Urinary responses to acute moxonidine are inhibited by natriuretic peptide receptor antagonist

Moxonidine ameliorates cardiac injury in rats with metabolic syndrome by regulating autophagy

AIMS

Reduced cardiac autophagy, ischemic injury, sympathetic overactivity, and apoptosis all contribute to metabolic syndrome (MetS)-associated cardiovascular risks. NR4A2, an orphan nuclear receptor NR4A family member, induces autophagy while suppressing apoptosis in myocardial infarction. Moxonidine, a sympathoinhibitor imidazoline1 receptor (I1R) agonist, has beneficial metabolic and hemodynamic effects; however, whether autophagy and/or NR4A2 signaling are involved in moxonidine's cardiovascular effects via I1R activation, is unknown, and is the aim of this study.

MATERIALS AND METHODS

To induce MetS, rats were fed 3 % salt in their diet and 10 % fructose in their drinking water for 12 weeks. MetS-rats were given either moxonidine (6 mg/kg/day, gavage), efaroxan (I1R antagonist, 0.6 mg/kg/day, i.p), both treatments, or vehicles for the last two weeks. Blood pressure, lipid profile, and glycemic control were evaluated. Histopathological examination, circulating cardiac troponin I (c-TnI), proinflammatory interleukin-6 (IL-6), apoptosis (active caspase-3 and Fas-immunostaining), interstitial fibrosis [transforming growth factor-β1 (TGF-β1), Mallory's trichrome staining], and extracellular matrix remodeling [matrix metalloproteinase-9 (MMP-9)], were used to assess cardiac pathology. Cardiac NR4A2 and its downstream factor, p53, as well as autophagic flux markers, SQSTM1/p62, LC3, and Beclin-1 were also determined.

KEY FINDINGS

Moxonidine significantly ameliorated MetS-induced metabolic and hemodynamic derangements and the associated cardiac pathology. Moxonidine restored NR4A2 and p53 myocardial levels and enhanced autophagic flux via modulating SQSTM1/p62, LC3, and Beclin-1. Efaroxan reversed the majority of the moxonidine-induced improvements.

SIGNIFICANCE

The current study suggests that autophagy modulation via I1R activation is involved in moxonidine-mediated cardiac beneficial effects in MetS.

Inefficient constitutive inhibition of P2X3 receptors by brain natriuretic peptide system contributes to sensitization of trigeminal sensory neurons in a genetic mouse model of familial hemiplegic migraine

BACKGROUND

On trigeminal ganglion neurons, pain-sensing P2X3 receptors are constitutively inhibited by brain natriuretic peptide via its natriuretic peptide receptor-A. This inhibition is associated with increased P2X3 serine phosphorylation and receptor redistribution to non-lipid raft membrane compartments. The natriuretic peptide receptor-A antagonist anantin reverses these effects. We studied whether P2X3 inhibition is dysfunctional in a genetic familial hemiplegic migraine type-1 model produced by introduction of the human pathogenic R192Q missense mutation into the mouse CACNA1A gene (knock-in phenotype). This model faithfully replicates several properties of familial hemiplegic migraine type-1, with gain-of-function of CaV2.1 Ca(2+) channels, raised levels of the algogenic peptide calcitonin gene-related peptide, and enhanced activity of P2X3 receptors in trigeminal ganglia.

RESULTS

In knock-in neurons, anantin did not affect P2X3 receptor activity, membrane distribution, or serine phosphorylation level, implying ineffective inhibition by the constitutive brain natriuretic peptide/natriuretic peptide receptor-A pathway. However, expression and functional properties of this pathway remained intact together with its ability to downregulate TRPV1 channels. Reversing the familial hemiplegic migraine type-1 phenotype with the CaV2.1-specific antagonist, ω-agatoxin IVA restored P2X3 activity to wild-type level and enabled the potentiating effects of anantin again. After blocking calcitonin gene-related peptide receptors, P2X3 receptors exhibited wild-type properties and were again potentiated by anantin.

CONCLUSIONS

P2X3 receptors on mouse trigeminal ganglion neurons are subjected to contrasting modulation by inhibitory brain natriuretic peptide and facilitatory calcitonin gene-related peptide that both operate via complex intracellular signaling. In the familial hemiplegic migraine type-1 migraine model, the action of calcitonin gene-related peptide appears to prevail over brain natriuretic peptide, thus suggesting that peripheral inhibition of P2X3 receptors becomes insufficient and contributes to trigeminal pain sensitization.

Terminalia arjuna bark extract improves diuresis and attenuates acute hypobaric hypoxia induced cerebral vascular leakage

ETHNOPHARMACOLOGICAL RELEVANCE

Terminalia arjuna (Roxb. ex DC.) Wight & Arn. (T. arjuna) has been widely used in the traditional ayurvedic system of medicine as a cardioprotectant and for acute and chronic renal diseases supporting its ethnopharmacological use.

AIM OF THE STUDY

The present study aimed at evaluating the diuretic action of an alcoholic extract of T. arjuna and its possible use as a prophylactic to prevent vascular leakage during acute mountain sickness at high altitude.

MATERIALS AND METHODS

Rats were exposed to hypobaric hypoxia simulated to an altitude of 27,000 ft. in a decompression chamber for 12h. T. arjuna bark extract was administered at a single dose of 150 mg/kg (p.o.) to male Sprague Dawley rats (200 ± 20 g) 30 min prior to exposure. Total urine volume was measured during exposure to hypobaric hypoxia. The animals were then investigated for cerebral vascular leakage and serum concentration of sodium, potassium, renin, angiotensin-II, aldosterone and atrial natriuretic peptide (ANP).

RESULTS

T. arjuna ameliorated acute hypobaric hypoxia induced decrease in glomerular filtration rate (p<0.5), increased total urine output (p<0.5) and prevented cerebral vascular leakage in hypoxic rats. T. arjuna treated animals also showed decrease in serum levels of renin (p<0.001) and angiotensin-II (p<0.5) as compared to placebo treated animals. Administration of T. arjuna attenuated acute hypobaric hypoxia induced oxidative stress, improved aldosterone levels and altered electrolyte balance in animals through ANP dependent mechanism.

CONCLUSION

Results of the present study indicate towards diuretic potential of hydro-alcoholic extract of T. arjuna bark and provide evidence for its novel application as a prophylactic to attenuate acute hypobaric hypoxia induced cerebral vascular leakage through ANP mediated modulation of renin-angiotensin-aldosterone system.

Role of atrial natriuretic peptide in mediating the blood pressure-independent natriuresis elicited by systemic inhibition of nitric oxide

While it is clearly recognized that increased intrarenal nitric oxide (NO) levels elicit natriuresis, confounding data showing that systemic nitric oxide synthase inhibition (NOSi) also increases sodium excretion (U_NaV) poses a conundrum. This response has been attributed to the associated increases in arterial pressure (AP); however, the increases in AP and in U_NaV are temporally dissociated. The changes in regional renal haemodynamics induced by NOSi could also contribute to the alterations of U_NaV. To evaluate the roles of AP and non-AP mechanisms mediating the natriuresis, N_ω-nitro-l-arginine methyl ester hydrochloride (L-NAME) was infused i.v. at doses ranging from 5 to 50 μg/kg/min in anaesthetized rats. U_NaV, perfusion of the cortex (cortical blood flow, CBF) and medulla (medullary blood flow, MBF) with laser-Doppler flowmetry and glomerular filtration rate (GFR) were measured. U_NaV increased from 0.6 ± 0.2 to 1.6 ± 0.1 μmol/kg/min (P < 0.05) with the lower nonpressor doses. With the higher doses, AP increased from 116 ± 4 to 122 ± 4 mmHg and U_NaV increased from 1.1 ± 0.3 to 3.3 ± 0.7 μmol/min/g (P < 0.002). U_NaV increased similarly in a group where renal AP was maintained at baseline levels. The associated reductions in CBF (17 ± 5 and 38 ± 5 %) and MBF (27 ± 6 and 52 ± 6 %) would be expected to attenuate rather than contribute to the natriuresis. Plasma atrial natriuretic peptide (ANP) concentrations increased significantly following NOSi. Anantin, a natriuretic peptide receptor-A blocker, prevented or reversed the L-NAME-induced natriuresis without altering the L-NAME-induced changes in AP or CBF. The results indicate that increased ANP and related natriuretic peptides mediate the AP-independent natriuresis, at least partly, elicited by systemic L-NAME infusion and help resolve the conundrum of natriuresis during systemic NOSi.

B-type natriuretic peptide-induced delayed modulation of TRPV1 and P2X3 receptors of mouse trigeminal sensory neurons

Important pain transducers of noxious stimuli are small- and medium-diameter sensory neurons that express transient receptor vanilloid-1 (TRPV1) channels and/or adenosine triphosphate (ATP)-gated P2X3 receptors whose activity is upregulated by endogenous neuropeptides in acute and chronic pain models. Little is known about the role of endogenous modulators in restraining the expression and function of TRPV1 and P2X3 receptors. In dorsal root ganglia, evidence supports the involvement of the natriuretic peptide system in the modulation of nociceptive transmission especially via the B-type natriuretic peptide (BNP) that activates the natriuretic peptide receptor-A (NPR-A) to downregulate sensory neuron excitability. Since the role of BNP in trigeminal ganglia (TG) is unclear, we investigated the expression of BNP in mouse TG in situ or in primary cultures and its effect on P2X3 and TRPV1 receptors of patch-clamped cultured neurons. Against scant expression of BNP, almost all neurons expressed NPR-A at membrane level. While BNP rapidly increased cGMP production and Akt kinase phosphorylation, there was no early change in passive neuronal properties or responses to capsaicin, α,β-meATP or GABA. Nonetheless, 24 h application of BNP depressed TRPV1 mediated currents (an effect blocked by the NPR-A antagonist anantin) without changing responses to α,β-meATP or GABA. Anantin alone decreased basal cGMP production and enhanced control α,β-meATP-evoked responses, implying constitutive regulation of P2X3 receptors by ambient BNP. These data suggest a slow modulatory action by BNP on TRPV1 and P2X3 receptors outlining the role of this peptide as a negative regulator of trigeminal sensory neuron excitability to nociceptive stimuli.

Effect of moxonidine and amlodipine on serum YKL-40, plasma lipids and insulin sensitivity in insulin-resistant hypertensive patients-a randomized, crossover trial

Moxonidine is a selective imidazoline receptor agonist with comparable blood pressure-lowering efficacy to first-line antihypertensives and favorable metabolic effects. YKL-40 (chitinase-3-1-protein) has been proposed as a new marker of inflammation, atherosclerosis and endothelial dysfunction in neoplastic, cardiovascular and metabolic diseases but has not yet been studied in the context of essential hypertension. Fifteen patients (10 M, 5 F; age 48+/-14 years) with arterial hypertension and insulin resistance (HOMA-IR index >2.77) on at least two antihypertensive drugs were randomized to receive either moxonidine (0.4 mg) or amlodipine (10 mg) for two 8-week periods with a 7-day wash-out. Serum insulin, glucose, C-reactive protein (CRP), lipids, uric acid, YKL-40 and blood pressure were measured and insulin sensitivity was calculated (HOMA) at the beginning and end of each study phase. Mean BP decreased significantly with both moxonidine and amlodipine (-9.8+/-7.6 and -10.4+/-7.3 mm Hg, respectively). Serum high-density lipoprotein cholesterol increased with both therapies, but only moxonidine-affected serum triglycerides. No significant changes in serum uric acid, CRP, YKL-40 (2.3 and 3.3 ng ml(-1), respectively) or HOMA index (0.70+/-2.4 and 0.76+/-2.8) were observed. There was a strong negative correlation between serum uric acid and YKL-40 concentration at baseline (r=-0.77, P=0.01). Serum YKL-40 did not correlate with blood pressure, biochemical parameters or HOMA index. Moxonidine is an effective adjunctive antihypertensive agent for use in patients with hypertension and insulin resistance that induces beneficial effects on serum lipid profile but does not reduce insulin resistance, inflammation or serum YKL-40 concentration.

Antagonistic effects of atipamezole and yohimbine on medetomidine-induced diuresis in healthy dogs

This study aimed to investigate and compare the antagonistic effects of atipamezole and yohimbine on medetomidine-induced diuresis in healthy dogs. Five dogs were used repeatedly in each of 8 groups. One group was not medicated. Dogs in the other groups received 20 microg/kg of medetomidine intramuscularly and, 0.5 h later, saline (as the control injection), 50, 100, or 300 microg/kg of atipamezole, or 50, 100, or 300 microg/kg of yohimbine intramuscularly. Urine and blood samples were taken 11 times over 24 h for measurement of the following: urine volume, specific gravity, and creatinine concentration; urine and plasma osmolality; urine and plasma concentrations of electrolytes and arginine vasopressin (AVP); and the plasma concentration of atrial natriuretic peptide (ANP). Both atipamezole and yohimbine antagonized the diuretic effect of medetomidine, inhibiting medetomidine-induced decreases in urine specific gravity, osmolality, and concentrations of creatinine, sodium, potassium, chloride, and AVP and reversing both the medetomidine-induced increase in plasma concentrations of sodium, potassium, and chloride and the medetomidine-induced decrease in the plasma AVP concentration. Atipamezole significantly stimulated ANP release. The antidiuretic action of yohimbine was more potent than that of atipamezole but was not dose-dependent, in contrast to the action of atipamezole. The effects of these drugs may not be due only to actions mediated by alpha(2)-adrenoceptors.

Diuretic effects of medetomidine compared with xylazine in healthy dogs

This study aimed to investigate and compare the effects of medetomidine and xylazine on diuretic and hormonal variables in healthy dogs. Five dogs, used in each of 11 groups, were injected intramuscularly with physiological saline solution (control), 5, 10, 20, 40, and 80 microg/kg of medetomidine, and 0.25, 0.5, 1, 2, and 4 mg/kg of xylazine. Urine and blood samples were taken 11 times over 24 h. Both medetomidine and xylazine increased urine production in a dose-dependent manner up to 4 h after injection, but the increase was much less with medetomidine than with xylazine at the tested doses. Urine specific gravity, pH, osmolality, and concentrations of creatinine, sodium, potassium, chloride, and arginine vasopressin (AVP) were decreased in a dose-dependent manner with both medetomidine and xylazine. Plasma osmolality and concentrations of sodium, potassium, and chloride were increased significantly with both drugs. Total amounts of urine AVP excreted and plasma AVP concentrations were significantly decreased by higher doses of medetomidine but were not significantly decreased by xylazine. Higher doses of both drugs significantly increased the plasma concentration of atrial natriuretic peptide (ANP), but the effect was greater with medetomidine than with xylazine. The results revealed that both drugs induce a profound diuresis, but medetomidine's effect is less dose-dependent than xylazine's effect. Although changes in plasma concentrations of AVP and ANP may partially influence the diuresis induced by medetomidine, other factors may be involved in the mechanism of the diuretic response to both drugs. Thus, both agents can be used clinically for transient but effective diuresis accompanied by sedation.

Antagonistic effects of atipamezole and yohimbine on xylazine-induced diuresis in healthy dogs

The aim of this study was to investigate and compare the antagonistic effects of atipamezole and yohimbine on xylazine-induced diuresis in healthy dogs. Five healthy male beagles were assigned to each of the 8 treatment groups in a randomized design at 1-week intervals in the same dog. One group was not medicated. The dogs in the other groups received 2 mg/kg xylazine intramuscularly (IM) and a treatment of saline (control), 50, 100 or 300 microg/kg of each atipamezole or yohimbine IM 0.5 hr later. Urine and blood samples were collected 11 times over the course of 24 hr. Urine volume, pH, specific gravity and creatinine values; osmolality, electrolyte and arginine vasopressin (AVP) values in both urine and plasma; and plasma atrial natriuretic peptide (ANP) concentration were measured. Both atipamezole and yohimbine antagonized xylazine-induced diuresis. The reversal effect of yohimbine was more potent, but not dose-dependent at the tested doses, in contrast with atipamezole. Both atipamezole and yohimbine exhibited similar potency in reversing the decreases in urine specific gravity, osmolality, creatinine, sodium and chloride concentrations and the increase in the plasma potassium concentration induced by xylazine. Both also inhibited xylazine-induced diuresis without significantly altering the hormonal profile in the dogs. A higher dose of atipamezole tended to increase the plasma ANP concentration. This may not be due only to actions mediated by alpha(2)-adrenoceptors. Both drugs can be used as antagonistic agents against xylazine-induced diuresis in healthy dogs.

Involvement of central α1- and α2-adrenoceptors on cardiovascular responses to moxonidine

In the present study we compared the effects produced by moxonidine (alpha2-adrenoceptor/imidazoline agonist) injected into the 4th cerebral ventricle and into the lateral cerebral ventricle on mean arterial pressure, heart rate and on renal, mesenteric and hindquarter vascular resistances, as well as the possible action of moxonidine on central alpha1- or alpha2-adrenoceptors to produce cardiovascular responses. Male Holtzman rats (n=7-8) anesthetized with urethane (0.5 g/kg, intravenously-i.v.) and alpha-chloralose (60 mg/kg, i.v.) were used. Moxonidine (5, 10 and 20 nmol) injected into the 4th ventricle reduced arterial pressure (-19+/-5, -30+/-7 and -43+/-8 mmHg vs. vehicle: 2+/-4 mmHg), heart rate (-10+/-6, -16+/-7 and -27+/-9 beats per minute-bpm, vs. vehicle: 4+/-5 bpm), and renal, mesenteric and hindquarter vascular resistances. Moxonidine (5, 10 and 20 nmol) into the lateral ventricle only reduced renal vascular resistance (-77+/-17%, -85+/-13%, -89+/-10% vs. vehicle: 3+/-4%), without changes on arterial pressure, heart rate and mesenteric and hindquarter vascular resistances. Pre-treatment with the selective alpha2-adrenoceptor antagonist yohimbine (80, 160 and 320 nmol) injected into the 4th ventricle attenuated the hypotension (-32+/-5, -25+/-4 and -12+/-6 mmHg), bradycardia (-26+/-11, -23+/-5 and -11+/-6 bpm) and the reduction in renal, mesenteric and hindquarter vascular resistances produced by moxonidine (20 nmol) into the 4th ventricle. Pre-treatment with yohimbine (320 nmol) into the lateral ventricle did not change the renal vasodilation produced by moxonidine (20 nmol) into the lateral ventricle. The alpha1-adrenoceptor antagonist prazosin (320 nmol) injected into the 4th ventricle did not affect the cardiovascular effects of moxonidine. However, prazosin (80, 160 and 320 nmol) into the lateral ventricle abolished the renal vasodilation (-17+/-4, -6+/-9 and 2+/-11%) produced by moxonidine. The results indicate that the decrease in renal vascular resistance due to moxonidine action in the forebrain is mediated by alpha1-adrenoceptors, while the cardiovascular effects produced by moxonidine acting in the brainstem depend at least partially on the activation of alpha2-adrenoceptors.

Response of cardiac mast cells to atrial natriuretic peptide

Previously, our laboratory demonstrated that cardiac mast cell degranulation induces adverse ventricular remodeling in response to chronic volume overload. The purpose of this study was to investigate whether atrial natriuretic peptide (ANP), which is known to be elevated in chronic volume overload, causes cardiac mast cell degranulation. Relative to control, ANP induced significant histamine release from peritoneal mast cells, whereas isolated cardiac mast cells were not responsive. Infusion of ANP (225 pg/ml) into blood-perfused isolated rat hearts produced minimal activation of cardiac mast cells, similar to that seen in the control group. ANP also did not increase matrix metalloproteinase-2 activity, reduce collagen volume fraction, or alter diastolic or systolic cardiac function compared with saline-treated controls. In a subsequent study to evaluate the effects of natriuretic peptide receptor antagonism on volume overload-induced ventricular remodeling, anantin was administered to rats with an aortocaval fistula. Comparable increases of myocardial MMP-2 activity in treated and untreated rats with an aortocaval fistula were associated with equivalent decreases in ventricular collagen (P < 0.05 vs. sham-operated controls). Cardiac functional parameters and left ventricular hypertrophy were unaffected by anantin. We conclude that ANP is not a cardiac mast cell secretagogue and is not responsible for the cardiac mast cell-mediated adverse ventricular remodeling in response to volume overload.

Moxonidine: a review of its use in essential hypertension

Moxonidine (Physiotens, Moxon, Cynt) is an orally administered imidazoline compound with selective agonist activity at imidazoline I1 receptors and only minor activity at alpha2-adrenoceptors. Moxonidine acts centrally to reduce peripheral sympathetic activity, thus decreasing peripheral vascular resistance. In patients with mild to moderate hypertension, moxonidine reduces blood pressure (BP) as effectively as most first-line antihypertensives when used as monotherapy and is also an effective adjunctive therapy in combination with other antihypertensive agents. It improves the metabolic profile in patients with hypertension and diabetes mellitus or impaired glucose tolerance, is well tolerated, has a low potential for drug interactions and may be administered once daily in most patients. Thus, moxonidine is a good option in the treatment of patients with mild to moderate hypertension, particularly as adjunctive therapy in patients with the metabolic syndrome.

Receptors involved in moxonidine-stimulated atrial natriuretic peptide release from isolated normotensive rat hearts

Imidazoline I1-receptors are present in the heart and may be involved in atrial natriuretic peptide (ANP) release. The following studies investigated whether moxonidine (an antihypertensive imidazoline I1-receptor and alpha2-adrenoceptor agonist) acts directly on the heart to stimulate ANP release, and to characterize the receptor type involved in this action. Perfusion of rat (200-225 g) isolated hearts with moxonidine (10(-6) and 10(-5) M), for 30 min, resulted in ANP release (83+/-29 and 277+/-70 ng/30 min, above basal, respectively), significantly (P<0.01) different from perfusion with buffer (-6+/-31 ng/30 min). ANP release stimulated by moxonidine (10(-6) M) was inhibited by co-perfusion with the antagonists, AGN192403 (imidazoline I1-receptor), phenoxybenzamine (alpha2>alpha1-adrenoceptors), and prazosin (alpha1>alpha2-adrenoceptors), but increased by rauwolscine (alpha2-adrenoceptors). Perfusion with 10(-5) M brimonidine (full alpha2-adrenoceptor agonist) inhibited moxonidine-stimulated ANP release. Similarly, moxonidine (10(-6) M) tended to reduce coronary flow, but significantly increased coronary flow in the presence of brimonidine, which was vasoconstrictive when perfused alone. Coronary flow was reduced by 10(-5) M each, brimonidine>clonidine>moxonidine; while similar bradycardia was observed with clonidine and moxonidine, but not with brimonidine. In conclusion, these results argue in favor of moxonidine acting primarily on imidazoline I1-receptors to release ANP, with both alpha2-adrenoceptor and imidazoline I1-receptors exerting inhibitory inter-relation. In contrast, the coronary vasodilatory effect of moxonidine requires full activation of alpha2-adrenoceptor. The sympatholytic and ANP-releasing effects of moxonidine appear to be mediated by cardiac imidazoline receptors that may be differentially localized. Most importantly, moxonidine can stimulate ANP release from the heart without contribution of the central nervous system.

Natriuretic peptides as therapeutic targets

Natriuretic peptides (atrial natriuretic peptide, brain natriuretic peptide and C-type natriuretic peptide) are cardiac and vascular peptides with vasodilatory, diuretic, natriuretic, anti-inflammatory, antifibrotic and antimitogenic actions. Natriuretic peptides are implicated in normal pressure and volume homeostasis and in the defence against excessive increases in overload-related factors, vasopressive and cardiotoxic factors and their impact on the heart, blood vessels and brain. Genetic manipulation studies confirmed the importance of natriuretic peptides in these functions. Natriuretic peptides are metabolised by NPR-C (clearance receptors) and by enzymatic degradation by neutral endopeptidase. Natriuretic peptide levels (mainly brain natriuretic peptide) correlate with left ventricular hypertrophy and with the severity of heart failure, and are reduced by effective treatment, thus used as diagnostic and prognostic tools. Based on the multiple protective effects of natriuretic peptides, pharmacological therapy has been approved and includes potentiating natriuretic peptide levels by intravenous infusion or by inhibition of endogenous natriuretic peptide degradation. Because each approach has its limitations, the field remains open for improvement.

Modulation by brain natriuretic peptide of GABA receptors on rat retinal ON-type bipolar cells

Natriuretic peptides (NPs) may work as neuromodulators through their associated receptors [NP receptors (NPRs)]. By immunocytochemistry, we showed that NPR-A and NPR-B were expressed abundantly on both ON-type and OFF-type bipolar cells (BCs) in rat retina, including the dendrites, somata, and axon terminals. Whole-cell recordings made from isolated ON-type BCs further showed that brain natriuretic peptide (BNP) suppressed GABAA receptor-, but not GABAC receptor-, mediated currents of the BCs, which was blocked by the NPR-A antagonist anantin. The NPR-C agonist c-ANF [des(Gln18, Ser19, Gln20, Leu21, Gly22)ANF(4-23)-NH2] did not suppress GABAA currents. The BNP effect on GABAA currents was abolished with preincubation with the pGC-A/B antagonist HS-142-1 but mimicked by application of 8-bromoguanosine-3',5'-cyclomonophosphate. These results suggest that elevated levels of intracellular cGMP caused by activation of NPR-A may mediate the BNP effect. Internal infusion of the cGMP-dependent protein kinase G (PKG) inhibitor KT5823 essentially blocked the BNP-induced reduction of GABAA currents. Moreover, calcium imaging showed that BNP caused a significant elevation of intracellular calcium that could be caused by increased calcium release from intracellular stores by PKG. The BNP effect was blocked by the ryanodine receptor modulators caffeine, ryanodine, and ruthenium red but not by the IP3 receptor antagonists heparin and xestospongin-C. Furthermore, the BNP effect was abolished after application of the blocker of endoplasmic reticulum Ca2+-ATPase thapsigargin and greatly reduced by the calmodulin inhibitors W-7 and calmidazolium. We therefore conclude that the increased calcium release from ryanodine-sensitive calcium stores by BNP may be responsible for the BNP-caused GABAA response suppression in ON-type BCs through stimulating calmodulin.