Takahiro Moriyama's research while affiliated with The University of Tokyo and other places

The effect of reserpine on vascular and intestinal smooth muscles was examined. In these muscles, reserpine inhibited the high K(+)-induced contraction, and this inhibitory effect was antagonized by the increase in external Ca2+ concentration and also by a Ca2+ channel activator, Bay k8644. In rabbit aorta, increases in cytosolic Ca2+ level and muscle tension due to high K+ were inhibited in parallel by reserpine. These results suggest that reserpine inhibits L-type Ca2+ channels to inhibit smooth muscle contraction.

The relaxant effects of amiloride and its analogues, benzamil, 5-(N', N'-diethyl)-amiloride (DEAM) and 5-(N-ethyl-N-isopropyl)-amiloride (EIAM), were investigated using smooth muscle of guinea-pig taenia caeci and chicken gizzard. High K+-induced contractions of intact taenia and gizzard were inhibited by these compounds (1–100 μM) with the order of potency; benzamil ≥ EIAM > DEAM > amiloride. Contractions of permealized taenia and gizzard were also inhibited by these compounds at concentrations 8–35 times higher than those needed to inhibit the contractions of intact tissues. These compounds inhibited 20 K myosin light chain (MLC) phosphorylation at the concentrations needed to inhibit the contraction in the permealized muscles. Calmodulin (CaM) activity, as monitored by erythrocyte membrane (Ca2+ + Mg2+)-ATPase and phosphodiesterase activities, was inhibited by DEAM and EIAM at similar concentrations as those to inhibit the MLC phosphorylation. Benzamil also inhibited CaM activity at concentrations 4–8 times higher than those required to inhibit MLC phosphorylation. However, amiloride failed to inhibit CaM activity. Among these compounds, amiloride and benzamil inhibited Ca2+/CaM-independent MLC phosphorylation due to trypsin-treated MLC kinase. Taenia tissue gradually accumulated these compounds and the tissue/medium ratio exceeded 3.5–17 after a 3-hr incubation period. These results indicate that amiloride and its analogues inhibit smooth muscle contraction mainly by the direct inhibition of MLC phosphorylation. The inhibitory effect of amiloride may be attributable to the inhibition of MLC kinase, whereas the inhibitory effect of DEAM and EIAM may largely be attributable to the inhibition of CaM. Benzamil may inhibit contraction by the inhibition of both MLC kinase and CaM. Differences in the drug-sensitivity between intact and permealized tissues may be attributable to the difference in drug accumulation by the cell.

The relaxant action of amiloride was investigated in the smooth muscles of guinea pig taenia ceci and chicken gizzard. Amiloride inhibited the contractions induced by high K+ (45.4 mM) and carbachol (10 microM) in the taenia with the concentrations needed to induce 50% inhibition (IC50) of approximately 41 microM. A prolonged incubation period (greater than 1 hr) was necessary to obtain the full inhibition of these contractions. The taenia gradually accumulated amiloride and the tissue/medium ratio exceeded 2.0 after a 120-min incubation period. Amiloride had no effect on the high K+-stimulated 45Ca++ uptake or the ATP content of the taenia. Amiloride inhibited the Ca++-induced contraction of the saponin-treated taenia with an IC50 of 186 microM. Amiloride (10-1000 microM) also inhibited superprecipitation and Mg++-adenosine triphosphatase activity of the gizzard native actomyosin as well as the phosphorylation of myosin light chain. The inhibition of the phosphorylation was antagonized competitively by ATP. Amiloride (1 mM) had no effect on the dephosphorylation of myosin light chain upon removal of Ca++ from reaction medium. Amiloride, at concentrations up to 1 mM, had not effect on calmodulin activity as monitored by the Ca++-calmodulin-activated erythrocyte membrane (Ca++ + Mg++)-adenosine triphosphatase and phosphodiesterase activities. In contrast to this, trifluoperazine inhibited the calmodulin activity at the concentration needed to inhibit the Ca++-induced contraction of the permeabilized taenia and the superprecipitation and the phosphorylation of myosin light chain of gizzard. We conclude that amiloride, unlike trifluoperazine, may inhibit directly the myosin light chain kinase activity to induce muscle relaxation.