Receptor-mediated Smooth Muscle Contraction Research Articles

Phospholipase C-δ1modulates sustained contraction of rat mesenteric small arteries in response to noradrenaline, but not endothelin-1

Vasoconstrictors activate phospholipase C (PLC), which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP(2)), leading to calcium mobilization, protein kinase C activation, and contraction. Our aim was to investigate whether PLC-delta(1), a PLC isoform implicated in alpha(1)-adrenoreceptor signaling and the pathogenesis of hypertension, is involved in noradrenaline (NA) or endothelin (ET-1)-induced PIP(2) hydrolysis and contraction. Rat mesenteric small arteries were studied. Contractility was measured by pressure myography, phospholipids or inositol phosphates were measured by radiolabeling with (33)Pi or myo-[(3)H]inositol, and caveolae/rafts were prepared by discontinuous sucrose density centrifugation. PLC-delta(1) was localized by immunoblot analysis and neutralized by delivery of PLC-delta(1) antibody. The PLC inhibitor U73122, but not the negative control U-73342, markedly inhibited NA and ET-1 contraction but had no effect on potassium or phorbol ester contraction, implicating PLC activity in receptor-mediated smooth muscle contraction. PLC-delta(1) was present in caveolae/rafts, and NA, but not ET-1, stimulated a rapid twofold increase in PLC-delta(1) levels in these domains. PLC-delta(1) is calcium dependent, and removal of extracellular calcium prevented its association with caveolae/rafts in response to NA, concomitantly reducing NA-induced [(33)P]PIP(2) hydrolysis and [(3)H]inositol phosphate formation but with no effect on ET-1-induced [(33)P]PIP(2) hydrolysis. Neutralization of PLC-delta(1) by PLC-delta(1) antibody prevented its caveolae/raft association and attenuated the sustained contractile response to NA compared with control antibodies. In contrast, ET-1-induced contraction was not affected by PLC-delta(1) antibody. These results indicate the novel and selective role of caveolae/raft localized PLC-delta(1) in NA-induced PIP(2) hydrolysis and sustained contraction in intact vascular tissue.

Pressor responses to platelet-activating factor and thromboxane are mediated by Rho-kinase.

Platelet-activating factor (PAF) contracts smooth muscle of airways and vessels primarily via release of thromboxane. Contraction of smooth muscle is thought to be mediated either by calcium and inositol trisphosphate (IP(3))-dependent activation of the myosin light chain kinase or, alternatively, via the recently discovered Rho-kinase pathway. Here we investigated the contribution of these two pathways to PAF and thromboxane receptor-mediated broncho- and vasoconstriction in two different rat models: the isolated perfused lung (IPL) and precision-cut lung slices. Inhibition of the IP(3) receptor (1-10 microM xestospongin C) or inhibition of phosphatidylinositol-specific PLC (30 microM L-108) did not affect bronchoconstriction but attenuated the sustained vasoconstriction by PAF. Inhibition of myosin light chain kinase (35 microM ML-7) or of calmodulin kinase kinase (26 microM STO609), which regulates the phosphorylation of the myosin light chain, had only a small effect on PAF- or thromboxane-induced pressor responses. Similarly, calmidazolium (10 microM), which inhibits calmodulin-dependent proteins, only weakly reduced the airway responses. In contrast, Y-27632 (10 microM), a Rho-kinase inhibitor, attenuated the thromboxane release triggered by PAF and provided partial or complete inhibition against PAF- and thromboxane-induced pressor responses, respectively. Together, our data indicate that PAF- and thus thromboxane receptor-mediated smooth muscle contraction depends largely on the Rho-kinase pathway.

P-54 - Development of Ca2+-dependent and Ca2+-independent desensitization induced with varied excess carbachol in muscarinic receptor-mediated smooth muscle contraction of guinea-pig taenia caeci.

P-54 - Development of Ca2+-dependent and Ca2+-independent desensitization induced with varied excess carbachol in muscarinic receptor-mediated smooth muscle contraction of guinea-pig taenia caeci.

Effect of extracellular calcium concentration on angiotensin AT 1 receptor-mediated smooth muscle contraction and antagonism in rabbit aorta

Angiotensin-II (AII) stimulates smooth muscle contraction by activating angiotensin AT 1 receptor which induces intracellular Ca 2+ release and Ca 2+ influx from extracellular space. In this study, effect of extracellular Ca 2+ concentration ([Ca 2+] 0) on angiotensin AT 1 receptor-mediated contractile response to AII has been examined in the absence and presence of [Sar 1, Ala 8]AII in rabbit aorta. A decrease in agonist potency and an increase in antagonist potency for depression were observed in low [Ca 2+] 0. Data were interpreted by applying an explanatory model developed previously. The result indicates that [Ca 2+] 0 is linked to the efficacy expression of AII at angiotensin AT 1 receptor and this prompted speculation about the underlying mechanism.

Desensitization of α-Adrenergic Receptor-Mediated Smooth Muscle Contraction

Desensitization of alpha-adrenergic receptor-mediated smooth muscle contraction occur in aortas from New England Deaconess Hospital (NEDH) rats harboring pheochromocytoma (PHEO) and following chronic exposure to the alpha-adrenergic agonist phenylephrine in vitro. Endothelium is known to release an endothelial cell-derived relaxing factor that promotes smooth muscle relaxation. We wondered if the endothelium might contribute to the desensitization of contraction. The role of the endothelium in desensitization was studied using aortic rings with endothelium [E(+)] and with endothelium removed [E(-)]. Maximal phenylephrine (PE)-induced contraction (Emax) for E(+) was 1.7 +/- 0.3 g in controls and 0.4 +/- 0.1 g in PHEO (p less than 0.001), demonstrating desensitization; however, for E(-), Emax was 2.4 +/- 0.2 g in PHEO vs. 2.5 +/- 0.2 g in controls, demonstrating restoration of maximal contraction when the endothelium was removed. However, sensitivity [-log EC50(M)] to PE in E(-) remained significantly lower in PHEO compared to controls (6.94 +/- 0.12 vs. 7.51 +/- 0.14, respectively, p less than 0.001). Similarly, in aortic ring segments desensitized in vitro with phenylephrine, the maximal contraction in phenylephrine-exposed aortas was 60% of that seen in controls. Removal of the endothelium from the vessels pretreated with phenylephrine fully restored the maximal response and sensitivity of these vessels. Treatment of desensitized vessels with hemoglobin (5 x 10(-5) M) restored the maximal contraction and sensitivity to phenylephrine. When the endothelium was removed prior to chronic exposure to phenylephrine, the sensitivity to phenylephrine decreased while the Emax remained similar to controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of action of transmitters and other substances on smooth muscle.

Mechanisms of action of transmitters and other substances on smooth muscle.