Shinjiro Nakajyo

The difference in the inhibitory mechanisms of papaverine on vascular and intestinal smooth muscles

Papaverine (0.3-100 microM) more potently inhibited phenylephrine (1 microM)-induced contraction than 65 mM K+-induced contraction of the aorta, while it equally inhibited contractions induced by 65 mM K+ and carbachol (1 microM) in ileal smooth muscle. In phenylephrine-treated aorta, papaverine (1-10 microM) increased the cAMP and cGMP content. However, in carbachol-treated ileum, 30 microM papaverine partially increased the cAMP content while it maximally relaxed the preparation. In fura2-loaded aorta, papaverine (0.3-10 microM) inhibited both the contraction and the increase in intracellular Ca2+ level ([Ca2+]i) induced by phenylephrine in parallel. However, papaverine inhibited carbachol-induced contraction with only a small decrease in [Ca2+]i. Papaverine (1-30 microM) inhibited the carbachol-induced increase in oxidized flavoproteins, an indicator of increased mitochondrial oxidative phosphorylation, in ileal smooth muscle whereas it did not change the phenylephrine-induced increase in the aorta. These results suggest that papaverine inhibits smooth muscle contraction mainly by the accumulation of cAMP and/or cGMP due to the inhibition of phosphodiesterase in the aorta whereas, in ileal smooth muscle, papaverine inhibits smooth muscle contraction mainly by the inhibition of mitochondrial respiration.

Effects of Various Selective Phosphodiesterase Inhibitors on Carbachol-Induced Contraction and Cyclic Nucleotide Contents in the Guinea Pig Gall Bladder

The effects of various selective phosphodiesterase (PDE) inhibitors on muscle contractility and cyclic nucleotide contents in the guinea pig gall bladder were investigated. Various selective PDE inhibitors, vinpocetine (type 1), erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, type 2), milrinone (type 3), Ro20-1724 (type 4), and zaprinast (type 5), inhibited CCh-induced contractions in a concentration-dependent manner. The rank order of potency for the gall bladder was Ro20-1724 > vinpocetine > EHNA > milrinone > zaprinast, which was different from that of the trachea, taenia coli, and aorta. In the presence of CCh (0.3 muM), vinpocetine, milrinone, and Ro20-1724 each increased cAMP content, but not cGMP. By contrast, zaprinast increased cGMP content, but not cAMP, and EHNA increased both cAMP and cGMP contents. These results suggest that vinpocetine-, milrinone-, and Ro20-1724-induced relaxation was correlated with cAMP, zaprinast-induced relaxation was correlated with cGMP, and that EHNA-induced relaxation was correlated with cAMP and cGMP in the guinea pig gall bladder. In conclusion, the effect of PDE inhibitors in the guinea pig gall bladder was different from those in smooth muscles, such as the trachea, taenia coli, and aorta.

Inhibitory effect of lithium on muscle contractions caused by various stimulants in guinea-pig ileum

The tonic contractions induced in the guinea-pig ileal longitudinal muscle by 60 mM KCl, carbachol, histamine or A23187 were inhibited similarly by Li+-substituted solution. There was a good correlation between the Li+-induced inhibition of contractions induced by these stimulants and the intracellularly accumulated Li+ content. In the experiment with fura-2, Li+-substituted solution inhibited the high-K+-induced contraction without decreasing the cytosolic Ca2+ level elevated by high-K+ solution. The concentration-response curve for Ca2+ in the skinned fiber was shifted to the right by pretreatment with Li+-substituted solution. These results suggest that the inhibitory mechanism of Li+ on the contractions by various stimulants partly involves the direct inhibition of the contractile proteins in the guinea-pig ileal longitudinal muscle.

The prostaglandin E2-induced contraction in the ileal longitudinal smooth muscle isolated from various animal species.

PGE2 was examined for the effect on contractile response of the ileal longitudinal muscle isolated from eight animal species, monkey, dog, cat, rabbit, guinea pig, vole, rat and mouse. In isotonic recording, PGE2 induced a sustained contraction in the ileum isolated from monkey and cat. It caused a transient contraction followed by a relaxation in the ileum isolated from other animal species. The sensitivity of ileal strips to the contractile effect of PGE2, as estimated from 50% effective dose (ED50) of PGE2, was in the order of guinea pig>vole>rabbit>rat>mouse>monkey>dog>cat. On the other hand, ED50 values of acetylcholine (ACh) were in the concentrations ranging from 1×10-7 M to 5×10-7 M except for guinea pig. Thus, there were species differences in the sensitivity of the ileal strips to PGE2 but not to ACh. Further, the contractile responses to PGE2 in the intestine from these animal species were divided into three groups using various inhibitors. In the group 1 animals consisting of herbivorous like rabbit, guinea pig and vole, PGE2 contractions were inhibited by TTX, atropine, scopolamine or SC-19220. In the group 2 animals consisting of carnivorous like dog and cat, PGE2 contractions were inhibited by scopolamine and SC-19220 but not by atropine or TTX. In the group 3 animals consisting of omnivorous like monkey, rat and mouse, PGE2 contractions were inhibited only by SC-19220.

Inhibitory Effects of Tiamulin on Contractile and Electrical Responses in Isolated Thoracic Aorta and Cardiac Muscle of Guinea‐pigs

Abstract— The inhibitory effect of tiamulin, an antibiotic produced by Pleurotus mutilis, on contractile and electrical responses in isolated thoracic aorta and cardiac muscle of guinea‐pigs was studied. In the thoracic aorta, tiamulin with an IC50 of 9·7 × 10−6 m inhibited sustained contractions induced by isosmotically added 60 Mm KCl. The inhibitory effect of tiamulin on a Ca2+‐induced contraction in a depolarized muscle was competitively antagonized by raising external Ca2+ concentration. Bay K 8644 (10−7 m) antagonized tiamulins inhibition of the Ca2+ ‐induced contraction. Tiamulin (2×10−5 m) decreased the elevated cytoplasmic Ca2+ level measured by the fura 2 AM method in the depolarized muscle. In high K+‐isoprenaline‐treated left atria, tiamulin (2 × 10−5–2 × 10−4 m) produced negative inotropic effects. On the other hand in the membrane action potential of papillary muscles, tiamulin (2 × 10−6–2 × 10−4 m) produced decreases in action potential and durations and 2 × 10−4 m tiamulin depressed the slow response action potential in depolarized muscles. Tiamulin produced prolongations of the PR interval in ECG, negative chrono‐ and inotropic effects, and an increase in perfusion flow in guinea‐pig isolated and perfused hearts. These effects of tiamulin on the aorta or cardiac muscle were similar to those of verapamil and nifedipine. These results suggest that both the inhibitory action of tiamulin on the high K+‐induced contraction in the aorta and the negative inotropic effect of tiamulin on the cardiac muscle are due to an inhibition of Ca2+ entry through the voltage‐dependent Ca2+ channels of cells of both these muscles.

Effects of Papaverine on Twitches in Mouse Diaphragm

This study examined the inhibitory effects of papaverine on twitches directly elicited by electrical stimulation of the mouse diaphragm. Papaverine (3–100 µM) inhibited twitches in a dose-dependent manner. Papaverine increased the cyclic adenosine monophosphate (cAMP) but not cyclic guanosine monophosphate (cGMP) content. IBMX, Db-cAMP and 8-br-cGMP did not affect twitches, whereas verapamil and NaCN inhibited twitches. Increases in extracellular Ca2+ removed the twitch inhibition caused by verapamil but not that caused by papaverine. Papaverine (30 and 100 µM) and NaCN (1 mM) decreased creatine phosphate and ATP contents. These results suggest that the relaxing effects of papaverine on mouse diaphragm are mainly due to inhibition of aerobic energy metabolism.