Inhibitory effect of dendroaspis natriuretic peptide on spontaneous contraction in gastric antral circular smooth muscles of guinea pigs

Paraoxon attenuates vascular smooth muscle contraction through inhibiting Ca2+ influx in the rabbit thoracic aorta

We investigated the effect of paraoxon on vascular contractility using organ baths in thoracic aortic rings of rabbits and examined the effect of paraoxon on calcium homeostasis using a whole-cell patch-clamp technique in isolated aortic smooth muscle cells of rabbits. The findings show that administration of paraoxon (30 μM) attenuated thoracic aorta contraction induced by phenylephrine (1 μM) and/or a high K⁺ environment (80 mM) in both the presence and absence of thoracic aortic endothelium. This inhibitory effect of paraoxon on vasoconstrictor-induced contraction was abolished in the absence of extracellular Ca²⁺, or in the presence of the Ca²⁺ channel inhibitor, verapamil. But atropine had little effect on the inhibitory effect of paraoxon on phenylephrine-induced contraction. Paraoxon also attenuated vascular smooth muscle contraction induced by the cumulative addition of CaCl₂ and attenuated an increase of intracellular Ca²⁺ concentration induced by K⁺ in vascular smooth muscle cells. Moreover, paraoxon (30 μM) inhibited significantly L-type calcium current in isolated aortic smooth muscle cells of rabbits. In conclusion, our results demonstrate that paraoxon attenuates vasoconstrictor-induced contraction through inhibiting Ca²⁺ influx in the rabbits thoracic aorta.

Atrial natriuretic peptide signal pathway upregulated in stomach of streptozotocin-induced diabetic mice

AIM

To investigate atrial natriuretic peptide (ANP) secretion from gastric mucosa and the relationship between the ANP/natriuretic peptide receptor type A (NPR-A) pathway and diabetic gastroparesis.

METHODS

Male imprinting control region (ICR) mice (4 wk old) were divided into two groups: control mice, and streptozotocin-induced diabetic mice. Eight weeks after injection, spontaneous gastric contraction was recorded by using physiography in control and streptozotocin-induced diabetic mice. The ANP-positive cells in gastric mucosa and among dispersed gastric epithelial cells were detected by using immunohistochemistry and flow cytometry, respectively. ANP and natriuretic peptide receptor type A (NPR-A) gene expression in gastric tissue was observed by using the reverse transcriptase polymerase chain reaction.

RESULTS

The frequency of spontaneous gastric contraction was reduced from 12.9 +/- 0.8 cycles/min in the control group to 8.4 +/- 0.6 cycles/min in the diabetic mice (n = 8, P < 0.05). However, the amplitude of contraction was not significantly affected in the diabetic group. The depletion of interstitial cells of Cajal in the gastric muscle layer was observed in the diabetic mice. ANP-positive cells were distributed in the gastric mucosal layer and the density index of ANP-positive cells was increased from 20.9 +/- 2.2 cells/field in control mice to 51.8 +/- 2.9 cells/field in diabetic mice (n = 8, P < 0.05). The percentage of ANP-positive cells among the dispersed gastric epithelial cells was increased from 10.0% +/- 0.9% in the control mice to 41.2% +/- 1.0% in the diabetic mice (n = 3, P < 0.05). ANP and NPR-A genes were both expressed in mouse stomach, and the expression was significantly increased in the diabetic mice.

CONCLUSION

These results suggest that the ANP/NPR-A signaling pathway is upregulated in streptozotocin-induced diabetic mice, and contributes to the development of diabetic gastroparesis.

Effects of Dendroaspis natriuretic peptide on delayed rectifier potassium currents and its mechanism

Dendroaspis natriuretic peptide (DNP), a newly-described natriuretic peptide, plays an inhibitory role in smooth muscle motility of the gastrointestinal tract. However, the effect of DNP on delayed rectifier potassium currents I(K(V)) is still unclear. In this study, we sought to investigate the effect of DNP on I(K(V)) and its mechanism in gastric antral circular smooth muscle cells using the whole-cell patch-clamp technique. DNP significantly inhibited I(K(V)) in a concentration-dependent manner. LY83583 (1 micromol/l), a guanylate cyclase inhibitor, significantly impaired DNP-induced inhibition of I(K(V)). Moreover, DNP-induced inhibition in I(K(V)) was potentiated by the cyclic guanosine monophosphate (cGMP) sensitive phosphoesterase inhibitor zaparinast (0.1 micromol/l). DNP-induced inhibition of I(K(V)) was completely blocked by KT5823, an inhibitor of cGMP-dependent protein kinase G(PKG), but not affected by KT-5720, a PKA-specific inhibitor. Taken together, our results suggest that DNP inhibits I(K(V)) via the cGMP/PKG-dependent signaling axis instead of the cAMP/PKA pathway.

Dendroaspis natriuretic peptide relaxes gastric antral circular smooth muscle of guinea-pig through the cGMP/cGMP-dependent protein kinase pathway

AIM: To systematically investigate if cGMP/cGMP-dependent protein kinase G (PKG) signaling pathway may participate in dendroaspis natriuretic peptide (DNP)-induced relaxation of gastric circular smooth muscle.

METHODS: The content of cGMP in guinea pig gastric antral smooth muscle tissue and perfusion solution were measured using radioimmunoassay; spontaneous contraction of gastric antral circular muscles recorded using a 4-channel physiograph; and Ca²⁺-activated K⁺ currents (I_K(Ca)) and spontaneous transient outward currents (STOCs) in isolated gastric antral myocytes were recorded using the whole-cell patch clamp technique.

RESULTS: DNP markedly enhanced cGMP levels in gastric antral smooth muscle tissue and in the perfusion medium. DNP induced relaxation in gastric antral circular smooth muscle, which was inhibited by KT5823, a cGMP-dependent PKG inhibitor. DNP increased I_K(Ca). This effect was almost completely blocked by KT5823, and partially blocked by LY83583, an inhibitor of guanylate cyclase to change the production of cGMP. DNP also increased STOCs. The effect of DNP on STOCs was abolished in the presence of KT5823, but not affected by KT-5720, a PKA-specific inhibitor.

CONCLUSION: DNP activates I_K(Ca) and relaxes guinea-pig gastric antral circular smooth muscle via the cGMP/PKG-dependent singling axis instead of cAMP/PKA pathway.

Effects of dendroaspis natriuretic peptide on calcium-activated potassium current and its mechanism

In this study, we sought to investigate the effect of dendroaspis natriuretic peptide (DNP) on calcium-activated potassium current (I K(Ca)) and its mechanism in gastric antral circular smooth muscle cells (SMCs) using the whole-cell patch-clamp technique. DNP concentration-dependently increased macroscopic I K(Ca) and spontaneous transient outward currents (STOCs) in freshly isolated guinea pig gastric antral circular SMCs. The effects of DNP on I K(Ca) and/or STOCs were not blocked by applying calcium-free bath solution or the ryanodine receptor (RyR) antagonist ryanodine (10 microM), but they were inhibited by the inositol triphosphate receptor (IP3R) inhibitor heparin or the guanylate cyclase inhibitor LY83583. Moreover, a DNP-induced increase in STOCs was potentiated by the cyclic guanosine monophosphate (cGMP)-sensitive phosphoesterase inhibitor zaprinast. In conclusion, our results suggest that DNP increases I K(Ca) in gastric antral circular SMCs by increasing cGMP production and activating IP3Rs.