H and R1239G shifted the G(V) curve ~5-10 mV to the right and dramatically slowed _act at depolarized test potentials.

4. The voltage dependence of steady-state inactivation was not significantly altered by any of the disease mutations.

5. Wild-type and mutant L-type currents were also measured in the presence of (-)-Bay K8644, which boosted the amplitude ~5- to 7-fold. The effects of the mutations on the position of the G(V) curve and the voltage dependence of _act were essentially the same as in the absence of agonist. Bay K-enhanced tail currents were slowed by R528H and accelerated by R1239H and R1239G.

6. We conclude that the domain IV mutations R1239H and R1239G have similar effects on the gating properties of the skeletal muscle L-type Ca²⁺ channel expressed in Xenopus oocytes, while the domain II mutation R528H has distinct effects. This result implies that the location of the substitutions is more important than their degree of conservation in determining their biophysical consequences.

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