Direct inhibition of contractile apparatus by analogues of amiloride in the smooth muscle of guinea-pig taenia caecum and chicken gizzard

Abstract

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 (Ca 2+ + Mg 2+)-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 Ca 2+/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.