Pharmacological activity of (-)-discretamine, a novel vascular alpha-adrenoceptor and 5-hydroxytryptamine receptor antagonist, isolated from Fissistigma glaucescens.
1. The pharmacological activity of (-)-discretamine, isolated from Fissistigma glaucescens, was determined in rat isolated thoracic aorta, cardiac tissues and ventricular myocytes and guinea-pig isolated trachea. 2. (-)-Discretamine was found to be an alpha 1-adrenoceptor blocking agent in rat thoracic aorta as revealed by its competitive antagonism of noradrenaline (pA2 = 7.20 +/- 0.10)- or phenylephrine (pA2 = 7.60 +/- 0.09)-induced vasoconstriction. It was as potent as phentolamine (pA2 = 7.51 +/- 0.10), but was more potent than yohimbine (pA2 = 6.18 +/- 0.06). Removal of endothelium significantly increased the antagonistic potency of (-)-discretamine on noradrenaline (pA2 = 7.52 +/- 0.09)- or phenylephrine (pA2 = 7.90 +/- 0.09)-induced vasoconstriction. 3. (-)-Discretamine was also an alpha 2-adrenoceptor blocking agent (pA2 = 6.30 +/- 0.15) and a 5-hydroxytryptamine antagonist (pA2 = 6.87 +/- 0.06), both in rat aorta denuded of endothelium. 4. (-)-Discretamine protected alpha-adrenoceptors from alkylation by the irreversible blocking agent, phenoxybenzamine. 5. [3H]-inositol monophosphate formation caused by noradrenaline (3 microM) in rat thoracic aorta was suppressed by (-)-discretamine (10 and 30 microM) and prazosin (3 microM). 6. A high concentration of (-)-discretamine (30 microM) did not affect the contraction induced by the thromboxane receptor agonist U-46619, prostaglandin F2 alpha (PGF2 alpha), angiotensin II, high K+ or endothelin in rat aorta denuded of endothelium. Neither cyclic AMP nor cyclic GMP content of rat thoracic aorta was changed by (-)-discretamine. 7. Contraction of guinea-pig trachea caused by histamine, leukotriene C4 or carbachol was not affected by (-)-discretamine (30 microM). (-)-Discretamine also did not block beta l- or beta2-adrenoceptor-mediated responses induced by isoprenaline in rat right atria and guinea-pig trachea.8. A voltage clamp study in rat ventricular single myocytes revealed that sodium inward current, slow inward Ca2+ current or transient (ItO) and steady state (I800) outward current was not affected by(-)-discretamine (30 microM).9. It is concluded that (-)-discretamine is a selective x-adrenoceptor and 5-HT receptor antagonist in vascular smooth muscle.
Ko
F N
FN
Pharmacological Institute, College of Medicine, National Taiwan University, Taipei.
Yu
S M
SM
Su
M J
MJ
Wu
Y C
YC
Teng
C M
CM
D000317:Adrenergic alpha-Antagonists / Q000302:isolation & purification / Q000494:pharmacology*; D058671:Adrenergic beta-1 Receptor Antagonists; D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects; D001600:Berberine Alkaloids / Q000302:isolation & purification / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism; D006152:Cyclic GMP / Q000378:metabolism; D016207:Cytokines / Q000037:antagonists & inhibitors; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007294:Inositol / Q000378:metabolism; D008297:Male; D009130:Muscle, Smooth / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects; D009638:Norepinephrine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D010946:Plants, Medicinal / Q000737:chemistry*; D015221:Potassium Channels / Q000187:drug effects; D051381:Rats; D011921:Rats, Inbred WKY; D017208:Rats, Wistar; D012702:Serotonin Antagonists; D015222:Sodium Channels / Q000187:drug effects; D014132:Trachea / Q000187:drug effects
D000317:Adrenergic alpha-Antagonists; D058671:Adrenergic beta-1 Receptor Antagonists; D001600:Berberine Alkaloids; D016207:Cytokines; D015221:Potassium Channels; D012702:Serotonin Antagonists; D015222:Sodium Channels; D007294:Inositol; C010941:discretamine; D000242:Cyclic AMP; D006152:Cyclic GMP; D009638:Norepinephrine