Dihydropyridine enantiomers block recombinant L-type Ca2+ channels by two different mechanisms.
1. The molecular basis of the state-dependent block of L-type Ca2+ channels by dihydropyridines is still poorly understood. Therefore, we studied the enantioselectivity of Ca2+ channel block by isradipine enantiomers at three holding potentials (-80, -60 and -40 mV) in Chinese hamster ovary (CHO) cells stably expressing the rabbit lung alpha1C-b-subunit. 2. The extent of enantioselectivity did not markedly change with the holding potential (IC50 ratios of 104-138), whereas the potency of both isradipine enantiomers increased with depolarisation of the holding potential. 3. In addition to its block of the peak Ca2+ channel current, Ipeak, (-)-isradipine inhibited the relative current at the end of the test pulse, the so-called Ilate, normalised to Ipeak (Ilate/Ipeak). This effect was unaffected by the holding potential and revealed distinct kinetics compared to the development of conventional block of Ipeak. 4. When these effects were studied using an alpha1C-b-mutant lacking the high-affinity dihydropyridine binding site, expressed in human embryonic kidney (HEK 293) cells, both enantiomers blocked Ilate/Ipeak to a similar degree. 5. Our data are discussed within the framework of the 'guarded receptor' and the 'modulated receptor' hypotheses. The very different properties of the block of Ilate/Ipeak compared to those of the conventional high-affinity block of Ipeak suggest the existence of an additional mechanism possibly mediated via a second, distinct binding site.
Handrock
R
R
Department of Pharmacology, University of Kiel, Hospitalstrasse 4, 24105 Kiel, Department of Pharmacology, University of Cologne, Gleueler Strasse 24, 50931 Cologne, Germany. renate.handrock@uni-koeln.de
Rao-Schymanski
R
R
Klugbauer
N
N
Hofmann
F
F
Herzig
S
S
D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D002121:Calcium Channel Blockers / Q000737:chemistry / Q000494:pharmacology*; D020746:Calcium Channels, L-Type / Q000187:drug effects* / Q000378:metabolism; D002460:Cell Line; D006224:Cricetinae; D006801:Humans; D017275:Isradipine / Q000737:chemistry / Q000494:pharmacology*; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D011817:Rabbits; D011994:Recombinant Proteins / Q000187:drug effects / Q000378:metabolism; D013237:Stereoisomerism
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D011994:Recombinant Proteins; D017275:Isradipine