Distinct roles for L- and T-type Ca(2+) channels in regulation of atrial ANP release

Oryeongsan (Wulingsan) ameliorates impaired ANP secretion of atria from spontaneously hypertensive rats

Oryeongsan (ORS), a herbal medicine formula, has long been used for the treatment of impaired body water balance in Asian countries. Recently, it was shown that ORS administration modulates the renin-angiotensin system (RAS). Purpose of the present study was to determine characteristics of atrial ANP secretion and effects of ORS on the secretion in the atria from spontaneously hypertensive rats (SHR). Normotensive WKY groups (WKY-V, WKY-ORS, WKY-LOS) and hypertensive SHR groups (SHR-V, SHR-ORS, SHR-LOS) treated with vehicle, ORS, and losartan as a positive control group, respectively, were used. Experiments were performed in perfused beating atria (1.3 Hz) allowing atrial distension, acetylcholine (ACh) stimulation, and serial collection of atrial perfusates. The secreted ANP concentration was measured using radioimmunoassay. Interstitial fluid (ISF) translocation was measured using [³H]inulin clearance. Stepwise increase in atrial distension by 1.1, 2.0, and 2.7 cmH₂O above basal distension further increased ANP secretion proportionally in the atria from WKY-V, but the response was significantly suppressed in the atria from SHR-V. Cardiomyocyte ANP release, the first step of atrial ANP secretion, was suppressed in the atria from SHR-V compared to those from WKY-V (-8.02 ± 2.86, -15.86 ± 2.27, and -20.09 ± 3.62%; n = 8, for SHR-V vs. 8.59 ± 2.81, 15.65 ± 7.14, and 38.12 ± 8.28%; n = 8, for WKY-V; p < 0.001 for all stepwise distension, respectively). Chronic treatment with ORS reversed the suppressed ANP release in atria from SHR-ORS group (6.76 ± 3.92, 9.12 ± 2.85, and 28.79 ± 1.79% for SHR-ORS; n = 5 vs. SHR-V; n = 8; p = 0.01, p < 0.001, p < 0.001, respectively). The effects of ORS were comparable to those of losartan. Trans-endocardial translocation of ISF, the second step of atrial ANP secretion was similar in the atria from the hypertensive SHR-V and normotensive WKY-V. ACh-induced ANP secretion and cardiomyocyte ANP release were also suppressed in the atria from SHR-V compared to WKY-V and ORS reversed the suppression. These findings were accompanied with accentuation of the AT₁ receptor expression and suppression of the AT₂/Mas receptor, M₂ mACh receptor and GIRK4, a molecular component of K_ACh channel, expression in the atria from SHR-V. Further, treatment with ORS or losartan reversed the expressions in the groups of SHR-ORS and SHR-LOS. These results show that ANP secretion is suppressed in the atria from SHR in association with accentuation of AT₁ receptor and suppression of AT₂/Mas receptor and K_ACh channel expression. Treatment with ORS ameliorates impaired ANP secretion through improving cardiomyocyte ANP release with modulation of the cardiac RAS and muscarinic signaling. These findings provide experimental evidence which supports the effect of ORS on the regulation of atrial ANP secretion in the atria from SHR.

Endogenous ACh tonically stimulates ANP secretion in rat atria

Exogenous acetylcholine (ACh) is known to stimulate atrial natriuretic peptide (ANP) secretion concomitantly with a decrease in atrial pulse pressure. However, the role of intrinsic ACh in the regulation of ANP secretion remains unknown. Recently, it was shown that nonneuronal and neuronal ACh is present in the cardiac atria. From this finding we hypothesize that endogenously released ACh is involved in the regulation of ANP secretion in an autocrine or paracrine manner in the atria. Experiments were performed in isolated beating rat atria. ANP was measured using radioimmunoassay. To increase the availability of the ACh in the extracellular space of the atrium, its degradation was inhibited with an inhibitor of acetylcholinesterase. Acetylcholinesterase inhibition with physostigmine increased ANP secretion concomitantly with a decrease in atrial dynamics in a concentration-dependent manner. Inhibitors of M2 muscarinic ACh receptor (mAChR), methoctramine, and ACh-activated K(+) (KACh(+)) channels, tertiapin-Q, abolished the physostigmine-induced changes. The effects were not observed in the atria from rats treated with pertussis toxin. Furthermore, the physostigmine-induced effects were attenuated by an inhibitor of high-affinity choline transporter, hemicholinium-3, which is a rate-limiting step of ACh synthesis. Inhibitors of the mAChR signaling pathway and ACh synthesis also attenuated the basal levels of ANP secretion and accentuated atrial dynamics. These findings suggest that endogenously released ACh tonically stimulates ANP secretion from atrial cardiomyocytes via activation of M2 mAChR-Gi/o-KACh(+) channel signaling. It is also suggested that the ACh-ANP signaling is implicated in cardiac physiology and pathophysiology.

Ouabain stimulates atrial natriuretic peptide secretion via the endothelin-1/ET(B) receptor-mediated pathway in beating rabbit atria

AIMS

Ouabain has been reported to increase the secretion of atrial natriuretic peptide (ANP) in vitro. However, the mechanism by which ouabain increases ANP secretion is not well known. Therefore, the purpose of the present study was to investigate the underlying mechanism of ouabain-stimulated ANP secretion.

MAIN METHODS

A perfused beating rabbit atrial model was used. The ANP and ET-1 levels in the atrial perfusates were measured by radioimmunoassays.

KEY FINDINGS

Ouabain (1.0, 3.0 and 6.0 μmol/L) significantly increased atrial ANP secretion in a dose-dependent manner, while the endothelin (ET)-1 levels were increased by the higher doses (3.0 and 6.0 μmol/L) of ouabain. Ouabain-increased atrial ET-1 release was blocked by PD98059 (30.0 μmol/L), an inhibitor of mitogen-activated protein kinase (MAPK). Nifedipine (1.0 μmol/L), an inhibitor of L-type Ca(2+) channels, completely abolished ouabain-increased ANP secretion without changing the ouabain-induced atrial dynamics. KB-R7943 (3.0 μmol/L), an inhibitor of Na(+)-Ca(2+) exchangers, completely blocked the effects of ouabain-increased atrial dynamics, but did not modulate ouabain-increased ANP secretion. ET-1 significantly stimulated atrial ANP release in a dose-dependent manner. The effects of ET-1 and ouabain on ANP secretion were completely blocked by BQ788 (0.3 μmol/L), an inhibitor of ET-1 type B (ET(B)) receptors, but not by BQ123 (0.3 μM), an inhibitor of ET-1 type A receptors. Ouabain-increased atrial ANP secretion was blocked by PD98059 and indomethacin (30.0 μmol/L), an inhibitor of cyclooxygenase.

SIGNIFICANCE

Ouabain significantly stimulated atrial ANP secretion via an ET-1-ET(B) receptor-mediated pathway involving MAPK signaling pathway activation and prostaglandin formation.

Different subcellular populations of L-type Ca2+ channels exhibit unique regulation and functional roles in cardiomyocytes

Influx of Ca(2+) through L-type Ca(2+) channels (LTCCs) contributes to numerous cellular processes in cardiomyocytes including excitation-contraction (EC) coupling, membrane excitability, and transcriptional regulation. Distinct subpopulations of LTCCs have been identified in cardiac myocytes, including those at dyadic junctions and within different plasma membrane microdomains such as lipid rafts and caveolae. These subpopulations of LTCCs exhibit regionally distinct functional properties and regulation, affording precise spatiotemporal modulation of L-type Ca(2+) current (I(Ca,L)). Different subcellular LTCC populations demonstrate variable rates of Ca(2+)-dependent inactivation and sometimes coupled gating of neighboring channels, which can lead to focal, persistent I(Ca,L). In addition, the assembly of spatially defined macromolecular signaling complexes permits compartmentalized regulation of I(Ca,L) by a variety of neurohormonal pathways. For example, β-adrenergic receptor subtypes signal to different LTCC subpopulations, with β(2)-adrenergic activation leading to enhanced I(Ca,L) through caveolar LTCCs and β(1)-adrenergic stimulation modulating LTCCs outside of caveolae. Disruptions in the normal subcellular targeting of LTCCs and associated signaling proteins may contribute to the pathophysiology of a variety of cardiac diseases including heart failure and certain arrhythmias. Further identifying the characteristic functional properties and array of regulatory molecules associated with specific LTCC subpopulations will provide a mechanistic framework to understand how LTCCs contribute to diverse cellular processes in normal and diseased myocardium. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Aqueous extract of Zanthoxylum schinifolium elicits contractile and secretory responses via beta1-adrenoceptor activation in beating rabbit atria

AIM OF STUDY

Although Zanthoxylum schinifolium has long been used in the traditional oriental medicine, cardiac effects have not well been documented. The aim of the present study was to investigate the effects of aqueous extract of leaves and stems from Zanthoxylum schinifolium (AZS) on inotropic effect and atrial natriuretic peptide (ANP) secretion.

MATERIALS AND METHODS

The AZS-induced changes in atrial dynamics, cAMP efflux and atrial ANP secretion were determined in isolated perfused beating rabbit atria.

RESULTS

AZS increased atrial pulse pressure, stroke volume, and cAMP efflux concomitantly with inhibition of ANP secretion in a concentration-dependent manner. The AZS-induced increases in atrial dynamics and cAMP efflux, and decrease in ANP secretion were attenuated by pretreatment with propranolol and CGP 20712 but not ICI 118,551. Also, the AZS-induced changes in atrial dynamics and ANP secretion were attenuated by diltiazem and KT 5720. Diltiazem and KT 5720 had not significant effect on the AZS-induced increase in cAMP efflux.

CONCLUSION

These results suggest that AZS elicits a positive inotropic effect and decrease in ANP secretion via beta(1)-adrenoceptor-cAMP-Ca(2+) signaling in beating rabbit atria.

cAMP produced by pituitary adenylate cyclase-activating polypeptide 27 inhibits atrial natriuretic peptide secretion in rabbit beating atria

The aim of the present study was to determine the effects of increased cAMP levels in response to pituitary adenylate cyclase-activating polypeptide 27 (PACAP27) on atrial atrial natriuretic peptide (ANP) secretion in rabbit atria. A perfused beating atrial model was used in the present study and cAMP efflux and ANP levels in atrial perfusates were measured by radioimmnoassay. At 100 nmol/L, PACAP27 increased cAMP production, which resulted in subsequent inhibition of ANP secretion. Nicardipine (1.0 micromol/L), an L-type Ca2+ channel blocker, attenuated inhibition of ANP secretion by PACAP27. Staurosporine (1.0 micromol/L), a non-specific protein kinase inhibitor, and H-89 (1.0 micromol/L), a cAMP-dependent protein kinase A (PKA) inhibitor, completely blocked the inhibition of ANP secretion in response to PACAP27 but had no effect on PACAP27-induced increases in cAMP. In conclusion, the results suggest that increased cAMP levels in response to PACAP27 negatively regulate ANP secretion via the adenylate cyclase-cAMP-PKA signalling pathway in rabbit atria and that L-type Ca2+ channels may be involved, in part, in the regulation of ANP secretion by cAMP.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Determinants of natriuretic peptide gene expression

The cardiac natriuretic peptides (NP) atrial natriuretic factor or peptide (ANF or ANP) and brain natriuretic peptide (BNP) are polypeptide hormones synthesized, stored and secreted mainly by cardiac muscle cells (cardiocytes) of the atria of the heart. Both ANF and BNP are co-stored in storage granules referred to as specific atrial granules. The biological properties of NP include modulation of intrinsic renal mechanisms, the sympathetic nervous system, the rennin-angiotensin-aldosterone system (RAAS) and other determinants, of fluid volume, vascular tone and renal function. Studies on the control of baseline and stimulated ANF synthesis and secretion indicate at least two types of regulated secretory processes in atrial cardiocytes: one is stretch-stimulated and pertussis toxin (PTX) sensitive and the other is Gq-mediated and is PTX insensitive. Baseline ANF secretion is also PTX insensitive. In vivo, it is conceivable that the first process mediates stimulated ANF secretion brought about by changes in central venous return and subsequent atrial muscle stretch as observed in acute extracellular fluid volume expansion. The second type of stimulation is brought about by sustained hemodynamic and neuroendocrine stimuli such as those observed in congestive heart failure.

Ablation of Cav2.3 / E?type voltage?gated calcium channel results in cardiac arrhythmia and altered autonomic control within the murine cardiovascular system

Voltage-gated calcium channels are key components in cardiac electrophysiology. We demonstrate that Ca(v)2.3 is expressed in mouse and human heart and that mice lacking the Ca(v)2.3 voltage-gated calcium channel exhibit severe alterations in cardiac function. Amplified cDNA fragments from murine heart and single cardiomyocytes reveal the expression of three different Ca(v)2.3 splice variants. The ablation of Ca(v)2.3 was found to be accompanied by a compensatory upregulation of the Ca(v)3.1 T-type calcium channel, while other voltage-gated calcium channels remained unaffected. Telemetric ECG recordings from Ca(v)2.3 deficient mice displayed subsidiary escape rhythm, altered atrial activation patterns, atrioventricular conduction disturbances and alteration in QRS-morphology. Furthermore, time domain analysis of heart rate variability (HRV) in Ca(v)2.3(-/-) mice exhibited a significant increase in heart rate as well as in the coefficient of variance (CV) compared to control mice. Administration of atropin/propranolol revealed that increased heart rate was due to enhanced sympathetic tonus and that partial decrease of CV in Ca(v)2.3(-/-) mice after autonomic block was in accordance with a complete abolishment of 2(nd) degree atrioventricular block. However, escape rhythms, atrial activation disturbances and QRS-dysmorphology remained unaffected, indicating that these are intrinsic cardiac features in Ca(v)2.3(-/-) mice. We conclude that the expression of Ca(v)2.3 is essential for normal impulse generation and conduction in murine heart.

Osmoregulation of atrial myocytic ANP release: osmotransduction via cross-talk between L-type Ca2+ channel and SR Ca2+ release

Hyperosmolality has been known to increase ANP release. However, its physiological role in the regulation of atrial myocytic ANP release and the mechanism by which hyperosmolality increases ANP release are to be defined. The purpose of the present study was to define these questions. Experiments were performed in perfused beating rabbit atria. Hyperosmolality increased atrial ANP release, cAMP efflux, and atrial dynamics in a concentration-dependent manner. The osmolality threshold for the increase in ANP release was as low as 10 mosmol/kgH2O (approximately 3%) above the basal levels (1.55 +/- 1.71, 17.19 +/- 3.11, 23.15 +/- 5.49, 54.04 +/- 11.98, and 62.00 +/- 13.48% for 10, 20, 30, 60, and 100 mM mannitol, respectively; all P < 0.01). Blockade of sarcolemmal L-type Ca2+ channel activity, which increased ANP release, attenuated hyperosmolality-induced increases in ANP release (-13.58 +/- 4.68% vs. 62.00 +/- 13.48%, P < 0.001) and cAMP efflux but not atrial dynamics. Blockade of the Ca2+ release from the sarcoplasmic reticulum, which increased ANP release, attenuated hyperosmolality-induced increases in ANP release (13.44 +/- 7.47% vs. 62.00 +/- 13.48%, P < 0.01) and dynamics but not cAMP efflux. Blockades of Na+-K+-2Cl- cotransporter, Na+/H+ exchanger, and Na+/Ca2+ exchanger had no effect on hyperosmolality-induced increase in ANP release. The present study suggests that hyperosmolality regulates atrial myocytic ANP release and that the mechanism by which hyperosmolality activates ANP release is closely related to the cross-talk between the sarcolemmal L-type Ca2+ channel activity and sarcoplasmic reticulum Ca2+ release, possibly inactivation of the L-type Ca2+ channels.

Histamine inhibits atrial myocytic ANP release via H2 receptor-cAMP-protein kinase signaling

Changes in cyclic nucleotide production and atrial dynamics have been known to modulate atrial natriuretic peptide (ANP) release. Although cardiac atrium expresses histamine receptors and contains histamine, the role of histamine in the regulation of ANP release has to be defined. The purpose of the present study was to define the effect of histamine on the regulation of ANP release in perfused beating rabbit atria. Histamine decreased ANP release concomitantly with increases in cAMP efflux and atrial dynamics in a concentration-dependent manner. Histamine-induced decrease in ANP release was a function of an increase in cAMP production. Blockade of histamine H2 receptor with cimetidine but not of H1 receptor with triprolidine abolished the responses of histamine. Cell-permeable cAMP analog, 8-Br-cAMP, mimicked the effects of histamine, and the responses were dose-dependent and blocked by a protein kinase A (PKA)-selective inhibitor, KT5720. Nifedipine failed to modulate histamine-induced decrease in ANP release. Protein kinase nonselective inhibitor staurosporine blocked histamine-induced changes in a concentration-dependent manner. KT5720 and RP-adenosine 3',5'-cyclic monophosphorothioate, another PKA-selective inhibitor, attenuated histamine-induced changes. These results suggest that histamine decreases atrial ANP release by H2 receptor-cAMP signaling via PKA-dependent and -independent pathways.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Protein kinase-dependent and Ca(2+)-independent cAMP inhibition of ANP release in beating rabbit atria

Regulation of atrial release of atrial natriuretic peptide (ANP) is coupled to changes in atrial dynamics. However, the mechanism by which mechanical stretch controls myocytic ANP release must be defined. The purpose of this study was to define the mechanism by which cAMP controls myocytic ANP release in perfused, beating rabbit atria. The cAMP-elevating agents forskolin and 3-isobutyl-1-methylxanthine (IBMX) inhibited myocytic ANP release. The activation of adenylyl cyclase with forskolin inhibited ANP release, which was a function of an increase in cAMP production. Inhibitors for L-type Ca(2+) channels and protein kinase A (PKA) attenuated a minor portion of the forskolin-induced inhibition of ANP release. Gö-6976 and KN-62, which are specific inhibitors for protein kinase C-alpha and Ca(2+)/calmodulin kinase, respectively, failed to modulate forskolin-induced inhibition of ANP release. The nonspecific protein kinase inhibitor staurosporine blocked forskolin-induced inhibition of ANP release in a dose-dependent manner. Staurosporine but not nifedipine shifted the relationship between cAMP and ANP release. Inhibitors for L-type Ca(2+) channels and PKA and staurosporine blocked forskolin-induced accentuation of atrial dynamics. These results suggest that cAMP inhibits atrial myocytic release of ANP via protein kinase-dependent and L-type Ca(2+)-channel-dependent and -independent signaling pathways.

Plasma atrial natriuretic peptide levels in essential hypertension after treatment with verapamil

The aim of this study was to evaluate the long term effects of the selective Ca2+-blocker verapamil on atrial natriuretic peptide (ANP) levels in patients with moderate essential hypertension. The drug was given orally in a daily dose of 300 mg for 30 days. At the end of this clinical trial, plasma ANP levels increased by 16.14% despite the drop in blood pressure while left atrial and ventricular diameters remained unchanged. These findings indicate that the increase of ANP plasma levels is not the result of a mechanical load on the left cardiac chambers but the result of a pharmacological action. These observations also indicate that verapamil exerts part of its antihypertensive action by increasing ANP plasma levels.