Some effects of nipradilol, a beta-antagonist possessing a nitroxy group, on smooth muscle of the pig coronary artery.
1. The effects of nipradilol, a beta-adrenoceptor antagonist which possesses a nitroxy group, on cytosolic Ca2+ concentration ([Ca2+]i), and on tension development were simultaneously measured by front-surface fluorometry and fura-2-loaded strips in the proximal portion of pig coronary arteries. 2. Nipradilol reduced in a concentration-dependent manner both the [Ca2+]i and tension, irrespective of whether the strips were unstimulated or exposed to either high K+ or histamine containing solutions. However, both in the case of contractions induced by high K+-depolarization and histamine stimulation, for a given [Ca2+]i elevation the tension which developed in the presence of nipradilol was smaller than that generated in its absence, so that the [Ca2+]i-tension curves during the contraction were shifted to the right. 3. In the absence of extracellular Ca2+, the [Ca2+]i elevation due to the release of Ca2+ from histamine-sensitive store was inhibited by nipradilol. Nipradilol had no effect on the [Ca2+]i elevation due to the release of Ca2+ from caffeine-sensitive stores; however, it did inhibit the caffeine-induced increase in tension. A derivative of nipradilol, which lacked a nitroxy molecule (Nip(-N)), had no effect on the [Ca2+]i and tension elevated by histamine or caffeine in the absence of extracellular Ca2+. 4. The beta-adrenoceptor agonist, isoprenaline, reduced [Ca2+]i tension when applied to steady state contractions induced by high K+, or at the peak level of tension to histamine. The reduction of [Ca2+]i and tension induced by isoprenaline was inhibited by Nip(-N) in a concentration-dependent manner and nipradilol inhibited the isoprenaline-induced relaxation with bell-shaped concentration-response curves. At lower concentrations, nipradilol acted as a beta-blocker, the IC50- value being smaller than that of Nip(-N), and at higher concentrations, it acted as a nitrovasodilator. 5. Thus, it is suggested that, at lower concentrations, nipradilol, an antianginal drug, acts as a beta-adrenoceptor antagonist. At higher concentrations, it relaxes the proximal portion of the coronary artery by directly reducing [Ca2+]i and the Ca2+-sensitivity of the myofilaments, apparently due to the presence of the nitroxy molecule.
Abe
S
S
Division of Clinical Nutrition, Graduate School of Health and Nutrition Sciences, Nakamura-Gakuen, Japan.
Nakamura
M
M
Kanaide
H
H
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000737:chemistry / Q000494:pharmacology*; D000704:Analysis of Variance; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D003331:Coronary Vessels / Q000187:drug effects / Q000378:metabolism; D005456:Fluorescent Dyes / Q000494:pharmacology; D016257:Fura-2 / Q000494:pharmacology; D006632:Histamine; D007545:Isoproterenol / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000378:metabolism; D011412:Propanolamines / Q000737:chemistry / Q000494:pharmacology*; D013552:Swine
D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D000697:Central Nervous System Stimulants; D005456:Fluorescent Dyes; D011412:Propanolamines; D002110:Caffeine; D006632:Histamine; C036089:nipradilol; D007545:Isoproterenol; D002118:Calcium; D016257:Fura-2