The effect of trifluoperazine on muscle tension and cytoplasmic Ca2+ level in guinea pig ileum

Inhibitory mechanism of monensin on high K+-induced contraction in guniea-pig urinary bladder

In this study, we examined the inhibitory mechanism of monensin on high K+-induced contraction in guinea-pig urinary bladder. The relaxant effect of monensin (0.001 - 10 microM) was more potent than those of NaCN (100 microM - 1 mM) and forskolin (3 - 10 microM). Monensin (0.1 microM), NaCN (300 microM), or forskolin (10 microM) inhibited high K+-induced contraction without decreasing [Ca2+]i level. Monensin and NaCN remarkably decreased creatine phosphate and ATP contents. Monensin and NaCN inhibited high K+-induced increases in flavoprotein fluorescence, which is involved in mitochondrial respiration. Forskolin increased cAMP content but monensin did not. Monensin increased Na+ content at 10 microM but not at 0.1 microM that induced maximum relaxation. In the alpha-toxin-permeabilized muscle, forskolin significantly inhibited the Ca2+-induced contraction, but monensin did not affect it. These results suggest that the relaxation mechanism of monensin in smooth muscle of urinary bladder may be an inhibition of oxidative metabolism.

Length-dependent regulation of basal myosin phosphorylation and force in detrusor smooth muscle

Urinary bladder (detrusor) smooth muscle is active in the absence of an external stimulus. Tone occurs even "at rest" during the filling phase, and it is elevated in patients with overactive bladder. This study examined the role of muscle length on tone and the level of basal myosin light chain phosphorylation (MLC(20P)). MLC(20P) was 23.9 +/- 1% (n = 58) at short lengths (zero preload; L(z)). An increase in length from L(z) to the optimal length for contraction (L(o)) caused a reduction in MLC(20P) to 15.8 +/- 1% (n = 49). Whereas 10 microM staurosporine reduced MLC(20P) at L(z), 1 microM staurosporine, a Ca(2+)-free solution, and inhibitors of MLC kinase, protein kinase C (PKC) and RhoA kinase (ROK) did not. However, 1 microM staurosporine and inhibitors of ROK inhibited MLC(20P) and tone at L(o). These data support the hypothesis that a Ca(2+)-independent kinase, possibly ZIP-like kinase, regulates MLC(20P) at L(z), whereas in detrusor stretched to L(o), additional kinases, such as ROK, participate.

Regulation of ERK phosphorylation in differentiated arterial muscle of rabbits

Extracellular signal-regulated kinases (ERK) and mitogen-activated protein (MAP) kinases participate in cell signaling, regulating cell growth. In differentiated cells, the role ERK plays is less well known. This study quantified the degree of basal and stimulated ERK phosphorylation and contraction in freshly isolated arteries. The level of basal ERK phosphorylation was identical in preloaded and slack arteries, was greater in media than in the whole artery, and was reduced by the MAP or ERK kinase (MEK) inhibitor PD-98059. Chemical denudation using 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one did not elevate basal ERK phosphorylation. PD-98059 reduced maximum phenylephrine (PE)-stimulated ERK phosphorylation but not force. Pervanadate elevated ERK phosphorylation without causing contraction. Contractions produced by PE and relaxations produced by PE washout preceded the ERK phosphorylation. K(+) depolarization, muscle stretch, and angiotensin II elevated ERK phosphorylation transiently, whereas PE maintained ERK phosphorylation for 30 min. The alpha(1A)-adrenergic receptor antagonist WB-4101 reduced PE-stimulated force by 70% and abolished PE-induced ERK phosphorylation. Afterloaded and zero-load contractions produced by K(+) depolarization displayed identical increases in ERK phosphorylation. These data indicate that ERK was active basally in the differentiated artery but regulated by the endothelium and that ERK phosphorylation was not load dependent. A strong correlation between PE-induced force and ERK phosphorylation supports the hypothesis that ERK activation may reflect a signal "notifying" the cell of the degree of alpha(1)-adrenergic receptor-induced contraction.

Chlorpromazine at comparatively low concentrations potentiates carbachol-induced tonic contraction of ileal longitudinal smooth muscle of the guinea-pig

The neuroleptic phenothiazine derivative chlorpromazine (CPZ) at high concentration (1 x 10(-5) M) decreased either the phasic or tonic contraction in response to carbachol and the carbachol-induced increase in [Ca2+]i in both phases in ileal muscle. In contrast, CPZ at low concentrations (8 x 10(-7) - 5 x 10(-6) M) decreased only the phasic contraction and potentiated the tonic contraction induced by carbachol. However, CPZ at these concentrations dose-dependently decreased the carbachol-induced increase in [Ca2+]i in both phases. These results suggested that CPZ dose-dependently decreased the initial phasic contraction in response to carbachol by inhibition of Ca2+ release from the intracellular storage sites. CPZ at low concentrations appears to increase Ca2+ sensitivity to contractile proteins in the carbachol-induced tonic phase. CPZ dose-dependently reduced the 60 mM K(+)-induced phasic and tonic responses and a concomitant decrease in [Ca2+]i in ileal muscle.

Hypoxia and smooth muscle function: key regulatory events during metabolic stress

Hypoxia rapidly reduces force in many smooth muscles and we review recent data that shed light on the mechanisms involved. As many regulated cellular processes are integrated to co-ordinate smooth muscle contractility, the processes responsible for decreased force output with altered metabolism are also likely to be many, acting in concert, rather than the actions of one altered parameter. Nevertheless the aim of this study is to elucidate the hierarchical series of events that contribute to reduced smooth muscle force production during altered metabolism. We conclude that in many phasic smooth muscles the decrease in force can be attributed to impaired electro-mechanical coupling whereby the Ca2+ transient is reduced. A direct effect of hypoxia on the Ca2+ channel may be of key importance. In tonic vascular smooth muscles KATP channels may also play a role in the integrated functional responses to hypoxia. There are also many examples of force being reduced, in tonically activated preparations, without a fall in steady-state Ca2+; indeed it usually increases. We examine the roles of altered [ATP], pH, myosin phosphorylation, inorganic phosphate and proteolytic activity on the [Ca2+]-force relationship during hypoxia. We find no defining force-inhibitory role for any one factor acting alone, and suggest that force most probably falls as a result of the combination of myriad factors.

Effects of hypoxia on [Ca2+]i, pHi and myosin light chain phosphorylation in guinea-pig taenia caeci

1. Hypoxia (achieved by bubbling with N2 instead of O2) reduces the force of a KCl (40 mM)-induced contracture to approximately 10% of the control value in guinea-pig taenia caeci. The underlying mechanism of this relaxation in response to hypoxia was investigated by measuring the major cell signalling parameters, intracellular Ca2+ concentration ([Ca2+]i) and myosin regulatory light chain (LC20) phosphorylation (MLC-P1), as well as intracellular pH (pHi), a factor often suggested to mediate hypoxic relaxation of muscle. 2. [Ca2+]i, measured using the ratiometric fluorescent dye fura-2, increased when 40 mM KCl was added to physiological saline solution (PSS) (peak value assigned 100%), and the steady state after 15 min was 92.8%. There were no detectable decreases in [Ca2+]i during hypoxia. 3. MLC-Pi, measured using isoelectric focusing-polyacrylamide gel electrophoresis and identified using Western blotting, increased from 9% of the total LC20 in Ca(2+)-free PSS to a peak value of 51% in 40 mM KCl-PSS. The steady-state value in hypoxia of 43% was not significantly different from that in control oxygenated conditions at the same point in time. 4. pHi, measured using the ratiometric fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxy-fluorescein (BCECF), under quiescent conditions (Ca(2+)-free PSS) was 7.23 and increased to 7.36 with 40 mM KCl. After imposition of hypoxia pHi remained unchanged despite the known increase in both lactate content and production. 5. As [Ca2+]i and MLC-Pi, key factors in activation, were not decreased by hypoxia and changes in pHi were minor, hypoxic relaxation in guinea-pig taenia caeci appears to be directly related to energy limitation rather than any oxygen-sensing mechanism.

Blockade by calmodulin inhibitors of Ca2+ channels in smooth muscle from rat vas deferens

1. Effects of three compounds which are used as calmodulin inhibitors (trifluoperazine, W-7 and calmidazolium) on Ca2+ channels were investigated in smooth muscle from rat vas deferens. 2. All three calmodulin inhibitors relaxed the smooth muscle precontracted by a high concentration of KCl (63.7 mM). The order of potency for the relaxation was trifluoperazine > W-7 > calmidazolium. 3. In binding studies using a microsomal fraction of vas deferens, all these calmodulin inhibitors displaced specific [3H]-nimodipine binding. Trifluoperazine and W-7 inhibited the binding at concentrations that relaxed the smooth muscle whereas calmidazolium inhibited at concentrations much lower than those necessary for muscle relaxation. 4. Ba2+ current flowing through voltage-gated Ca2+ channels was measured under whole-cell voltage-clamp conditions in isolated smooth muscle cells. The Ba2+ current was suppressed by the three calmodulin inhibitors in the concentration-range where inhibition of [3H]-nimodipine binding was observed. Neither voltage-dependence nor the inactivation time course of Ba2+ current were affected by these compounds. 5. The results suggest that the calmodulin inhibitors directly block Ca2+ channels in the smooth muscle cells. The channel inhibition by trifluoperazine and W-7, but perhaps not that by calmidazolium, may be responsible for the muscle relaxation observed with these compounds.

Ca2+ localization and sensitivity in vascular smooth muscle

An increase in cytosolic Ca2+ level ([Ca2+]i) is a prerequisite for smooth muscle contraction. Simultaneous measurements of [Ca2+]i and muscle tension give direct information on the Ca2+ regulation of smooth muscle. The photoprotein aequorin and the fluorescent Ca2+ indicator fura-2 are widely used for this purpose. Although there are some inconsistencies between the results obtained with these two indicators, comparison between [Ca2+]i and muscle tension in vascular smooth muscle indicates that stimulation of alpha-adrenoceptors increases, whereas stimulation of beta-adrenoceptors decreases, both the Ca2+ sensitivity of contractile elements and [Ca2+]i. Thus, as Hideaki Karaki explains, contractility of vascular smooth muscle may be regulated not only by [Ca2+]i but also by the Ca2+ sensitivity of the contractile elements.