Activation of L-type calcium channel by tolazoline derivatives: role of isothiocyanate moiety.
Studies have investigated the pharmacologic mechanism of 2-(4'-isothiocyanatobenzyl) imidazoline (IBI) and analogs for interaction with imidazoline receptors (IRs), alpha-adrenergic receptors (alpha-ARs), and calcium channels in cardiovascular muscle systems. IBI differs from tolazoline by substitution of an electrophilic isothiocyanato (NCS) group. Unlike tolazoline, which is a partial alpha-AR agonist, IBI produced an irreversible, slow-onset, and sustained contraction of rat aorta with an median effective concentration (EC50) value of 5 microM, and a maximal contraction (116%) greater than that of phenylephrine (100%) and tolazoline (59%). The IBI-induced contractions were dependent on calcium channels and independent of alpha-ARs or IRs. Similarly, structure-activity relation studies in rat aortic smooth muscles on a series of synthesized IBI analogs indicated that NCS analogs, but not those without the NCS group, exhibited effects by a non-alpha-AR, non-IR, but a calcium channel-dependent mechanism. Thus the presence of an intact IBI ring in these analogs is not a requirement for these activities. Further, IBI inhibited dihydropyridine (DHP, [3H]PN 200-110 and [3H]Bay K 8644) binding to L-type calcium channels of T-tubule membranes in rabbit skeletal muscle. In contrast to nifedipine, IBI and NCS derivatives (nifedipine-NCS, naphazoline-NCS) only partially (50-88%) displaced specific binding of these radioligands. A single site of noncooperative interaction was observed for nifedipine (nH = 0.97), whereas tolazoline-NCS (IBI, nH = 1.46) and nifedipine-NCS (nH = 1.37) exhibited a positive cooperativity in binding to DHP sites. These receptor-binding data indicate that NCS derivatives bind to L-type calcium channels and interact allosterically with DHP-binding sites. Direct binding of the NCS group to specific nucleophilic protein sites of the calcium channel may be responsible for its activation and the subsequent contractile effects of IBI.
Lei
L
L
Division of Pharmacology, College of Pharmacy, The Ohio State University, Columbus, USA.
Vaghy
P L
PL
Slavica
M
M
De Los Angeles
J
J
Smith
B
B
Miller
D D
DD
Feller
D R
DR
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D001011:Aorta / Q000187:drug effects / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D019329:Idazoxan / Q000494:pharmacology; D007093:Imidazoles / Q000494:pharmacology; D017879:Isothiocyanates / Q000494:pharmacology*; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000502:physiology; D010643:Phenoxybenzamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D017207:Rats, Sprague-Dawley; D013329:Structure-Activity Relationship; D014043:Tolazoline / Q000031:analogs & derivatives / Q000494:pharmacology*; D014661:Vasoconstriction / Q000187:drug effects*
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D007093:Imidazoles; D017879:Isothiocyanates; D010643:Phenoxybenzamine; D010656:Phenylephrine; C037152:isothiocyanic acid; D014043:Tolazoline; C014282:cirazoline; D019329:Idazoxan