Modulation of slow inactivation in class A Ca 2+ channels by β‐subunits

1 β‐subunit modulation of slow inactivation of class A calcium (Ca 2+ ) channels was studied with two‐microlectrode voltage clamp after expression of the α 1A ‐ (BI‐2) together with β 1a ‐, β 2a ‐, β 3 ‐ or β 4 ‐subunits in Xenopus oocytes. 2 On‐ and off‐rates of slow inactivation were estimated from the kinetics of recovery from slow inactivation. Ca 2+ channels with an α 1A /β‐subunit composition inducing the slower rate of fast inactivation displayed the faster rate of slow inactivation. The corresponding order of slow inactivation time constants (τ onset ) was: α 1A /β 2a , 33 ± 3 s; α 1A /β 4 , 42 ± 4 s; α 1A /β 1a , 59 ± 4 s; α 1A /β 3 , 67 ± 5 s ( n ≥ 7 ). 3 Recovery of class A Ca 2+ channels from slow inactivation was voltage dependent and accelerated at hyperpolarized voltages. At a given holding potential recovery kinetics were not significantly modulated by different β‐subunits. 4 Two mutations in segment IIIS6 (IF1612/1613AA) slowed fast inactivation and accelerated the onset of slow inactivation in the resulting mutant (α 1A/IF‐AA /β 3 ) in a similar manner as coexpression of the β 2a ‐subunit. Recovery from slow inactivation was slightly slowed in the double mutant. 5 Our data suggest that class A Ca 2+ channels enter the ‘slow inactivated’ state more willingly from the open than from the ‘fast inactivated’ state. The rate of slow inactivation is, therefore, indirectly modulated by different β‐subunits. 6 Fast and slow inactivation in class A Ca 2+ channels appears to represent structurally independent conformational changes. Fast inactivation is not a prerequisite for slow inactivation.