Activation of muscarinic receptors in porcine airway smooth muscle elicits a transient increase in phospholipase D activity

Signal transduction underlying the control of urinary bladder smooth muscle tone by muscarinic receptors and beta-adrenoceptors

The normal physiological contraction of the urinary bladder, which is required for voiding, is predominantly mediated by muscarinic receptors, primarily the M₃ subtype, with the M₂ subtype providing a secondary backup role. Bladder relaxation, which is required for urine storage, is mediated by β-adrenoceptors, in most species involving a strong β₃-component. An excessive stimulation of contraction or a reduced relaxation of the detrusor smooth muscle during the storage phase of the micturition cycle may contribute to bladder dysfunction known as the overactive bladder. Therefore, interference with the signal transduction of these receptors may be a viable approach to develop drugs for the treatment of overactive bladder. The prototypical signaling pathway of M₃ receptors is activation of phospholipase C (PLC), and this pathway is also activated in the bladder. Nevertheless, PLC apparently contributes only in a very minor way to bladder contraction. Rather, muscarinic-receptor-mediated bladder contraction involves voltage-operated Ca²⁺ channels and Rho kinase. The prototypical signaling pathway of β-adrenoceptors is an activation of adenylyl cyclase with the subsequent formation of cAMP. Nevertheless, cAMP apparently contributes in a minor way only to β-adrenoceptor-mediated bladder relaxation. BK_Ca channels may play a greater role in β-adrenoceptor-mediated bladder relaxation. We conclude that apart from muscarinic receptor antagonists and β-adrenoceptor agonists, inhibitors of Rho kinase and activators of BK_Ca channels may have potential to treat an overactive bladder.

Muscarinic receptor stimulation activates a Ca(2+)-dependent Cl(-) conductance in rat distal colon

Recently, it was observed that the acetylcholine analogue carbachol induces a transient stimulation of an apical Cl(-) conductance in basolaterally depolarized rat distal colonic epithelium (Schultheiss et al., 2003). The further characterization of this conductance was the aim of the present study. All experiments were performed at basolaterally depolarized tissues (111.5 mmol.l(-1) KCl buffer at the serosal side); in the absence of a K(+) gradient, a Cl(-) current was driven across the apical membrane (107 mmol.l(-1) K gluconate/4.5 mmol.l(-1) KCl buffer on the mucosal side). Under these conditions, carbachol evoked an atropine-sensitive biphasic change in short-circuit current (I(SC)), consisting of a transient increase followed by a long-lasting decrease, suggesting a stimulation of apical Cl(-) conductance followed by an inhibition. This conductance was inhibited by SITS, but was resistant against glibenclamide, a blocker of CFTR. The carbachol-induced I(SC) was dependent on the presence of mucosal Ca(2+). Ionomycin, a Ca(2+) ionophore, mimicked the effect of carbachol. An antibody against bovine Ca(2+)-activated Cl(-) channel ClCa 1 stained rat colonic epithelial cells both at the cell membrane as well as intracellularly, suggesting that the action of Ca(2+) may be caused by a stimulation of a ClC a-type anion channel. The activation of apical Cl(-) conductance by carbachol was resistant against any blockers of the phospholipase C/IP3/protein kinase C pathway tested (e.g., U-73122, 2-ABP, Li(+), staurosporine), but was inhibited by the NO-synthase blocker L: -NNA. Vice versa, NO-donating compounds such as GEA 3162 or sodium nitroprusside evoked a transient increase of I(SC). Consequently, NO seems to be involved in the transient stimulation of apical Ca(2+)-dependent Cl(-) conductance after muscarinic receptor stimulation.

Activation of apical K+ conductances by muscarinic receptor stimulation in rat distal colon: fast and slow components

In the epithelium of rat distal colon the acetylcholine analogue carbachol induces a transient increase of short-circuit current (Isc) via stimulation of cellular K+ conductances. Inhibition of the turnover of inositol-1,4,5-trisphosphate (IP3) by LiCl significantly reduced both the amplitude and the duration of this response. When the apical membrane was permeabilized with nystatin, LiCl nearly abolished the carbachol-induced activation of basolateral K+ conductances. In contrast, in epithelia, in which the basolateral membrane was bypassed by a basolateral depolarization, carbachol induced a biphasic increase in the K+ current across the apical membrane consisting of an early component carried by charybdotoxin- and tetraethylammonium-sensitive K+ channels followed by a sustained plateau carried by channels insensitive against these blockers. Only the latter was sensitive against LiCl or inhibition of protein kinases. In contrast, the stimulation of the early apical K+ conductance by carbachol proved to be resistant against inhibition of phospholipase C or protein kinases. However, apical dichlorobenzamil, an inhibitor of Na+/Ca2+ exchangers, or a Ca2+-free mucosal buffer solution significantly reduced the early component of the carbachol-induced apical K+ current. The presence of an apically localized Na+/Ca2+-exchanger was proven immunohistochemically. Taken together these experiments reveal divergent regulatory mechanisms for the stimulation of apical Ca2+-dependent K+ channels in this secretory epithelium, part of them being activated by an inflow of Ca2+ across the apical membrane.

Regulation of phospholipase D by muscarinic receptors in rat submandibular ductal cells

The muscarinic agonist carbachol stimulated phospholipase D (PLD) in rat submandibular gland (RSMG) ductal cells in a time and concentration-dependent manner. This effect was inhibited by chelation of extracellular calcium with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). PLD could also be activated by epinephrine and AlF(4)(-), two polyphosphoinositide-specific phospholipase C (PPI-PLC) activators, and by the phorbol ester o-tetradecanoylphorbol 13-acetate (TPA) which activates protein kinase C (PKC). Ionomycin and thapsigargin only slightly increased PLD activity. Ortho-vanadate, a tyrosine phosphatase inhibitor, also stimulated PLD activity. Both carbachol and o-vanadate increased the formation of inositol phosphates and the tyrosine phosphorylation of at least two proteins (55-60 and 120 kDa). Calphostin C (a PKC inhibitor), U73122 (a PPI-PLC inhibitor) and genistein (a tyrosine kinase inhibitor) blocked the activation of PLD, of PLC and the phosphorylation of tyrosyl residues in response to carbachol and vanadate. Taken together, these results suggest that rat submandibular gland ductal cells express a calcium-dependent PLD activity. This enzyme is regulated by carbachol via a PLC-PKC-tyrosine kinase pathway.

Activation of phospholipase D in porcine tracheal smooth muscle: role of phosphatidylinositol 3-kinase and RhoA activation

Muscarinic receptor agonists transiently activate phospholipase D in tracheal smooth muscle. Muscarinic activation of phospholipase D in this tissue is dependent on activation of protein kinase C and an unidentified pathway that is not protein kinase C dependent. Cholinergic agents have also been shown to activate phospholipase D by pathways linked to the small G protein, RhoA. This study explores the relationship between muscarinic activation of phophatidylinositol 3-kinase and activation of RhoA, and examines whether phospholipase D activation is dependent on either pathway in tracheal smooth muscle. Wortmannin or 2-(4-morphonyl)-8-phenyl-4H-1-benzopyran-4-one (LY-294002), putative specific inhibitors of phophatidylinositol 3-kinase, significantly inhibit acetylcholine-induced formation of phosphatidylethanol and also block acetylcholine-induced translocation of RhoA to the membrane. In previous experiments calphostin C, a protein kinase C inhibitor, partially inhibited both acetylcholine-induced and phorbol-12-myristate-13-acetate (PMA)-induced phosphatidylethanol formation. In the present study calphostin C did not block acetylcholine-induced RhoA translocation to the membrane. However, the Rho kinase inhibitor, N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide (Y-27632), significantly inhibited acetylcholine-induced phosphatidylethanol formation, but had no effect on activation of phospholipase D by PMA. Acetylcholine treatment also stimulated the phosphorylation of the 110-kDa subunit of phosphatidylinositol 3-kinase. Phosphorylation of phosphatidylinositol 3-kinase 110-kDa subunit could be blocked by wortmannin in a concentration-dependent manner, and acetylcholine-induced phosphatidylinositol 3-kinase activity was significantly inhibited by wortmannin. LY-294002 also inhibited acetylcholine-induced phosphorylation of 110-kDa subunit and activation of phosphatidylinositol 3-kinase. These results suggest that acetylcholine stimulation translocates RhoA to the membrane by a phosphatidylinositol 3-kinase-dependent mechanism and acetylcholine-induced phospholipase D stimulation is at least partly mediated via phosphatidylinositol 3-kinase, however, protein kinase C appears to activate phospholipase D independent of phosphatidylinositol 3-kinase or RhoA activation in porcine tracheal smooth muscle.

PLD pathway involved in carbachol-induced Cl- secretion: possible role of TNF-alpha

In a previous study, it was found that exposure to tumor necrosis factor-alpha (TNF-alpha) potentiated the electrophysiological response to carbachol in a time-dependent and cycloheximide-sensitive manner. It was deduced that the potentiation could be due to protein kinase C activity because of increased 1,2-diacylglycerol. It was also observed that propranolol could decrease the electrophysiological response to carbachol (Oprins JC, Meijer HP, and Groot JA. Am J Physiol Cell Physiol 278: C463-C472, 2000). The aim of the present study was to investigate whether the phospholipase D (PLD) pathway plays a role in the carbachol response and the potentiating effect of TNF-alpha. The transphosphatidylation reaction in the presence of the primary alcohol 1-butanol [leading to stable phosphatidylbutanol (Pbut) formation] was used to measure activity of PLD. The phosphatidic acid (PA) levels were also measured. Muscarinic stimulation resulted in an increased formation of Pbut and PA. TNF-alpha decreased levels of PA.