Altered role of C-type natriuretic peptide-activated pGC-cGMP-PDE3-cAMP signaling in hyperthyroid beating rabbit atria

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

B-type natriuretic peptide overexpression ameliorates hepatorenal fibrocystic disease in a rat model of polycystic kidney disease

Polycystic kidney disease (PKD) involves progressive hepatorenal cyst expansion and fibrosis, frequently leading to end-stage renal disease. Increased vasopressin and cAMP signaling, dysregulated calcium homeostasis, and hypertension play major roles in PKD progression. The guanylyl cyclase A agonist, B-type natriuretic peptide (BNP), stimulates cGMP and shows anti-fibrotic, anti-hypertensive, and vasopressin-suppressive effects, potentially counteracting PKD pathogenesis. Here, we assessed the impacts of guanylyl cyclase A activation on PKD progression in a rat model of PKD. Sustained BNP production significantly reduced kidney weight, renal cystic indexes and fibrosis, in concert with suppressed hepatic cystogenesis in vivo. In vitro, BNP decreased cystic epithelial cell proliferation, suppressed fibrotic gene expression, and increased intracellular calcium. Together, our data demonstrate multifaceted effects of sustained activation of guanylyl cyclase A on polycystic kidney and liver disease. Thus, targeting the guanylyl cyclase A-cGMP axis may provide a novel therapeutic strategy for hepatorenal fibrocystic diseases.

The natriuretic peptides BNP and CNP increase heart rate and electrical conduction by stimulating ionic currents in the sinoatrial node and atrial myocardium following activation of guanylyl cyclase-linked natriuretic peptide receptors

Natriuretic peptides (NPs) are best known for their ability to regulate blood vessel tone and kidney function whereas their electrophysiological effects on the heart are less clear. Here, we measured the effects of BNP and CNP on sinoatrial node (SAN) and atrial electrophysiology in isolated hearts as well as isolated SAN and right atrial myocytes from mice. BNP and CNP dose-dependently increased heart rate and conduction through the heart as indicated by reductions in R-R interval, P wave duration and P-R interval on ECGs. In conjunction with these ECG changes BNP and CNP (100 nM) increased spontaneous action potential frequency in isolated SAN myocytes by increasing L-type Ca(2+) current (I(Ca,L)) and the hyperpolarization-activated current (I(f)). BNP had no effect on right atrial myocyte APs in basal conditions; however, in the presence of isoproterenol (10nM), BNP increased atrial AP duration and I(Ca,L). Quantitative gene expression and immunocytochemistry data show that all three NP receptors (NPR-A, NPR-B and NPR-C) are expressed in the SAN and atrium. The effects of BNP and CNP on SAN and right atrial myocytes were maintained in mutant mice lacking functional NPR-C receptors and blocked by the NPR-A antagonist A71915 indicating that BNP and CNP function through their guanylyl cyclase-linked receptors. Our data also show that the effects of BNP and CNP are completely absent in the presence of the phosphodiesterase 3 inhibitor milrinone. Based on these data we conclude that NPs can increase heart rate and electrical conduction by activating the guanylyl cyclase-linked NPR-A and NPR-B receptors and inhibiting PDE3 activity.

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.