alpha 1D (Cav1.3) subunits can form l-type Ca2+ channels activating at negative voltages.
In cochlea inner hair cells (IHCs), L-type Ca(2+) channels (LTCCs) formed by alpha1D subunits (D-LTCCs) possess biophysical and pharmacological properties distinct from those of alpha1C containing C-LTCCs. We investigated to which extent these differences are determined by alpha1D itself by analyzing the biophysical and pharmacological properties of cloned human alpha1D splice variants in tsA-201 cells. Variant alpha1D(8A,) containing exon 8A sequence in repeat I, yielded alpha1D protein and L-type currents, whereas no intact protein and currents were observed after expression with exon 8B. In whole cell patch-clamp recordings (charge carrier 15-20 mm Ba(2+)), alpha1D(8A) - mediated currents activated at more negative voltages (activation threshold, -45.7 versus -31.5 mV, p < 0.05) and more rapidly (tau(act) for maximal inward currents 0.8 versus 2.3 ms; p < 0.05) than currents mediated by rabbit alpha1C. Inactivation during depolarizing pulses was slower than for alpha1C (current inactivation after 5-s depolarizations by 90 versus 99%, p < 0.05) but faster than for LTCCs in IHCs. The sensitivity for the dihydropyridine (DHP) L-type channel blocker isradipine was 8.5-fold lower than for alpha1C. Radioligand binding experiments revealed that this was not due to a lower affinity for the DHP binding pocket, suggesting that differences in the voltage-dependence of DHP block account for decreased sensitivity of D-LTCCs. Our experiments show that alpha1D(8A) subunits can form slowly inactivating LTCCs activating at more negative voltages than alpha1C. These properties should allow D-LTCCs to control physiological processes, such as diastolic depolarization in sinoatrial node cells, neurotransmitter release in IHCs and neuronal excitability.
Koschak
A
A
Institut für Biochemische Pharmakologie, Peter-Mayr-Strasse 1, A-6020 Innsbruck, Austria.
Reimer
D
D
Huber
I
I
Grabner
M
M
Glossmann
H
H
Engel
J
J
Striessnig
J
J
D000595:Amino Acid Sequence; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D003001:Cloning, Molecular; D018076:DNA, Complementary; D006801:Humans; D017275:Isradipine / Q000494:pharmacology; D008969:Molecular Sequence Data
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D018076:DNA, Complementary; D017275:Isradipine