L-type Ca channel block by highly hydrophilic dihydropyridines in single ventricular cells of guinea-pig hearts.
Blocking of L-type Ca channels by highly hydrophilic dihydropyridines, NKY-722 and KV-1360, was investigated in single ventricular cells of guinea-pig hearts using the whole-cell voltage clamp technique. At a holding potential of -30 mV, NKY-722 (1-100 nM) decreased the amplitude of the L-type Ca channel current (ICa) in a concentration-dependent manner. NKY-722 did not change the time constants of the decay of ICa. In the presence of NKY-722 (1 microM), the steady-state inactivation curve was shifted toward a more negative potential (by -33.0 +/- 2.0 mV) without changing its slope factor. The use-dependent block was elicited at a pulse frequency of 3.3 Hz or more. Even after washing out the drug at -80 mV for 20 min, ICa inhibited by NKY-722 (100 nM) at -30 mV was scarcely recovered when the membrane potential was clamped back to -30 mV. A permanently charged compound KV-1360 (0.1-1 microM), a quaternary amine derivative of NKY-722, hardly affected ICa by intracellular and extracellular application. These results suggest that, in spite of the high degree of ionization (91% in the charged form at pH 7.4), the mode of the L-type Ca channel block by NKY-722 is quite similar to that by lipophilic dihydropyridines. Consequently, the neutral form of NKY-722 is the active compound and this reaches the dihydropyridine receptor by "membranous approach".
Department of Pharmacology, Akita University School of Medicine, Japan.
H
H
Watanabe
M
M
Nishio
M
M
Miura
T
T
Iijima
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000145:classification / Q000187:drug effects* / Q000378:metabolism; D004095:Dihydropyridines / Q000494:pharmacology*; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004095:Dihydropyridines; C097374:KV 1360; C058351:NKY 722