Mechanisms underlying the inhibitory effect of dibutyryl cyclic AMP in vascular smooth muscle.
The mechanism by which dibutyryl cyclic AMP (db-cAMP) induces vasodilatation was examined in isolated rat aorta. The contraction induced by norepinephrine (NE) was more sensitive to the inhibitory effect of db-cAMP than that induced by high K+, and the contraction induced by lower concentrations of each stimulant was more sensitive to db-cAMP than that induced by higher concentrations. Db-cAMP at 10 microM inhibited the increases in muscle tension and cytosolic Ca2+ level ([Ca2+]i) without changing the [Ca2+]i-tension relationship, suggesting that the inhibitory effect is mainly due to a decrease in [Ca2+]i. A higher concentration (300 microM) of db-cAMP inhibited muscle tension more strongly than [Ca2+]i suggesting that db-cAMP decreases Ca2+ sensitivity of contractile elements. In contrast, 10 microM verapamil inhibited the NE-stimulated [Ca2+]i more strongly than the NE-induced contraction. The verapamil-insensitive portion of the NE-stimulated [Ca2+]i and contraction was inhibited by db-cAMP, suggesting that db-cAMP and verapamil act by different mechanisms. In Ca(2+)-free solution, 1 microM NE induced transient increases in muscle tension and [Ca2+]i. The transient contraction was inhibited by 1 mM db-cAMP more strongly than [Ca2+]i. An activator of adenylate cyclase, forskolin, showed inhibitory effects similar to those of db-cAMP. The inhibitory effects of db-cAMP and forskolin were inversely proportional to [Ca2+]i before the addition of these inhibitors. These results suggest that db-cAMP inhibits smooth muscle contraction by decreasing [Ca2+]i and the Ca2+ sensitivity of contractile elements, and that both of these effects are stronger when [Ca2+]i is lower.
Abe
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A
Department of Veterinary Pharmacology, Faculty of Agriculture, University of Tokyo, Japan.
Karaki
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H
D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D003994:Bucladesine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D009638:Norepinephrine / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains
D003994:Bucladesine; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine