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

1.A truncated form of the rabbit alpha(1S) Ca2+ channel subunit (alpha(1S D elta C)) was expressed with the beta(1b), alpha(2)delta and gamma 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 aux iliary subunits were necessary to record significant L-type current. A rapi dly inactivating, dihydropyridine-insensitive endogenous Ba2+ current was o bserved in oocytes expressing the auxiliary subunits without an exogenous a lpha subunit. Expression of full-length alpha(1S) gave 10-fold smaller curr ents than the truncated form. 2. Three missense mutations causing hypokalaemic periodic paralysis (R528H in domain II S4 of the alpha(1S) subunit; R1239H and R1239G in domain IV S4 ) were introduced into alpha(1S Delta C) and expressed in oocytes. L-type c urrent was separated from the endogenous current by nimodipine subtraction. All three of the mutations reduced L-type current amplitude (similar to 40 % for R528H, similar to 60-70 % for R1239H and R1239G). 3. The disease mutations altered the activation properties of L-type curren t. R528H shifted the G(V) curve similar to 5 mV to the left and modestly re duced the voltage dependence of the activation time constant, tau(act). R12 39H and R1239G shifted the Q(V) curve similar to 5-10 mV to the right and d ramatically slowed tau(act) at depolarized test potentials. 4. The voltage dependence of steady-state inactivation was not significantl y 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 similar to 5- to 7-fold, The effects of the mutations on the position of the G(V) curve and the voltage dependence of tau(act) were essentially the same as in the absence of agoni st. Bay K-enhanced tail currents were slowed by R528H and accelerated by R1 239H 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 disti nct effects. This result implies that the location of the substitutions is more important than their degree of conservation in determining their bioph ysical consequences.