Mechanism of vascular relaxation by thaligrisine: functional and binding assays.
In the present study we examine the mechanism by which thaligrisine, a bisbenzyltetrahydroisoquinoline alkaloid, inhibits the contractile response of vascular smooth muscle. The work includes functional studies on rat isolated aorta and tail artery precontracted with noradrenaline or KCl. In other experiments rat aorta was precontracted by caffeine in the presence or absence of extracellular Ca2+. In order to assess whether thaligrisine interacts directly with calcium channel binding sites or with alpha-adrenoceptors we examined the effect of the alkaloid on [3H]-(+)-cis diltiazem, [3H]-nitrendipine and [3H]-prazosin binding to cerebral cortical membranes. The functional studies showed that the alkaloid inhibited in a concentration-dependent manner the contractile response induced by depolarization in rat aorta (IC50 = 8.9+/-2.9 microM, n=5) and in tail artery (IC50 = 3.04+/-0.3 microM, n=6) or noradrenaline induced contraction in rat aorta (IC50 = 23.0+/-0.39 microM, n=9) and in tail artery (IC50 = 3.8+/-0.9 microM, n=7). In rat aorta, thaligrisine concentration-dependently inhibited noradrenaline-induced contraction in Ca2+-free solution (IC50 = 13.3 microM, n=18). The alkaloid also relaxed the spontaneous contractile response elicited by extracellular calcium after depletion of noradrenaline-sensitive intracellular stores (IC50 = 7.7 microM, n=4). The radioligand receptor-binding study showed that thaligrisine has higher affinity for [3H]-prazosin than for [3H]-(+)-cis-diltiazem binding sites, with Ki values of 0.048+/-0.007 microM and 1.5+/-1.1 microM respectively. [3H]-nitrendipine binding was not affected by thaligrisine. The present work provides evidence that thaligrisine shows higher affinity for [3H]-prazosin binding site than [3H]-(+)-cis-diltiazem binding sites, in contrast with tetrandrine and isotetrandrine that present similar affinity for both receptors. In functional studies thaligrisine, acted as an alpha1-adrenoceptor antagonist and as a Ca2+ channel blocker, relaxing noradrenaline or KCl-induced contractions in vascular smooth muscle. This compound specifically inhibits the refilling of internal Ca2+-stores sensitive to noradrenaline, by blocking Ca2+-entry through voltage-dependent Ca2+-channels.
Tur
R
R
Departamento de Farmacología, Facultat de Farmàcia, Universitat de València, Spain.
Magraner
J
J
Catret
M
M
Elorriaga
M
M
Ivorra
M D
MD
D'Ocon
P
P
Bermejo
A
A
Cabedo
N
N
Cortes
D
D
Anselmi
E
E
D000316:Adrenergic alpha-Agonists / Q000378:metabolism / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000378:metabolism / Q000494:pharmacology; D000470:Alkaloids / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D001158:Arteries / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D001665:Binding Sites; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000378:metabolism / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism; D004110:Diltiazem / Q000378:metabolism / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D009126:Muscle Relaxation / Q000187:drug effects* / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D009568:Nitrendipine / Q000378:metabolism / Q000494:pharmacology; D009638:Norepinephrine / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D011224:Prazosin / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism; D013623:Tail / Q000098:blood supply; D014197:Trees / Q000737:chemistry; D014316:Tritium; D014662:Vasoconstrictor Agents / Q000378:metabolism / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D000470:Alkaloids; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D011942:Receptors, Adrenergic, alpha; D014662:Vasoconstrictor Agents; C413291:thaligrisine; D014316:Tritium; D002110:Caffeine; D011189:Potassium Chloride; D009568:Nitrendipine; D004110:Diltiazem; D002118:Calcium; D009638:Norepinephrine; D011224:Prazosin