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

摘要

A truncated form of the rabbit α1S Ca2+ channel subunit (α1SΔC) was expressed with the β1b, α2δ and γ 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 α subunit. Expression of full-length α1S gave 10-fold smaller currents than the truncated form.Three missense mutations causing hypokalaemic periodic paralysis (R528H in domain II S4 of the α1S subunit; R1239H and R1239G in domain IV S4) were introduced into α1SΔC 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).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, τact. R1239H and R1239G shifted the G(V) curve ∼5–10 mV to the right and dramatically slowed τact at depolarized test potentials.The voltage dependence of steady-state inactivation was not significantly altered by any of the disease mutations.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 τact 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.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.