Isoproterenol changes the relationship between cytosolic Ca2+ and contraction in guinea pig taenia caecum

Forskolin Changes the Relationship between Cytosolic Ca and Contraction in Guinea Pig Ileum

This study was designed to clarify the mechanism of the inhibitory effect of forskolin on contraction, cytosolic Ca(2+) level ([Ca(2+)](i)), and Ca(2+) sensitivity in guinea pig ileum. Forskolin (0.1 nM~10 microM) inhibited high K(+) (25 mM and 40 mM)- or histamine (3 microM)-evoked contractions in a concentration-dependent manner. Histamine-evoked contractions were more sensitive to forskolin than high K(+)-evoked contractions. Spontaneous changes in [Ca(2+)](i) and contractions were inhibited by forskolin (1 microM) without changing the resting [Ca(2+)](i). Forskoln (10 microM) inhibited muscle tension more strongly than [Ca(2+)](i) stimulated by high K(+), and thus shifted the [Ca(2+)](i)-tension relationship to the lower-right. In histamine-stimulated contractions, forskolin (1 microM) inhibited both [Ca(2+)](i) and muscle tension without changing the [Ca(2+)](i)-tension relationship. In alpha-toxin-permeabilized tissues, forskolin (10 microM) inhibited the 0.3 microM Ca(2+)-evoked contractions in the presence of 0.1 mM GTP, but showed no effect on the Ca(2+)-tension relationship. We conclude that forskolin inhibits smooth muscle contractions by the following two mechanisms: a decrease in Ca(2+) sensitivity of contractile elements in high K(+)-stimulated muscle and a decrease in [Ca(2+)](i) in histamine-stimulated muscle.

Ibudilast-induced decreases in cytosolic Ca(2+) level and contraction in rat aorta

The mechanism by which ibudilast induces vasodilation was examined in isolated endothelium-denuded rat aorta. Ibudilast inhibited the contractions induced by phenylephrine (PE) and high K(+) with decrease of [Ca(2+)](i) level in a concentration-dependent manner, to the same degree. 3-Isobutyl-1-methylxanthine (IBMX) inhibited PE-induced contraction and [Ca(2+)](i) level in a concentration-dependent manner, but it inhibited high K(+)-induced contraction without decrease of [Ca(2+)](i) level. In comparison with IBMX, the increases of cAMP and cGMP contents in ibudilast were much smaller than that of muscle tension. Ibudilast did not inhibit 12-deoxyphorbol 13-isobutyrate (DPB)-induced contraction in the presence of verapamil. Treatment with 30 microM ibudilast inhibited the extracellularly added Ca(2+)-induced muscle tension and increases in [Ca(2+)](i) level during high K(+) depolarization. These results suggested that ibudilast inhibited PE- and high K(+)-induced muscle contractions mainly by the inhibition of [Ca(2+)](i) level in endothelium-denuded rat aorta.

Effects of various selective phosphodiesterase inhibitors on carbachol-induced contraction and cyclic nucleotide contents in guinea pig taenia coli

Effects of various selective phosphodiesterase (PDE) inhibitors on muscle contractility and cyclic nucleotide contents in guinea pig taenia coli were investigated. Forskolin and sodium nitroprusside inhibited carbachol (CCh)-induced contraction in a concentration-dependent manner. Various selective PDE inhibitors, vinpocetine (type 1), erythro -9-(2-hydroxy-3-nonyl)adenine (EHNA, type 2), milrinone (type 3), Ro20-1724(type 4) and zaprinast (type 5) inhibited CCh-induced contraction in a concentration-dependent manner, but the inhibition of milrinone was noticeably smaller than that of the other PDE inhibitors. The rank order of potency was zaprinast > vinpocetine > EHNA > Ro20-1724 > milrinone. In the presence of CCh (0.3 microM), vinpocetine and Ro20-1724 both increased cAMP content, but not cGMP. By contrast, EHNA and zaprinast both increased cGMP content, but not cAMP. Pretreatment with ODQ (30 microM), a soluble guanylyl cyclase inhibitor, decreased the inhibition of CCh-induced contraction by EHNA or zaprinast. Pretreatment with SQ22536 (100 microM), an adenylyl cyclase inhibitor, decreased the inhibition of CCh-induced contraction by vinpocetine or Ro20-1724. In conclusion, it was indicated that vinpocetine- or Ro20-1724-induced relaxation was correlated with cAMP but EHNA- or zaprinast- induced relaxation was correlated with cGMP.

Lack of cyclic nucleotide regulation of MBCQ-induced relaxation of rat ileal smooth muscle

The effects of the type V phosphodiesterase (PDE V) inhibitors, MBCQ, zaprinast and dipyridamole, on the relationship between relaxation and cyclic nucleotide content were investigated in rat ileal smooth muscle. Each of MBCQ (0.01-10 microM), zaprinast (0.1-100 microM) and dipyridamole (0.1-100 microM) inhibited carbachol (CCh; 10 microM)-induced contractions in a concentration-dependent manner. When compared with the concentrations of these agents producing 50% relaxation (IC50) of CCh-induced contraction, MBCQ was 14-20 fold more potent than the other agents. The inhibitory potency of these agents against high K+ (65 mM KCl)-induced contractions were similar to that for CCh. MBCQ (1, 10 microM) did not significantly increase the cGMP content above control levels in the presence of CCh (10 microM). Both Zaprinast (1-100 microM) and dipyridamole (1-100 microM) increased the cGMP content of smooth muscle preparations in a concentration-dependent manner. There was a positive correlation between the inhibition of the CCh-induced contraction and the increase in cGMP content elicited by zaprinast and dipyridamole (zaprinast; r=0.72, P<0.05, dipyridamole; r=0.92, P<0.05). However, MBCQ at a concentration which induced a medium-sized relaxation did not significantly increase the cGMP content. Neither MBCQ, zaprinast nor dipyridamole significantly increased the cAMP content of the preparations above control. In summary, it is suggested that the inhibition of CCh-induced contractions by zaprinast and dipyridamole involves increases in cGMP content via inhibition of PDE V. However the inhibition of CCh-induced contraction by MBCQ may not involve cyclic nucleotides in rat ileal smooth muscle.

8-Bromo-cAMP decreases the Ca2+ sensitivity of airway smooth muscle contraction through a mechanism distinct from inhibition of Rho-kinase

To clarify whether cyclic AMP (cAMP)/cAMP-dependent protein kinase (PKA) activation and Rho-kinase inhibition share a common mechanism to decrease the Ca2+ sensitivity of airway smooth muscle contraction, we examined the effects of 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), a stable cAMP analog, and (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexane carboxamide dihydrochloride, monohydrate (Y-27632), a Rho-kinase inhibitor, on carbachol (CCh)-, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-, 4beta-phorbol 12,13-dibutyrate (PDBu)-, and leukotriene D4 (LTD4)-induced Ca2+ sensitization in alpha-toxin-permeabilized rabbit tracheal and human bronchial smooth muscle. In rabbit trachea, CCh-induced smooth muscle contraction was inhibited by 8-BrcAMP and Y-27632 to a similar extent. However, GTPgammaS-induced smooth muscle contraction was resistant to 8-BrcAMP. In the presence of a saturating concentration of Y-27632, PDBu-induced smooth muscle contraction was completely reversed by 8-BrcAMP. Conversely, PDBu-induced smooth muscle contraction was resistant to Y-27632. In the presence of a saturating concentration of 8-BrcAMP, GTPgammaS-induced Ca2+ sensitization was also reversed by Y-27632. The 8-BrcAMP had no effect on the ATP-triggered contraction of tracheal smooth muscle that had been treated with calyculin A in rigor solutions. The 8-BrcAMP and Y-27632 additively accelerated the relaxation rate of PDBu- and GTPgammaS-treated smooth muscle under myosin light chain kinase-inhibited conditions. In human bronchus, LTD4-induced smooth muscle contraction was inhibited by both 8-BrcAMP and Y-27632. We conclude that cAMP/PKA-induced Ca2+ desensitization contains at least two mechanisms: 1) inhibition of the muscarinic receptor signaling upstream from Rho activation and 2) cAMP/PKA's preferential reversal of PKC-mediated Ca2+ sensitization in airway smooth muscle.

Receptor signaling mechanisms underlying muscarinic agonist-evoked contraction in guinea-pig ileal longitudinal smooth muscle

1 In guinea-pig ileal longitudinal muscle, muscarinic partial agonists, 4-(N-[3-chlorophenyl]-carbomoyloxy)-2-butynyl-trimethylammonium (McN-A343) and pilocarpine, each produced parallel increases in tension and cytosolic Ca(2+) concentration ([Ca(2+)]c) with a higher EC(50) than that of the full agonist carbachol. The maximum response of [Ca(2+)]c or tension was not much different among the three agonists. The Ca(2+) channel blocker nicardipine markedly inhibited the effects of all three agonists 2 The contractile response to any agonist was antagonized in a competitive manner by M(2) receptor selective antagonists (N,N'-bis[6-[[(2-methoyphenyl)methyl]amino]hexyl]-1,8-octanediamine tetrahydrochloride and 11-[[2-[(diethlamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4] benzodiazepine-6-one), and the apparent order of M(2) antagonist sensitivity was McN-A343>pilocarpine>carbachol. M(3) receptor selective antagonists, 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide and darifenacin, both severely depressed the maximum response for McN-A343, while darifenacin had a similar action in the case of pilocarpine. Both M(3) antagonists behaved in a competitive manner in the case of the carbachol response. 3 McN-A343 failed to release Ca(2+) from the intracellular stores, and the Ca(2+)-releasing action of pilocarpine was very weak compared with that of carbachol. All three agonists were capable of increasing Ca(2+) sensitivity of the contractile proteins. 4 McN-A343 rarely produced membrane depolarization, but always accelerated electrical spike discharge. Pilocarpine effect was more often accompanied by membrane depolarization, as was usually seen using carbachol. 5 The results suggest that muscarinic agonist-evoked contractions result primarily from the integration of Ca(2+) entry associated with the increased spike discharge and myofilaments Ca(2+) sensitization, and that Ca(2+) store release may contribute to the contraction indirectly via potentiation of the electrical membrane responses. They may also support the idea that an interaction of M(2) and M(3) receptors plays a crucial role in mediating the contraction response.

Mitogen-activated protein kinases partially regulate endothelin-1-induced contractions through a myosin light chain phosphorylation-independent pathway

Endothelin (ET), derived from the endothelium of blood vessels, is a potent vasoactive peptide. Although it has been reported to be involved in cardiovascular diseases, such as hypertension, the mechanism by which ET evokes vasoconstriction is still unclear. On the other hand, p42/p44 mitogen-activated protein kinase (MAPK) and p38 MAPK are activated by a variety of growth factors and cellular stresses, respectively. However, the role of p42/p44 MAPK and p38 MAPK on the ET-1-induced vasoconstriction is not fully understood. This study was undertaken to determine whether p42/p44 MAPK and p38 MAPK participate in the regulation of vascular smooth muscle contraction by ET-1. The isometric vasoconstriction and intracellular Ca(2+) ([Ca(2+)](i)) were simultaneously measured using CAF-100. Phosphorylation of myosin light chain (MLC) and p42/p44 MAPK, p38 MAPK were determined by Western blots. In rat thoracic aorta, ET-1 induced a sustained contraction. In contrast, [Ca(2+)](i) was decreased with time. Both PD98059, an inhibitor of p42/p44 MAPK, and SB203580, an inhibitor of p38 MAPK, partially attenuated ET-1-induced contractions in concentration-dependent manners. ET-1 increased phosphorylation of both p42/p44 MAPK and p38 MAPK, and PD98059 and SB203580 completely decreased phosphorylation of p42/p44 MAPK and p38 MAPK in response to ET-1 stimulation, respectively. On the other hand, PD98059 and SB203580 did not affect MLC phosphorylation in response to ET-1 stimulation. These results indicate that p38 MAPK, as well as p42/p44 MAPK, may partially regulate the ET-1-induced contraction through a MLC phosphorylation-independent pathway.

Tyrosine kinase participates in vasoconstriction through a Ca(2+)- and myosin light chain phosphorylation-independent pathway

This study was undertaken to determine the role of tyrosine kinase on intracellular Ca(2+) ([Ca(2+)](i)), myosin light chain (MLC) phosphorylation, and contraction caused by norepinephrine (NE) in rat aorta. NE induced a sustained contraction with an increase of [Ca(2+)](i). On the other hand, NE increased the phosphorylation of the 20 kDa MLC transiently. Pretreatment with genistein and tyrophostin 25, tyrosine kinase inhibitors, significantly inhibited NE-induced contraction, but did not affect the increase of [Ca(2+)](i) and MLC phosphorylation. These results suggest that tyrosine kinase may regulate the NE-mediated contraction without altering [Ca(2+)](i) and MLC phosphorylation in rat aorta.

NO donor sodium nitroprusside inhibits excitation-contraction coupling in guinea pig taenia coli

In guinea pig taenia coli, the nitric oxide (NO) donor sodium nitroprusside (SNP, 1 microM) reduced the carbachol-stimulated increases in muscle force in parallel with a decrease in intracellular Ca(2+) concentration ([Ca(2+)](i)). A decrease in the myosin light chain phosphorylation was also observed that was closely correlated with the decrease in [Ca(2+)](i). With the patch-clamp technique, 10 microM SNP decreased the peak Ba(2+) current, and this effect was blocked by an inhibitor of soluble guanylate cyclase. Carbachol (10 microM) induced an inward current, and this effect was markedly inhibited by SNP. SNP markedly increased the depolarization-activated outward K(+) currents, and this current was completely blocked by 0.3 micorM iberiotoxin. SNP (1 microM) significantly increased cGMP content without changing cAMP content. Decreased Ca(2+) sensitivity by SNP of contractile elements was not prominent in the permeabilized taenia, which was consistent with the [Ca(2+)](i)-force relationship in the intact tissue. These results suggest that SNP inhibits myosin light chain phosphorylation and smooth muscle contraction stimulated by carbachol, mainly by decreasing [Ca(2+)](i), which resulted from the combination of the inhibition of voltage-dependent Ca(2+) channels, the inhibition of nonselective cation currents, and the activation of Ca(2+)-activated K(+) currents.

Effects of a prostaglandin I2 analog iloprost on cytoplasmic Ca2+ levels and muscle contraction in isolated guinea pig aorta

In the isolated guinea pig aorta, the prostaglandin I2 analog iloprost (0.01-10 microM) inhibited the contractions induced by the thromboxane A2 analog U46619 (9,11-dideoxy-11 alpha,9 alpha-epoxymethanoprostaglandin F2 alpha; 30 nM) and prostaglandin F2 alpha (PGF2 alpha, 1 microM) in a concentration-dependent manner. In contrast, iloprost only partially inhibited the high K+ (65.4 mM)-induced contraction. In the muscle stimulated with high K+, verapamil (0.3 and 10 microM) inhibited [Ca2+]i and muscle tension in parallel, whereas iloprost (1 microM) inhibited muscle tension with only a small decrease in [Ca2+]i. In the muscle stimulated with U46619 (30 nM), verapamil and iloprost decreased both [Ca2+]i and muscle tension. However, as compared with the effect of verapamil, iloprost more strongly inhibited muscle tension than [Ca2+]i. The iloprost (0.1-1 microM)-induced relaxation was accompanied by a concentration-dependent increase in cAMP content. It was further demonstrated that inhibition of the U46619-contractions was augmented in the presence of cycloxygenase inhibitors, such as indomethacin (10 microM), ibuprofen (10 microM) and aspirin (10 microM). In contrast, the inhibition of PGF2 alpha-induced contraction was not affected by indomethacin. Similarly, the inhibitory effect of forskolin on U46619-induced contractions, but not on PGF2 alpha-induced contraction, was enhanced by indomethacin. These results suggest that iloprost inhibits vascular smooth muscle contraction by decreasing [Ca2+]i and the Ca2+ sensitivity of contractile elements through a cAMP-dependent mechanism. The results also suggest that in U46619-stimulated muscle, vasoactive prostaglandins that counterbalance the relaxing action of cAMP may be generated.

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.