1,5-benzothiazepine binding domain is located on the extracellular side of the cardiac L-type Ca2+ channel.
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 (I[Ca(L)]) with an IC50 value of 1.2 +/- 0.02 microM, which was close to the IC50 value of diltiazem (0.63 +/- 0.01 microM). The suppression of I[Ca(L)] by DTZ417 was voltage and use dependent but lacked tonic block, which allowed us to investigate the onset of the effect on I[Ca(L)] by changing the holding potential (HP) from -90 to -50 mV in the presence of DTZ417. DTZ417 did not have significant effects on I[Ca(L)] at an HP of -90 mV. At -50 mV, DTZ417 (50 microM) applied from the extracellular side completely suppressed I[Ca(L)], whereas it had no effect from the intracellular side. DTZ323 (1 microM) also inhibited I[Ca(L)] only from the extracellular side, without any effects by the intracellular application of < or = 10 microM. 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.
Kurokawa
J
J
Department of Toxicology and Pharmacology, Faculty of Pharmaceutical Sciences, University of Tokyo, Bunkyo-ku, Japan.
Adachi-Akahane
S
S
Nagao
T
T
D000818:Animals; D015220:Calcium Channels / Q000187:drug effects*; D002462:Cell Membrane / Q000187:drug effects; D004110:Diltiazem / Q000031:analogs & derivatives* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D008297:Male; D013841:Thiazepines / Q000494:pharmacology*
C106746:1,5-benzothiazepine; D015220:Calcium Channels; C105375:DTZ 323; D013841:Thiazepines; D004110:Diltiazem