Polyphosphoinositide metabolism in canine tracheal smooth muscle (CTSM) in response to a cholinergic stimulus

Agonist-induced phasic and tonic responses in smooth muscle are mediated by InsP3

Many cellular functions are regulated by agonist-induced InsP3-evoked Ca2+ release from the internal store. In non-excitable cells, predominantly, the initial Ca2+release from the store by InsP3 is followed by a more sustained elevation in [Ca2+]i via store-operated Ca2+ channels as a consequence of depletion of the store. Here, in smooth muscle, we report that the initial transient increase in Ca2+, from the internal store, is followed by a sustained response also as a consequence of depletion of the store (by InsP3), but, influx occurs via voltage-dependent Ca2+ channels. Contractions were measured in pieces of whole distal colon and membrane currents and [Ca2+]i in single colonic myocytes. Carbachol evoked phasic and tonic contractions; only the latter were abolished in Ca2+-free solution. The tonic component was blocked by the voltage-dependent Ca2+ channel blocker nimodipine but not by the store-operated channel blocker SKF 96365. InsP3 receptor inhibition, with 2-APB, attenuated both the phasic and tonic components. InsP3 may regulate tonic contractions via sarcolemma Ca2+ entry. In single cells,depolarisation (to ∼-20 mV) elevated [Ca2+]i and activated spontaneous transient outward currents (STOCs). CCh suppressed STOCs, as did caffeine and InsP3. InsP3 receptor blockade by 2-APB or heparin prevented CCh suppression of STOCs; protein kinase inhibition by H-7 or PKC19-36did not. InsP3 suppressed STOCs by depleting a Ca2+ store accessed separately by the ryanodine receptor (RyR). Thus depletion of the store by RyR activators abolished the InsP3-evoked Ca2+ transient. RyR inhibition (by tetracaine) reduced only STOCs but not the InsP3transient. InsP3 contributes to both phasic and tonic contractions. In the former, muscarinic receptor-evoked InsP3 releases Ca2+ from an internal store accessed by both InsP3 and RyR. Depletion of this store by InsP3 alone suppresses STOCs, depolarises the sarcolemma and permits entry of Ca2+ to generate the tonic component. Therefore, by lowering the internal store Ca2+ content,InsP3 may generate a sustained smooth muscle contraction. These results provide a mechanism to account for phasic and tonic smooth muscle contraction following receptor activation.

The involvement of protein kinase C and tyrosine kinase in vanadate-induced contraction

Gastric smooth muscle of cats was used to investigate the involvement of protein kinase in vanadate-induced contraction. Vanadate caused a contraction of cat gastric smooth muscle in a dose-dependent manner. Vanadate-induced contraction was totally inhibited by 2 mM EGTA and 1.5 mM LaCl3 and significantly inhibited by 10 microM verapamil and 1 microM nifedipine, suggesting that vanadate-induced contraction is dependent on the extracellular Ca2+ concentration. and the influx of extracellular Ca2+ was mediated through voltage-dependent Ca2+ channel. Both protein kinase C inhibitor and tyrosine kinase inhibitor significantly inhibited the vanadate-induced contraction and the combined inhibitory effect of two protein kinase inhibitors was greater than that of each one. But calmodulin antagonists did not have any influence on the vanadate-induced contraction. On the other hand, both forskolin (1 microM) and sodium nitroprusside (1 microM) significantly inhibited vanadate-induced contraction. Therefore, these results suggest that both protein kinase C and tyrosine kinase are involved in the vanadate-induced contraction which required the influx of extracellular Ca2+ in cat gastric smooth muscle, and that the contractile mechanism of vanadate may be different from that of agonist binding to its specific receptor.

Regulation of protein kinases in steady-state contraction of cat gastric smooth muscle

Cat gastric smooth muscle strips were used to investigate the involvement of protein kinases in the steady-state contraction induced by 1 microM acetylcholine or 20 mM KCI. The steady-state contraction induced by acetylcholine or KCl was inhibited by EGTA dose dependently. Voltage-dependent Ca2+ channel antagonists dose dependently inhibited the contractions induced by KCI as well as by acetylcholine. Inhibitory effects of voltage-dependent Ca2+ channel antagonists were significantly more prominent on KCI-induced contractions than on acetylcholine-induced contractions. The acetylcholine-induced contraction was dose dependently inhibited by 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8, a blocker of intracellular Ca2+ release), but the KCl-induced contraction was not inhibited at all. Therefore both intracellular Ca2+ release and extracellular Ca2+ influx seem to be necessary for the acetylcholine-induced contraction, but intracellular Ca2+ release is not necessary for the KCl-induced contraction. Protein kinase C inhibitors, 10 microM 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine 2HCl (H-7) and 1 microM staurosporine, significantly inhibited the contraction induced by acetylcholine or KCl. Calmodulin antagonists, 30 microM trifluoperazine and 50 microM N-(6-aminohexyl)-5-chloro-2-naphthalenesulfonamide HCI (W-7), however, significantly inhibited the contraction induced by acetylcholine but not by KCl. A tyrosine kinase inhibitor, 50 microM genistein, did not affect the acetylcholine-induced contraction but significantly inhibited the KCl-induced contraction. These results strongly suggest that the involvement of protein kinases in regulation of the steady-state contraction may be agonist-dependent.

Phosphoinositide metabolism in airway smooth muscle

Agonist-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, which generates inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol, is thought to be one of the major mechanisms underlying pharmacomechanical coupling in airway smooth muscle. This article is a review of the currently available information on phosphoinositide and inositol 1,4,5-trisphosphate metabolism in this tissue and includes data on inositol 1,4,5-trisphosphate-induced Ca2+ release and the receptor mediating this effect. The final section outlines the potential mechanisms underlying physiological regulation of phosphoinositide metabolism by other second-messenger pathways operative in this tissue.

Effect of phospholipase C inhibitor U-73122 on antigen-induced airway smooth muscle contraction in guinea pigs

The importance of phospholipase C (PLC) in airway smooth muscle contraction was studied, using an inhibitor of PLC, 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl] amino]hexyl]-1H-pyrrole-2,5-dione (U-73122). Tracheas from ovalbumin (OA)-sensitized guinea pigs contracted rapidly after exposure to low concentrations of antigen (OA). However, tracheas treated with U-73122 for 10 min prior to the addition of antigen, demonstrated a 3 log rightward shift in the OA dose-response curve with an IC50 of 7 microM. The analogue of U-73122, 1-[6[[17 beta-3-methoxyestra-1,3,5 trien-17-yl]amino]hexyl]-2,5-pyrrolidine-dione (U-73433), was approximately 5-fold less active in inhibiting smooth muscle contraction. In addition to the inhibition of antigen-induced smooth muscle contraction, U-73122 inhibited carbachol- and leukotriene D4-induced smooth muscle contraction. Furthermore, U-73122 inhibited in a dose-dependent manner antigen-induced histamine release from guinea pig tracheal tissue. The inhibition of smooth muscle contraction by U-73122 correlated well with the inhibition of polyphosphoinositide mediates smooth muscle contractile responses to muscarinic agonists and leukotrienes as well as antigenic-induced contraction.

H1 receptor mediates inositol phosphates response to histamine in gastric smooth muscle of guinea pigs

The effect of histamine on [3H]-inositol phosphates (IPs) formation was investigated with [3H]-inositol-labeled gastric smooth-muscle cells in guinea pigs. Histamine (10(-5) M) increased the formation of [3H]-IPs in the muscle cells. The increase was significantly inhibited by pyrilamine (10(-5) M) but not by cimetidine (10(-5) M). The contractile response to histamine was also completely inhibited by pyrilamine but not by cimetidine. Phorbol ester 12-myristate 13-acetate (10 microM) significantly inhibited histamine-stimulated [3H]-IPs formation by 56%, whereas forskolin (10 microM) decreased it by 18%. This study demonstrates that the response of [3H]-IPs formation and contraction to histamine is mediated through H1 receptor, and the formation of [3H]-IPs is negatively regulated by protein kinase C in gastric smooth muscle of guinea pigs.

Intracellular signaling in vascular smooth muscle

The two major modalities of pharmacomechanical coupling, inositol 1,4,5, trisphosphate induced Ca2+ release and modulation of Ca(2+)-sensitivity, are reviewed. Recent studies show that although changes in cytoplasmic Ca2+ play the major role in regulating smooth muscle contraction, agonists can also significantly affect the contractile state by modifying Ca(2+)-sensitivity. Inhibition of myosin light chain kinase or myosin light chain phosphatase leads to, respectively, desensitization or sensitization of the contractile apparatus to Ca2+. G-protein linked inhibition of myosin light chain phosphatase and Ca2+ release mediated by the phosphatidylinol cascade are the two major pharmacomechanical coupling mechanisms.

Rapid increase in inositol phosphate levels in norepinephrine-stimulated vascular smooth muscle

We examined the correlation between agonist-stimulated increases in inositol phosphates and force development in vascular smooth muscle. Segments of rat tail artery were preincubated with [3H]inositol and treated with norepinephrine (10(-5) M) for 3-10 s. Tissue levels of inositol monophosphate (IP), inositol bisphosphate (IP2), and inositol trisphosphate (IP3) were measured. IP and IP2 increased significantly after 3 s of norepinephrine treatment. IP3 increased significantly after 5 s of norepinephrine treatment. Analysis of tissue extracts by high-pressure liquid chromatography demonstrated that the only isomer of IP3 present in any tissue extract was the 1,4,5-isomer [Ins(1,4,5)P3]. Contractile response to norepinephrine stimulation showed that the increase in inositol phosphates coincides well with the time course of force development. This is the first report demonstrating such an early increase in Ins(1,4,5)P3 in agonist-stimulated vascular smooth muscle. These results are consistent with the hypothetical role of Ins(1,4,5)P3 as a mediator linking agonist-receptor activation to increased intracellular calcium and force development in norepinephrine-stimulated vascular smooth muscle.

Effects of age on muscarinic agonist-induced contraction and IP accumulation in airway smooth muscle

The effects of age on carbachol-stimulated force development and [3H]inositol phosphate production was studied in tracheal rings from guinea pigs aged 1 month and 25 months of age. The pD2 for the contractile response to carbachol was significantly reduced in tracheal tissues from old animals as compared to that of the young tissues (6.49 +/- 0.04, 7.09 +/- 0.04, n = 12), respectively. In contrast, inositol phosphate formation was not altered with increasing age when stimulated by carbachol or NaF, a direct activator of G proteins. Carbachol-induced inositol phosphate accumulation was inhibited by treatment with 1 micrograms/ml pertussis toxin, suggesting that IP1 accumulation is coupled to a pertussis-toxin-sensitive protein. The pD2 values for contraction (7.09 +/- 0.09, 6.49 +/- 0.04) were significantly different from the pD2 values for IP1 accumulation (4.72 +/- 0.14, 5.10 +/- 0.18) in both young and old tissues, respectively. These data suggest that IP1 accumulation is not responsible for the decreased contractile ability in tracheal smooth muscle during aging.

Mechanism of endothelin-induced contraction in guinea-pig trachea: comparison with rat aorta

1. Endothelin (1 nM-0.3 microM) produced a concentration-dependent contraction of guinea-pig epithelium-containing (intact) trachea (EC50 = 30.9 nM). Endothelin was a less potent agonist than leukotriene D4 (LTD4; EC50 = 0.77 nM), but was more potent than carbachol (EC50 = 0.15 microM) or substance P (EC50 = 1.4 microM). Endothelin was a more potent contractile agent in rat endothelium-denuded aorta (EC50 = 2.1 nM) than in guinea-pig trachea. 2. Endothelin-induced contraction in guinea-pig trachea was unaffected by mepyramine (10 microM), atropine (1 microM), SK&F 104353 (10 microM), a leukotriene receptor antagonist, or SQ 29,548 (1 microM), a thromboxane receptor antagonist. The contraction produced by 0.3 microM endothelin was potentiated by cyclo-oxygenase inhibition with 5 microM indomethacin. 3. Nicardipine (0.01 or 0.1 microM) or incubation in calcium-free medium +0.1 mM EGTA for 30 min had a relatively minor or no effect on endothelin concentration-response curves in guinea-pig intact trachea, but markedly inhibited responses produced by endothelin in endothelium-denuded aorta of the rat. Increasing the EGTA concentration in calcium-free medium to 1 mM abolished endothelin-induced contraction in guinea-pig trachea. 4. In guinea-pig trachea, ryanodine (10 microM) produced a 2.1 fold shift to the right of endothelin concentration-response curves and reduced the maximum response elicited by 0.3 microM endothelin. 5. Staurosporine (0.01 microM and 0.1 microM), a protein kinase C inhibitor, was without effect on endothelin- or carbachol-induced contraction in guinea-pig trachea, but markedly inhibited the response produced by endothelin in rat aorta. 6. Endothelin (3 nM-0.3 microM) produced a concentration-dependent stimulation of phosphatidylinositol (PI) turnover in guinea-pig intact trachea, with an EC50 value of 45.9 nM. 7. Removal of the epithlium markedly potentiated endothelin-induced contraction in guinea-pig trachea, producing a 4.7 fold leftward shift in endothelin concentration-response curves and an increase in the contractile response elicited by 0.3 microM endothelin. 8. These data indicate that endothelin is a potent agonist in guinea-pig trachea whose response is markedly enhanced by removal of the airway epithelium. Endothelin-induced contraction is not mediated to a marked extent by calcium influx via dihydropyridine-sensitive calcium channels and does not involve the release of histamine, acetylcholine, leukotrienes or thromboxane. Rather, endothelin appears to produce contraction of guinea-pig trachea via a direct action which involves stimulation of PI turnover and utilization of calcium from intracellular stores and, also, calcium influx via a pathway that is not sensitive to dihydropyridine calcium channel inhibitors. Endothelin-induced contraction of rat aorta was more sensitive to the effects of incubation in Ca2 +-free medium, nicardipine or staurosporine, suggesting that differences exist in the relative mechanisms whereby endothelin produces contraction in different tissues.

The effect of M&B 22948 on methacholine- and histamine-induced contraction and inositol 1,4,5-trisphosphate levels in guinea-pig tracheal tissue

The effect of a cyclic GMP phosphodiesterase inhibitor, M&B 22948, on methacholine- and histamine-induced contraction and inositol 1,4,5-trisphosphate (IP3) elevation was studied in guinea-pig tracheal rings. After addition of methacholine or histamine the rise in IP3 content was rapid and transient reaching a maximum after 5-15 s, which coincided with the maximum rate of tension development. Cyclic GMP levels of the tissue were elevated by M&B 22948 before agonist stimulation and further elevated by addition of methacholine or histamine. Cyclic AMP levels were not altered by any of these agents. M&B 22948 abolished IP3 generation induced by methacholine or histamine, but did not alter the rate or magnitude of tension development. Thus, IP3 generation does not appear to be responsible for the contractions induced by methacholine or histamine in this tissue.

Characterization of agonist-stimulated incorporation of myo-[3H]inositol into inositol phospholipids and [3H]inositol phosphate formation in tracheal smooth muscle

The effects of the muscarinic agonist carbachol, histamine and bradykinin on incorporation of [3H]inositol into the phosphoinositides and the formation of [3H]InsPs were examined in bovine tracheal smooth-muscle (BTSM) slices labelled with [3H]inositol. These agonists result in substantial and dose-related increases in the incorporation of [3H]inositol into the phospholipids. Carbachol and histamine stimulated the incorporation of [3H]inositol into the phospholipids to the same degree, despite histamine being only 35% as effective as carbachol on [3H]InsP accumulation. Histamine and carbachol, at maximal concentrations, were non-additive with respect to both the stimulated incorporation of [3H]inositol and [3H]InsP formation. For carbachol this effect on incorporation was found to occur to a similar extent in PtdInsP and PtdInsP2 as well as PtdIns. The initial effect of carbachol on [3H]inositol incorporation was rapid (maximal by 10 min); however, with prolonged stimulation large secondary declines in PtdInsP and PtdInsP2 labelling were observed, with depletion of the much larger PtdIns pool only evident in the presence of Li+. Lowering buffer [Ca2+] increased the incorporation of [3H]inositol under basal conditions, but did not attenuate the subsequent agonist-stimulated incorporation effect. The large changes in specific radioactivity of the phosphoinositides, and consequently the [3H]InsP products, after carbachol stimulation resulted in the apparent failure of atropine to reverse the [3H]InsP response completely. Labelling muscle slices with [3H]inositol in the presence of carbachol or labelling for longer periods (greater than 6 h) prevented subsequent carbachol-stimulated effects on incorporation without significantly altering the dose-response relationship for carbachol-stimulated [3H]InsP formation and resulted in steady-state labelling conditions confirmed by the ability of atropine to reverse fully the [3H]InsP response to carbachol. This study demonstrates the profound effects of a number of agonists on [3H]inositol incorporation into the phospho- and polyphosphoinositides in BTSM with important consequent changes in the specific radioactivity of these lipids and the resulting [3H]InsP products. In addition, a selective depletion of PtdInsP and PtdInsP2 over PtdIns has been demonstrated with prolonged muscarinic-receptor stimulation.

Human airway smooth muscle in cell culture: control of the intracellular calcium store

The release of intracellularly stored calcium (Ca2+) contributes to the rise in cytoplasmic Ca2+ concentration during agonist-induced activation of airway smooth muscle. We describe a novel preparation--human airway smooth muscle cells cultured in monolayers--which has enabled the investigation of the identity and the function of this intracellular Ca2+ store. Cells, enzymatically dispersed from surgically resected human bronchi, were cultured in monolayers and confirmed as smooth muscle by positive immunocytochemical staining for actin and myosin. The release of intracellularly stored Ca2+ in response to bronchoconstrictor agonists, histamine and carbachol, was demonstrated by stimulation of Ca2+ efflux from 45Ca-labelled cells. The technique of permeabilisation of the plasmalemmal membrane by saponin allowed the measurement of the Ca2+ content of the intracellular store in 45Ca-labelled cells. Uptake of Ca2+ by the store was energy-dependent and was enhanced by cyclic AMP. The effects of inhibitors of sarcoplasmic reticulum and mitochondrial function on Ca2+ uptake identified the store as sarcoplasmic reticulum. Inositol 1,4,5-trisphosphate released stored Ca2+ in a time and dose-dependent manner, supporting its putative role as an intracellular messenger for Ca2+ release in human airway smooth muscle.

Evidence for a direct relationship between phosphoinositide metabolism and airway smooth muscle contraction induced by muscarinic agonists

The relationship between bovine tracheal muscle contraction and phosphoinositide metabolism was studied with the muscarinic agonists, methacholine, oxotremorine, and McN-A-343. Analysis of the dose-response curves for contraction and inositol phosphates accumulation with these agonists demonstrated a direct relationship between the two parameters, with a considerable reserve of inositol phosphate production for the full contractile agonists, methacholine and oxotremorine, and no reserve for the partial agonist, McN-A-343.

Differential inhibitory effects of forskolin, isoproterenol, and dibutyryl cyclic adenosine monophosphate on phosphoinositide hydrolysis in canine tracheal smooth muscle

A characteristic feature of airway smooth muscle is its relative sensitivity to relaxant effects of beta adrenergic agonists when contracted by inflammatory mediators, such as histamine, vs. resistance to these relaxant effects when contracted by muscarinic agonists. Because contractions presumably depend upon the hydrolysis of membrane phosphoinositides (PI) and the generation of inositol phosphates (IP), our goal was to test for the effects of forskolin, isoproterenol, and dibutyryl cAMP on histamine- vs. methacholine-induced IP accumulation in canine tracheal smooth muscle. Methacholine (10(-3) M) was a more effective stimulant of IP accumulation (9.6 +/- 2.1-fold increase) than equimolar histamine (3.6 +/- 0.5-fold increase) in this tissue. When responses to equieffective methacholine (4 x 10(-6) M) and histamine (10(-3) M) were compared, neither forskolin, isoproterenol, nor dibutyryl cAMP significantly decreased IP accumulation in response to methacholine. In contrast, each of these three agents significantly decreased responses to histamine (by 56 +/- 9, 52 +/- 2, and 61 +/- 2%, respectively). We concluded that, in canine tracheal smooth muscle, increased cAMP is associated with inhibition of PI hydrolysis in response to histamine but not methacholine. The findings suggest a novel mechanism for selective modulation by cAMP of receptor-mediated cellular activation.

Inositol trisphosphate, calcium and muscle contraction

The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.

In vivo electrical stimulation of the sympathetic nerve of the eye increases inositol phosphate production and prostaglandin release in the rabbit iris muscle

The effects of in vivo electrical stimulation of the sympathetic nerve of the eye on phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis in rabbit iris and release of arachidonate and prostaglandin (PG) E2 into aqueous humor were investigated. myo-[3H]Inositol or [1-14C]arachidonate was injected intracamerally into each eye 3 h before electrical stimulation of one of the sympathetic trunks. Tissue phosphoinositides were determined by TLC, and 3H-labeled inositol phosphates were analyzed by either ion-exchange chromatography or HPLC. The aqueous humor was analyzed for 14C-labeled arachidonate and PGE2 by radiochromatography and for unlabeled PGE2 by radioimmunoassay. The results obtained from this study can be summarized as follows: (a) The rates of in vivo incorporation of myo-[3H]inositol into phosphoinositides and accumulation of 3H-labeled inositol phosphates in the iris muscle increased with time and then leveled off between 3 and 5 h. (b) Distribution of 3H radioactivity in inositol phosphates, as determined by HPLC, showed that of the total radioactivity in inositol phosphates, 53.6% was recovered in myo-inositol 1-phosphate, 36% in myo-inositol bisphosphate, 0.95% in myo-inositol 1,3,4-trisphosphate (1,3,4-IP3), and 2.6% in 1,4,5-IP3. (c) Electrical stimulation of the sympathetic nerve resulted in a significant loss of 3H radioactivity from PIP2 and a concomitant increase of that in IP3, an observation indicating that PIP2 is the physiological substrate for alpha 1-adrenergic receptors in this tissue. (d) Release of IP3 and liberation of arachidonate for PGE2 synthesis are dependent on the duration of stimulation and the intensity (voltage) of stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium ion mechanisms in airway smooth muscle: potential targets for novel symptomatic drugs for asthma

Calcium ions (Ca2+) are fundamental to the processes responsible for the initiation and maintenance of contraction of airway smooth muscle cells. Recent developments in our understanding of signal transduction mechanisms relating to intracellular Ca2+ release have extended our knowledge of excitation-contraction coupling mechanisms in airway smooth muscle. Furthermore, these developments open up potential targets for the development of new drugs, with novel mechanisms of action, for the symptomatic treatment of asthma. This article reviews these recent advances and focusses upon those calcium ion mechanisms that are possible targets for drug action.

Electrophysiological and other aspects of excitation-contraction coupling and uncoupling in mammalian airway smooth muscle

In this article the electrophysiological events which are believed to underly agonist-induced contraction and relaxation of airway smooth muscle are reviewed, with special emphasis on the indispensable role of the Ca ion. The contribution made by Na, K, Ca and Cl to, and the role that the electrogenic Na:K-dependent ATPase plays in, the maintenance of the resting membrane potential in both normal and sensitised airway smooth muscle cells is described together with the permeability changes that occur in the plasmalemma in response to excitatory and inhibitory agonists. In addition, the currently available evidence for the existence of potential-sensitive and receptor-operated Ca channels in respiratory smooth muscle, and how such channels may be involved in the regulation of airway calibre, is critically assessed.