Effect of (+)-niguldipine on myocardial alpha 1-adrenoceptors in the rabbit.
The influence of the alpha 1A-adrenoceptor subtype-selective antagonist (+)-niguldipine on the alpha 1-mediated positive inotropic effect was assessed in the isolated rabbit ventricular myocardium. (+)-Niguldipine displaced the specific binding of [3H]prazosin to a membrane fraction derived from rabbit ventricular muscle with high (Ki = 64.6 pmol/l; RH = 23%) and low (Ki = 7.08 nmol/l) affinity. (+)-Niguldipine displaced specific [3H]CGP-121177 binding only at very high concentrations (Ki = 118 nmol/l). (+)-Niguldipine at 0.1 pmol/l and higher shifted the concentration-response curve for the alpha 1-mediated positive inotropic effect downwards, but at higher concentrations (up to 10 and 100 nmol/l) it did not cause a further shift of the curve. (+)-Niguldipine (1-100 nmol/l) did not affect the beta-mediated positive inotropic effect and the basal force of concentration. (-)-Niguldipine also showed a selective inhibitory action on the alpha 1-adrenoceptor-mediated positive inotropic effect, but its affinity and potency were approximately 1-2 log units lower than those of (+)-niguldipine. The present results indicate that the alpha 1A-adrenoceptor subtype is involved in the alpha 1-mediated positive inotropic effect. (+)-Niguldipine (or (-)-niguldipine with lower affinity) is able to antagonize selectively the cardiac alpha 1A-adrenoceptor-mediated positive inotropic effect. The magnitude of the alpha 1A-mediated inotropic effect, however, may be much less than that mediated by the alpha 1B-subtype in the rabbit ventricular myocardium.
Endoh
M
M
Department of Pharmacology, Yamagata University School of Medicine, Japan.
Takanashi
M
M
Norota
I
I
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000319:Adrenergic beta-Antagonists / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D004095:Dihydropyridines / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006321:Heart / Q000187:drug effects*; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects*; D011224:Prazosin / Q000378:metabolism; D011412:Propanolamines / Q000378:metabolism; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D011943:Receptors, Adrenergic, beta / Q000378:metabolism; D013268:Stimulation, Chemical
D000317:Adrenergic alpha-Antagonists; D000319:Adrenergic beta-Antagonists; D002121:Calcium Channel Blockers; D004095:Dihydropyridines; D011412:Propanolamines; D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta; C033394:CGP 12177; D011224:Prazosin; C054074:niguldipine