S. Berjukow et al., Molecular mechanism of calcium channel block by isradipine - Role of a drug-induced inactivated channel conformation, J BIOL CHEM, 275(29), 2000, pp. 22114-22120
The role of the inactivated channel conformation in the molecular mechanism
of Ca2+ channel block by the 1,4-dihydropyridine (DHP) (+)-isradipine was
analyzed in L-type channel constructs (alpha(1LC); Berjukow, S., Gapp, F.,
Aczel, S., Sinnegger, M. J., Mitterdorfer, J., Glossmann, H., and Hering, S
. (1999) J. Biol. Chem. 274, 6154-6160) and a DHP-sensitive class A Ca2+ ch
annel mutant (alpha(1A-DHP); Sinnegger, M. J,, Wang, Z,, Grabner, RI., Heri
ng, S., Striessnig, J., Glossmann, H., and Mitterdorfer, J. (1997) J. Biol
Chem. 272, 27686-27693) carrying the high affinity determinants of the DHP
receptor site but inactivating at different rates. Ca2+ channel inactivatio
n was modulated by coexpressing the alpha(1A-DHP)- or alpha(1LC)-subunits i
n Xenopus oocytes with either the beta(2a)- or the beta(1a)-subunit and ami
no acid substitutions in L-type segment IVS6 (I1497A, I1498A, and V1504A).
Contrary to a modulated receptor mechanism assuming high affinity DHP bindi
ng to the inactivated state we observed no clear correlation between steady
state inactivation and Ca2+ channel block by (+)-isradipine: (i) a 3-fold
larger fraction of alpha(1A-DHP/beta 1a), channels in steady state inactiva
tion at -80 mV (compared with alpha(1A-DHP)/beta(2a)) did not enhance the b
lock by (+)-isradipine; (ii) different steady state inactivation of alpha(1
Lc) mutants at -30 mV did not correlate with voltage-dependent channel bloc
k; and (iii) the midpoint-voltages of the inactivation curves of slowly ina
ctivating L-type constructs and more rapidly inactivating alpha(1Lc)/beta(1
a) channels were shifted to a comparable extent to more hyperpolarized volt
ages. A kinetic analysis of (+)-isradipine interaction with different L-typ
e channel constructs revealed a drug-induced inactivated state. Entry and r
ecovery from drug-induced inactivation are modulated by intrinsic inactivat
ion determinants, suggesting a synergism between intrinsic inactivation and
DHP block.