Modulation of slow inactivation in class A Ca2+ channels by beta-subunits.
beta-subunit modulation of slow inactivation of class A calcium (Ca2+) channels was studied with two-microlectrode voltage clamp after expression of the alpha1A- (BI-2) together with beta1a-, beta2a-, beta3- or beta4-subunits in Xenopus oocytes. On- and off-rates of slow inactivation were estimated from the kinetics of recovery from slow inactivation. Ca2+ channels with an alpha1A/beta-subunit composition inducing the slower rate of fast inactivation displayed the faster rate of slow inactivation. The corresponding order of slow inactivation time constants (tau[onset]) was: alpha1A/beta2a, 33 +/- 3 s; alpha1A/beta4, 42 +/- 4 s; alpha1A/beta1a, 59 +/- 4 s; alpha1A/beta3, 67 +/- 5 s (n >= 7). Recovery of class A Ca2+ 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 beta-subunits. Two mutations in segment IIIS6 (IF1612/1613AA) slowed fast inactivation and accelerated the onset of slow inactivation in the resulting mutant (alpha1A/IF-AA/beta3) in a similar manner as coexpression of the beta2a-subunit. Recovery from slow inactivation was slightly slowed in the double mutant. Our data suggest that class A Ca2+ 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 beta-subunits. Fast and slow inactivation in class A Ca2+ channels appears to represent structurally independent conformational changes. Fast inactivation is not a prerequisite for slow inactivation.
Sokolov
S
S
Institut fur Biochemische Pharmakologie, Peter-Mayr-Strasse 1, A-6020 Innsbruck, Austria.
Weiss
R G
RG
Timin
E N
EN
Hering
S
S
D000465:Algorithms; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000235:genetics / Q000378:metabolism*; D004594:Electrophysiology; D007700:Kinetics; D008564:Membrane Potentials / Q000502:physiology; D009154:Mutation / Q000235:genetics / Q000502:physiology; D018408:Patch-Clamp Techniques; D017354:Point Mutation / Q000235:genetics; D014982:Xenopus laevis
D002121:Calcium Channel Blockers; D015220:Calcium Channels