PT  - JOURNAL ARTICLE
AU  - Junko Kurokawa
AU  - Satomi Adachi-Akahane
AU  - Taku Nagao
TI  - 1,5-Benzothiazepine Binding Domain Is Located on the Extracellular Side of the Cardiac L-Type Ca<sup>2+</sup> Channel
AID  - 10.1124/mol.51.2.262
DP  - 1997 Feb 01
TA  - Molecular Pharmacology
PG  - 262--268
VI  - 51
IP  - 2
4099  - http://molpharm.aspetjournals.org/content/51/2/262.short
4100  - http://molpharm.aspetjournals.org/content/51/2/262.full
SO  - Mol Pharmacol1997 Feb 01; 51
AB  - To determine whether 1,5-benzothiazepine Ca2+ channel blocker approaches its binding domain within the cardiac L-type Ca2+ channel from inside or outside of the membrane, we tested the effects of a novel potent 1,5-benzothiazepine derivative (DTZ323) and its quaternary ammonium derivative (DTZ417) on guinea pig ventricular myocytes by using the whole-cell patch-clamp technique. The extracellular application of DTZ417 suppressed the L-type Ca2+ channel currents (ICa(L)) with an IC50 value of 1.2 ± 0.02 μm, which was close to the IC50 value of diltiazem (0.63 ± 0.01 μm). The suppression of ICa(L) by DTZ417 was voltage and use dependent but lacked tonic block, which allowed us to investigate the onset of the effect on ICa(L) by changing the holding potential (HP) from −90 to −50 mV in the presence of DTZ417. DTZ417 did not have significant effects on ICa(L)at an HP of −90 mV. At −50 mV, DTZ417 (50 μm) applied from the extracellular side completely suppressed ICa(L), whereas it had no effect from the intracellular side. DTZ323 (1 μm) also inhibited ICa(L) only from the extracellular side, without any effects by the intracellular application of ≤10 μm. However, a quaternary phenylalkylamine derivative, D890 (0.1 mm), acted only from the intracellular side. These results suggest that in contrast to the phenylalkylamine binding site, in cardiac myocytes the 1,5-benzothiazepine binding site is accessible from the extracellular side of the L-type Ca2+ channel.