Regulation of Atrial Natriuretic Factor-(99-126) Secretion from Neonatal Rat Primary Atrial Cultures by Activators of Protein Kinases A and C

Modification of levosimendan-induced suppression of atrial natriuretic peptide secretion in hypertrophied rat atria

This study aimed to determine the effects of levosimendan, a calcium sensitizer, on atrial contractility and atrial natriuretic peptide (ANP) secretion and its modification in hypertrophied atria. Isolated perfused beating rat atria were used from control and isoproterenol-treated rats. Levosimendan and its metabolite OR-1896 caused a positive inotropic effect and suppressed ANP secretion in rat atria. Similar to levosimendan, the selective phosphodiesterase 3 (PDE3) or PDE4 inhibitor also suppressed ANP secretion. Suppression of ANP secretion by 1 µM levosimendan was abolished by PDE3 inhibitor, but reversed by PDE4 inhibitor. Levosimendan-induced suppression of ANP secretion was potentiated by K_ATP channel blocker, but blocked by K_ATP channel opener. Levosimendan alone did not significantly change cyclic adenosine monophosphate (cAMP) efflux in the perfusate; however, levosimendan combined with PDE4 inhibitor markedly increased this efflux. The stimulation of ANP secretion induced by levosimendan combined with PDE4 inhibitor was blocked by the protein kinase A (PKA) inhibitor. In isoproterenol-treated atria, levosimendan augmented the positive inotropic effect and ANP secretion in response to an increased extracellular calcium concentration ([Ca⁺]_o). These results suggests that levosimendan suppresses ANP secretion by both inhibiting PDE3 and opening K_ATP channels and that levosimendan combined with PDE4 inhibitor stimulates ANP secretion by activating the cAMP-PKA pathway. Modification of the effects of levosimendan on [Ca⁺]_o-induced positive inotropic effects and ANP secretion in isoproterenol-treated rat atria might be related to a disturbance in calcium metabolism.

Carvedilol Increases Atrial Natriuretic Peptide Plasma Levels in Hypertensive Patients

Twenty five patients with moderate essential hypertension were studied for 30 days in order to evaluate the medium term effects of carvedilol on atrial natriuretic peptide (ANP) levels. This drug blocks the β1 and β2 adrenenergic receptors as well as α1 adrenergic receptors. In addition, it has strong antioxidative and antiproliferative properties. The drug was given orally at a dose of 12.5 mg b.i.d. Quantitative determination of human ANP was made by radioimmunoassay procedure (RIA). At the end of this clinical trial, mean plasma levels of ANP had risen during treatment by 21.18% (from 37.60 pg/ml to 45.83 pg/ml) while both systolic (SBP) and diastolic (DBP) blood pressure as well as diameters of the left cardiac cavities had decreased in a statistically significant way. The ratio ANP/SBP was also increased by 46.6% in a statistically significant way. These findings support the suggestion that the increase in plasma ANP following the administration of β-adrenergic blockers to hypertensive patients is a primary effect of beta blockade and not a mechanical one secondary to a negative inotropic action on the left ventricle and obviously contributes to the anti-hypertensive action.

Modification of atrial natriuretic peptide system in cold-induced hypertensive rats

Cold exposure induces hypertension and cardiac hypertrophy via sympathetic activation. The sympathetic nervous system is fundamentally important for the regulation of cardiac atrial natriuretic peptide (ANP) secretion. The present study aimed to define changes in ANP level with renal functions during cold exposure of rats. We also measured the direct effects of adrenergic stimulation on ANP secretion in cold-induced hypertensive rat atria. Sustained elevation of blood pressure and tachycardia were observed by 2-wk cold exposure. Cold exposure increased urine volume, UNaV, UKV and positive water balance. Atrial ANP content, its mRNA level, and plasma ANP concentration increased. Plasma norepinephrine level was increased but both alpha(1A)- and beta(1)-adrenoceptor (AR) mRNA levels in atrium were decreased. In isolated perfused atria from cold-exposed rats, basal ANP secretion increased and pulse pressure decreased. Phenylephrine (alpha(1)-AR agonist)-induced stimulation of ANP secretion, and isoproterenol (beta-AR agonist)-induced suppression of ANP secretion were significantly attenuated. These results suggest that an increased plasma and atrial ANP level by cold exposure may be a compensatory response to changes in hemodynamics and body fluid balance. The phenylephrine- and isoproterenol-induced attenuation of ANP secretion in cold-exposed rat atria may be due to the downregulation of alpha(1A)- and beta(1)-adrenoceptors mRNA levels.

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.

Different response of ANP secretion to adrenoceptor stimulation in renal hypertensive rat atria

Sympathetic nervous system and atrial natriuretic peptide (ANP) system play fundamental roles in the regulation of cardiovascular functions. Overactivity of sympathetic nervous system can lead into cardiovascular diseases such as heart failure and hypertension. The present study aimed to define which adrenergic receptors (ARs) affect atrial contractility and ANP release and to determine their modification in renal hypertensive rat atria. An alpha(1)-AR agonist, cirazoline increased ANP release with positive inotropism. These alpha(1)-AR agonist-mediated responses were attenuated by the alpha(1A)-AR antagonist, but not by the alpha(1B)- or alpha(1D)-AR antagonist. An alpha(2)-AR agonist, guanabenz and clonidine increased ANP release with negative inotropism and decreased cAMP level. The order of potency for the increased ANP release was cirazoline>>phenylephrine=guanabenz>>clonidine. In contrast, a beta-AR agonist, isoproterenol decreased ANP release with positive inotropism and these responses were blocked by the beta(1)-AR antagonist but not by the beta(2)-AR antagonist. The increased cAMP level by isoproterenol was suppressed by pretreatment with both beta(1)- and beta(2)-AR antagonists. In renal hypertensive rat atria, the effects of isoproterenol on atrial contractility, ANP release, and cAMP level were attenuated whereas the effect of cirazoline on ANP release was unaltered. Atrial beta(1)-AR mRNA level but not alpha(1A)-AR mRNA level was decreased in renal hypertensive rats. These findings suggest that alpha(1A)- and beta(1)-AR oppositely regulate atrial ANP release and that atrial beta(1)-AR expression/function is impaired in renal hypertensive rats.

The effects of medications on circulating levels of cardiac natriuretic peptides

Circulating cardiac natriuretic peptide levels are being used increasingly in a range of clinical circumstances. Since it is evident that drugs used in the treatment of cardiovascular disorders can modulate natriuretic peptide levels, we here review the literature documenting these effects. Diuretics, blockers of the renin-angiotensin system, vasodilator agents, dopamine-like agonists, amiodarone, and perhaps allopurinol and statins suppress natriuretic peptide levels, most obviously in heart failure. Beta-blockers stimulate natriuretic peptide concentrations in hypertensive subjects, whereas in heart failure they have little effect or are stimulatory in the short term and inhibitory with sustained therapy. Digitalis compounds and aspirin tend to increase natriuretic peptide levels, and calcium channel blocking agents have varying effects depending on the individual drug and duration of administration. The effects of other drugs are less clear. Additional information is needed regarding the effects of medications along with dissection of the role of altered cardiac secretion versus changes in plasma clearance as explanation for drug-induced perturbations in natriuretic peptide concentrations. In the meantime, clinicians need to consider the known effects of medications when interpreting plasma levels of the cardiac natriuretic peptides.

Atrial natriuretic peptide in hypoxia

A growing number of mammalian genes whose expression is inducible by hypoxia have been identified. Among them, atrial natriuretic peptide (ANP) synthesis and secretion is increased during hypoxic exposure and plays an important role in the normal adaptation to hypoxia and in the pathogenesis of cardiopulmonary diseases, including chronic hypoxia-induced pulmonary hypertension and vascular remodeling, and right ventricular hypertrophy and right heart failure. This review discusses the roles of ANP and its receptors in hypoxia-induced pulmonary hypertension. We and other investigators have demonstrated that ANP gene expression is enhanced by exposure to hypoxia and that the ANP so generated protects against the development of hypoxic pulmonary hypertension. Results also show that hypoxia directly stimulates ANP gene expression and ANP release in cardiac myocytes in vitro. Several cis-responsive elements of the ANP promoter are involved in the response to changes in oxygen tension. Further, the ANP clearance receptor NPR-C, but not the biological active NPR-A and NPR-B receptors, is downregulated in hypoxia adapted lung. Hypoxia-sensitive tyrosine kinase receptor-associated growth factors, including fibroblast growth factor (FGF) and platelet derived growth factor (PDGF)-BB, but not hypoxia per se, inhibit NPR-C gene expression in pulmonary arterial smooth muscle cells in vitro. The reductions in NPR-C in the hypoxic lung retard the clearance of ANP and allow more ANP to bind to biological active NPR-A and NPR-B in the pulmonary circulation, relaxing preconstricted pulmonary vessels, reducing pulmonary arterial pressure, and attenuating the development of hypoxia-induced pulmonary hypertension and vascular remodeling.

Plasma atrial natriuretic peptide in essential hypertension after treatment with terazocin

The aim of this study was to evaluate the medium term effects of the selective alpha1-adrenenergic- blocker terazocin on atrial natriuretic peptide (ANP) levels in patients with moderate essential hypertension. The drug was given orally for 30 days. The daily dose was 1 mg for the first 7 days, 2 mg for the next 7 days and 5 mg for the remaining period of this clinical trial. At the end of this clinical trial, plasma ANP levels increased by 16.40% 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 terazocin exerts part of its antihypertensive action by increasing ANP plasma levels.

Subtype-specific roles of cAMP phosphodiesterases in regulation of atrial natriuretic peptide release

cAMP is known to control the release of atrial natriuretic peptide. To define the roles of cyclic nucleotide phosphodiesterase subtypes in the regulation of atrial natriuretic peptide (ANP) release, experiments were done with perfused beating rabbit atria. Phosphodiesterase 3 subtype-specific inhibitors, milrinone and cilostamide, inhibited myocytic ANP release with a concomitant increase in cAMP efflux. Similarly, trequinsin, another phosphodiesterase 3 inhibitor, decreased ANP release. A phosphodiesterase 4 subtype-specific inhibitor, rolipram, did not significantly change ANP release but increased AMP efflux. Also, 4-[(3-butoxy-4-methoxyphenyl)methyl]-2-imidazolidinone (Ro 20-1724), another phosphodiesterase 4 inhibitor, did not significantly change ANP release. The cAMP efflux was higher in the atrium treated with rolipram than in the atrium treated with milrinone or cilostamide. The data show that the cAMP pool, which is metabolized by phosphodiesterase 3, but not phosphodiesterase 4, is closely related to the basal regulation of atrial ANP release. The results suggest that intracellular cAMP is compartmentalized in the regulation of atrial ANP release, and that the release is controlled by a phosphodiesterase subtype-specific mechanism.

Atrial natriuretic peptide contributes to the antihypertensive action of many drugs

Data derived from a 10 years research program of our team demonstrate that many categories of antihypertensive drugs like beta-adrenergic blockers, alpha1-adrenergic blockers, ACE inhibitors, AT1-receptor antagonists and calcium-entry blockers increase plasma atrial natriuretic peptide (ANP) levels after a medium-term treatment of patients suffering from moderate essential hypertension. ANP always increases despite the drop of the arterial pressure and the fact that the left atrial and ventricular diameters remain unchanged or slightly reduced. These findings indicate that the increase of ANP plasma levels is not the result of a mechanical overload in the left cardiac chambers but the result of a pharmacological action. In conclusion, ANP is a universal factor contributing to the antihypertensive action of many drugs.

The effect of pituitary adenylate cyclase activating polypeptide on cultured rat cardiocytes as a cardioprotective factor

In the cardiovascular system, pituitary adenylate cyclase activating polypeptide (PACAP) exhibits not only vasodilation but also positive inotropic action by increasing cardiac output. Then the effect of PACAP in cultured cardiovascular cells was examined. In neonatal rat myocytes, PACAP evoked concentration-dependent increase in intracellular cyclic AMP content more potently than vasoactive intestinal polypeptide (VIP). However, in neonatal rat nonmyocytes, PACAP and VIP showed equal potency. The characterization of the subtype of PACAP/VIP receptors by RT-PCR analysis revealed that PAC1 receptor mRNA is dominantly present in the myocytes, but VPAC2 receptor mRNA is abundant in the nonmyocytes. In the myocytes, PACAP did not change the protein synthesis stimulated by endothelin or by itself. However, PACAP moderately stimulated the secretion of atrial natriuretic polypeptide (ANP). On the other hand, PACAP inhibited the protein synthesis and DNA synthesis of the nonmyocytes. These indicate that PACAP might be involved in the regulation of cardiac hypertrophy and fibrosis as a cardioprotective factor.

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.

Reduced oxygen tension increases atrial natriuretic peptide release from atrial cardiocytes

To test the hypothesis that reduced oxygen tension stimulates cardiac atrial natriuretic peptide (ANP) secretion, we measured ANP release and expression in neonatal rat atrial and ventricular cardiac myocytes exposed to 45 min and 3, 6, and 24 hr of 3% or 21% oxygen. In atrial cardiocytes, the percentage of increase in culture media ANP concentration from baseline was greater in cells exposed to 3% than in cells exposed to 21% oxygen after 3 hr (814% +/- 52% vs. 567% +/- 33%, P < 0.05) and 6 hr of exposure (1639% +/- 91% vs. 1155% +/- 73%, P < 0.05). No differences in the percentage of increase in culture media ANP concentration was seen at 45 min (284% +/- 27% vs. 201% +/- 16%, P = NS) or 24 hr (2499% +/- 250% vs. 2426% +/- 195%). There was a significant increase in cellular ANP content between 3 and 24 hr in atrial cardiocytes exposed to 21% oxygen (105% +/- 40% vs. 296% +/- 60%, P < 0.05), but not in atrial cardiocytes exposed to 3% oxygen (118% +/- 20% vs. 180% +/- 26%, P = NS). Steady-state ANP mRNA levels in atrial cardiocytes were not affected by oxygen tension. In ventricular cardiocytes, oxygen tension did not affect ANP secretion, cellular ANP content, or steady-state ANP mRNA levels. We conclude that reduced oxygen tension increases release of ANP from atrial, but not ventricular cardiocytes and that this mechanism may contribute to the elevation in plasma ANP seen during acute hypoxia.

Role of cell contractions in cAMP-induced cardiomyocyte atrial natriuretic peptide release

Incubation of spontaneously beating ventricular cardiomyocytes from neonatal rats with prostaglandin E(2) (0.1 microM) or forskolin (0.1 microM) simultaneously increased the rate of cellular contraction and atrial natriuretic peptide (ANP) secretion. Both responses were maximal within 10-20 min of application and were accompanied by three- to fourfold increases in cAMP formation. By contrast, a higher regimen of forskolin (10 microM) promoted a 20- to 30-fold increase in basal cAMP production, which was accompanied by the abolition of contractile activity and ANP release. Low regimens of forskolin (0.1 microM) doubled the occurrence of cytosolic Ca(2+) transients associated with monolayer contraction, whereas higher regimens of forskolin (10 microM) completely suppressed Ca(2+) transients. Moreover, in quiescent cultures that were pretreated with ryanodine, tetrodotoxin, nifedipine, or butanedione monoxime, prostaglandin E(2) (0.1 microM) and forskolin (0.1 microM) failed to elicit significant ANP secretion, suggesting that cAMP-elevating agents promote ANP secretion to a great extent via an increase in cellular contraction frequency in ventricular cardiomyocytes.

Effects of adrenomedullin on cultured rat cardiac myocytes and fibroblasts

The direct effects of adrenomedullin, a novel vasorelaxant peptide, on protein synthesis and atrial natriuretic peptide release in myocytes and on DNA and collagen syntheses in fibroblasts were examined using cultured ventricular cardiocytes. The protein synthesis of cardiac myocytes was not affected by adrenomedullin under non-stimulated conditions. Endothelin-1-induced protein synthesis in myocytes was slightly but significantly elevated by adrenomedullin. Likewise, the secretion of atrial natriuretic peptide from myocytes stimulated by endothelin-1 was increased by adrenomedullin. In cardiac fibroblasts, adrenomedullin clearly inhibited DNA synthesis and collagen production in a dose-dependent manner under both basal and angiotensin II-stimulated conditions. DNA and collagen syntheses by cardiac fibroblasts were suppressed by both 8-bromo cAMP and forskolin. Furthermore, a cAMP-specific phosphodiesterase inhibitor decreased DNA and collagen syntheses in fibroblasts and enhanced the inhibitory effects of adrenomedullin on these syntheses. Our observations suggest that adrenomedullin has opposite effects on cultured cardiac myocytes and fibroblasts and that the effects of adrenomedullin at least on fibroblasts are probably mediated through a cAMP-dependent pathway. As adrenomedullin is produced and secreted from both types of cardiac cells, adrenomedullin may play a role as an autocrine/paracrine modulator in the process of cardiac remodeling, mainly by suppressing mitogenesis and collagen synthesis in fibroblasts.

Alpha1-adrenergic regulation of peptidylglycine alpha-amidating monooxygenase gene expression in cultured rat cardiac myocytes: transcriptional studies and messenger ribonucleic acid stability

Peptidylglycine alpha-amidating monooxygenase (PAM; EC 1.14.17.3) is a bifunctional protein containing two enzymes that act sequentially to catalyse the alpha-amidation of neuroendocrine peptides. Previous studies have demonstrated that alpha-adrenergic stimulation results in an increase in intracellular volume and protein content of cultured neonatal rat myocardial cells. The present study examined the regulated expression of PAM during alpha-adrenergic stimulation. Alpha1-adrenergic stimulation activates the expression and release of PAM from myocytes. Following phenylephrine treatment, myocardial cells displayed a several fold increase in PAM activity, and a 2-4-fold increase in the steady state levels of PAM mRNA. This effect of alpha-adrenergic stimulation was dependent on the concentration and duration of exposure to the agonist, and displayed alpha1-adrenergic receptor specificity. The transcription rate experiments indicated that these alpha-adrenergic effects were not due to increased PAM gene activity, suggesting that a post-transcriptional mechanism was involved. The most common mechanism of post-transcriptional regulation affects cytoplasmic mRNA stability. Cardiomyocytes cultures from atria and ventricles in the presence of 5,6 dichloro-1-beta ribofuranosyl benzamidazole (DRB) showed that phenylephrine treatment increased the half-life of PAM mRNA from 13 +/- 1 to 21 +/- 1 h in atrial cells and from 8 +/- 1 to 12 +/- 1 h in ventricle cells. Analysis of nuclear RNA with probes specific for PAM intron sequences shows that increased PAM expression after phenylephrine treatment was not due to intranuclear stabilisation of the primary transcript. Protein kinase C inhibitors H7 and GF109203x, completely blocked the phenylephrine stimulated PAM expression. These results suggest that alpha-adrenergic agonist induces PAM mRNA levels by increasing its stability in the cytoplasm. They indicate that PAM gene expression augments through a H7 and GF109203x sensitive pathway, involving the activation of protein kinase C.

Potentiation of stretch-induced atrial natriuretic peptide secretion by intracellular acidosis

We sought to investigate whether atrial myocyte contraction and secretion of the atrial natriuretic peptide (ANP) are affected in the same manner by intervention in intracellular Ca(2+) handling by acidosis. The effects of propionate (20 mM)-induced intracellular acidosis on the stretch-induced changes in ANP secretion, contraction force, and intracellular Ca(2+) concentration ([Ca(2+)](i)) were studied in the isolated rat atrium. The stretch of the atrium was produced by increasing the intra-atrial pressure of the paced and superfused preparation. Contraction force was estimated from pressure pulses generated by the contraction of the atrium. Intracellular Ca(2+) was measured from indo 1-AM-loaded atria, and ANP was measured by radioimmunoassay from the perfusate samples collected during interventions. Intracellular pH of the atrial myocytes was measured by a fluorescent indicator (BCECF)-based imaging system. Intracellular acidification caused by 20 mM propionic acid (0.18 pH units) potentiated the stretch-induced (intra-atrial pressure from 1 to 4 mmHg) ANP secretion, causing a twofold secretion compared with nonacidotic controls. Simultaneously, the responsiveness of the atrial contraction to stretch was reduced (P < 0.05, n = 7). Stretch augmented the systolic indo 1-AM transients in acidic (P < 0.05, n = 6) and nonacidic atria (P < 0.05, n = 6). However, during acidosis this was accompanied by an increase of the diastolic indo 1-AM ratio (P < 0.05, n = 6). Cooccurrence of stretch and acidosis caused an increase in systolic and diastolic [Ca(2+)](i) and potentiated the stretch-induced ANP secretion, whereas the contraction force and its stretch sensitivity were decreased. This mechanism may be involved in ischemia-induced ANP secretion, suggesting a role for ANP secretion as an indicator of contractile dysfunction.

Tissue distribution and localization of natriuretic peptide receptor subtypes in stroke-prone spontaneously hypertensive rats

OBJECTIVE

To investigate tissue distribution and localization of the natriuretic peptide receptor (NPR) subtypes' messenger RNA (mRNA) and to compare their expression between stroke-prone spontaneously hypertensive rats (SHR-SP) and Wistar-Kyoto (WKY) rats.

METHODS

Total RNA was extracted from organs of SHR-SP and WKY rats aged 13 weeks. The mRNA level was examined by RNase protection assay. The localization of the transcripts was determined by in-situ hybridization.

RESULTS

In SHR-SP aged 13 weeks, NPR-A was expressed most abundantly in the adrenal gland, lung and aorta, in that order. NPR-B was expressed highly in the uterus and ovary, and also in the lung, adrenal, and brain. NPR-C was expressed predominantly in the atrium and mesentery, less so in the lung, vein, and kidney. In the adrenal gland, NPR-A was expressed mainly in zona glomerulosa cells. In the atrium, NPR-C was expressed throughout the wall. In the mesentery, NPR-C mRNA was detected mainly in adipocytes. In the kidney, NPR-C was found predominantly in podocytes. Whereas the levels of expression of NPR subtypes in most tissues examined did not differ between SHR-SP and WKY rats, the NPR-C mRNA level was significantly greater in the kidneys of SHR-SP than it was in those of WKY rats.

CONCLUSIONS

These results indicated that each NPR subtype had a distinct tissue distribution pattern and that the expression of NPR-C in the kidneys of SHR-SP was greater than that in the kidneys of WKY rats.

Adrenomedullin stimulates cAMP accumulation and inhibits atrial natriuretic peptide gene expression in cardiomyocytes

Adrenomedullin (ADM) is a novel vasodilating and natriuretic peptide which may play an important role in cardiovascular regulation. In neonatal cardiomyocyte cultures we have shown that ADM leads to dose-dependent inceases in cAMP accumulation and subsequent inhibition of atrial natriuretic peptide (ANP) gene expression and secretion. Forskolin-mediated elevation of intracellular cAMP levels led to a qualitatively similar inhibitory effect on both ANP gene expression and secretion. These data show that ADM has direct effects on expression of ANP in the cardiomyocyte by a mechanism that may involve the activation of adenylate cyclase, lending further support to the hypothesis that ADM may act in vivo as an important endocrine or paracrine modulator of cardiovascular function.

Localization of alpha 1-adrenoceptors in rat and human hearts by immunocytochemistry

The localization of the alpha 1 adrenoceptors (alpha 1-AR) in the heart tissues from rat and human and in the cultured heart cells from neonatal rats was studied by indirect immunofluorescence and postembedding electronmicroscopical immuno-gold technique. With antipeptide antibodies directed against the second extracellular loop of the human alpha 1-AR (AS sequence 192-218), this receptor was found to be localized along the sarcolemma in both human and rat hearts. Similar localization sites were detected in cultivated rat neonatal cardiomyocytes. Beside the localization in cardiomyocytes, alpha 1-AR were identified in endothelial cells of capillaries and smooth muscle cells of coronary vessels, in neuronal endings, in mast cells of cultivated heart cells but not, or in less amount in fibroblasts. Interestingly, in the right atrium of rat heart the localization of alpha 1-AR was found to be near or on atrial natriuretic factor (ANF) granules, providing the basis for the alpha-adrenergic influence on ANF release. The immunocytochemical studies further confirm and complete the findings known by using autoradiographic binding studies with specific ligands.

Effect of neuromimetics upon the release of atrial natriuretic peptide granules: are multiple pathways involved in secretion?

The release of atrial natriuretic peptide (ANP) in response to the application of neurohumoral agonists (neuromimetics) is directly demonstrated and quantified at the cellular level, using an ultrastructural assay developed to quantify secretion. The assay uses an in situ tannic acid perfusion technique to arrest the exocytosis of atrial secretory granules in the anesthetized rat. The animal is perfused with the neuromimetic, and secretory granules, which retain the capacity to undergo exocytosis throughout the subsequent 30 min tannic acid perfusion, accumulate at the cell surface in a state of fusion with the plasma membrane. Quantification of arrested granules thus provides a measure of the rate of granule release and allows the responses to different agents to be assessed. The actions of three different agents were investigated: isoproterenol, phenylephrine, and acetylcholine. In previously published studies, investigations of the actions of these agents on ANP release has produced unclear and sometimes contradictory results. Using our ultrastructural assay, it was found that during the 30 min perfusion period neither isoprenaline nor phenylephrine caused a significant change in the rate of secretory granule release, whereas acetylcholine significantly decreased the rate of granule release. A new model of secretion is proposed to integrate these findings with previous results and help clarify the complex picture of atrial natriuretic peptide release.

Effects of ANP receptor antagonists on ANP secretion from adult rat cultured atrial myocytes

Atrial natriuretic peptide (ANP) is a hormone-secreted predominantly by atrial myocytes. ANP exerts many of its actions via activation of the particulate guanylyl cyclase receptor ANPR-A and the formation of guanosine 3',5'-cyclic monophosphate (cGMP), which serves as a second messenger in the target cells. Using membrane-permeable cGMP analogues (8-bromo-cGMP and dibutyryl- cGMP), we first tested the hypothesis that ANP secretion by adult rat cultured atrial myocytes can be modulated through the second messenger cGMP. Second, we examined the effects of two competitive ANPR-A receptor antagonists, namely HS-142-1 and anantin, on cGMP formation and ANP secretion from cultured atrial myocytes. Cultured atrial myocytes secreted large quantities of immunoreactive (ir) ANP under basal conditions. We found that cGMP analogues inhibited basal irANP secretion from cultured atrial myocytes, whereas HS-142-1 and anantin had stimulating effects. HS-142-1 and anantin reduced cGMP formation in cultured atrial myocytes at basal conditions. These results suggest an autoregulatory mechanism of ANP secretion by atrial myocytes in an autocrine/paracrine fashion.

Calcium influx in platelet activating factor-induced atrial natriuretic peptide release in rat cardiomyocytes

Atrial natriuretic peptide (ANP) is released from the myocardium after the activation of protein kinase C and/or ischemia, events that are associated with an increase in platelet activating factor (PAF) production in this tissue. In this study we demonstrate that PAF, but not lyso-PAF, induces a concentration-dependent increase in ANP secretion in spontaneously beating neonatal rat cardiomyocytes, a response associated with increases in cellular adenosine 3',5'-cyclic monophosphate (cAMP) formation, calcium influx, and the mobilization of calcium from intracellular stores. cAMP formation and calcium influx appear to play major roles in PAF-induced ANP secretion in this system, insofar as PAF-induced ANP release was substantially reduced in the presence of the (R)-p-diastereoisomer of adenosine 3',5'-cyclic monophosphorothioate (10 microM), whereas both PAF-induced calcium influx and ANP secretion were abolished in the presence of the calcium channel antagonist nifedipine (0.1 microM). Consistent with these results, N6-2'-O-dibutyryl cAMP (DBcAMP, 10 microM) and/or forskolin (0.1 microM) simultaneously increased cAMP production, calcium influx, and ANP release in these cells, with both DBcAMP- and forskolin-induced ANP secretion being fully abolished in the presence of 0.1 microM nifedipine. Taken together, these results suggest that PAF, DBcAMP, and forskolin promote ANP secretion in spontaneously beating cardiomyocytes via the activation of a cAMP-dependent, nifedipine-sensitive myocardial calcium channel and that calcium influx is a major requirement for cAMP-induced ANP secretion in this system.

Regulation of hypertrophy and atrophy in cultured adult heart cells

Mechanical loading and alpha-adrenergic receptor stimulation have both been shown to induce hypertrophy in isolated neonatal heart cells. The present study examined the effects of adrenergic hormones and contractile activity on the hypertrophic response in isolated adult feline cardiomyocytes maintained for more than 14 days in insulin- and serum-supplemented medium. Measurements of the hypertrophic response included cell size, total protein content, myosin heavy chain content, and the time course of activation of increased protein synthesis. Reactivation of the "fetal" gene program was evaluated by secretion of atrial natriuretic factor (ANF) into the medium. Significant myocyte hypertrophy was induced in both quiescent myocytes treated with alpha 1-adrenergic agonists and in beating myocytes treated with beta-adrenergic agonists. However, there were both quantitative and qualitative differences in the response to each type of stimulation. alpha-Adrenergic agonists promoted an increase in cell size, protein content, and ANF secretion but not myofibrillar reorganization, which was observed only in beating myocytes. In contrast to results reported for neonatal heart cells, determinants of hypertrophy in beating myocytes exceeded those in nonbeating alpha 1-adrenergic agonist-treated heart cells in every parameter examined. In addition, in the case of both beating and alpha-adrenergic stimulation, there were marked time-dependent variations in rates of protein synthesis over the interval of 4 hours to 7 days of treatment with each type of stimulus. Differences were also encountered in correlations between rates of protein synthesis and protein accumulation over this interval. The effect of beating was particularly important both to the reorganization of myofibrillar structure and the metabolism of myosin heavy chain. In cultures in which beating was inhibited with the calcium channel antagonist nifedipine, the loss of myosin heavy chain was significantly greater than that of total protein.

Release of atrial natriuretic factor

The cardiac atria synthesize and store a hormone termed atrial natriuretic factor (ANF). ANF is released into the systemic circulation, and the circulating 28 amino acid peptide can be measured by radioimmunoassay. The hormone participates in body fluid homeostasis through its effect on renal sodium excretion and by inducing a shift of circulating fluid to the interstitial space. Release of ANF is mainly regulated by mechanical changes in the left and right atrial wall. It has been demonstrated that ANF release is related to changes in atrial wall tension occurring during each atrial cycle, and therefore, release of ANF will increase with increasing heart rate. Not only the increase in wall tension during passive atrial distension (v wave), but also the increase in tension during atrial systole (a wave) are determinants of ANF release. The mechanochemical transducer is most likely located in the atrial myocytes, but its nature is unknown. There is no evidence to suggest that efferent cardiac nerves are essential in the regulation of ANF release. Humoral factors have been suggested as regulators of ANF release, particularly catecholamines and angiotensin II. A receptor-mediated direct stimulatory effect of alpha-adrenergic stimulation and an inhibitory effect of beta-adrenergic stimulation have been demonstrated, but these direct effects are small compared to the effect of changes in atrial wall tension. Circulating catecholamines and angiotensin II stimulate ANF release mainly through their haemodynamic effects.

Cellular mechanisms for synthesis and secretion of atrial natriuretic peptide and brain natriuretic peptide in cultured rat atrial cells

To investigate the cellular mechanism for the synthesis and secretion of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), we examined the effects of vasoactive agents on the secretion rates and gene expression of ANP and BNP in cultured rat atrial cells. Endothelin (10(-7) M, +61%), 12-O-tetradecanoylphorbol 13-acetate (TPA, 10(-6) M, +62%), the calcium ionophore A23187 (10(-6) M, +95%), and Bay K 8644 (10(-6) M, +34%) (p < 0.05 each) all increased the secretion of ANP into the culture media in a dose-dependent fashion. On the other hand, endothelin (10(-7) M, +57%) and TPA (10(-6) M, +55%) (p < 0.01 each) increased the secretion of BNP in a dose-dependent manner, whereas A23187 (10(-6) M, -45%, p < 0.001) suppressed the secretion of BNP in a dose-dependent manner, and Bay K 8644 caused no significant effects on BNP secretion. The molecular forms of intracellular ANP were exclusively gamma-ANP, whereas those of BNP were gamma-BNP and its carboxy terminal 45-amino-acid peptide, BNP-45. The ratio of media to cell contents was much higher in BNP than in ANP. Northern blot analysis revealed that both ANP mRNA and BNP mRNA levels were significantly increased by 10(-7) M endothelin (ANP mRNA, +52%; BNP mRNA, +36%; p < 0.05 each) and 5 x 10(-5) M 1-oleoyl-2-acetylglycerol (ANP mRNA, +296%; BNP mRNA, +133%; p < 0.01 each) but not by 10(-6) M A23187. Thus, the secretion of ANP is stimulated by both the elevation of [Ca2+]i and the activation of protein kinase C, whereas its synthesis is increased mainly by the activation of protein kinase C. The synthesis and secretion of BNP are augmented by the activation of protein kinase C rather than the elevation of [Ca2+]i. Furthermore, the processing and secretion of ANP and BNP may be regulated in different manners.

Secretion of immunoreactive ANF from FRTL-5 rat thyroid cells: regulation by calcium ionophore A23187

Atrial natriuretic factor-like immunoreactivity (ir-ANF) was characterized in a continuous line of rat thyroid follicular cells (FRTL-5) and the influence of the calcium ionophore A23187 on ir-ANF secretion was examined. Ir-ANF was identified by immunohistochemical staining as primarily reticular and juxtanuclear in short-term cultures, and more peripheral and granular in longer-term cultures, suggesting a process of ir-ANF packaging into secretory granules. The accumulation of ir-ANF granules was dependent upon the presence of thyrotropin (TSH) in the medium. Secreted ir-ANF was characterized using reversed-phase, high-performance liquid chromatography (RP-HPLC) and radioimmunoassay as a single peak eluting one fraction earlier than 125I-labeled rat ANF (99-126) (i.e., circulating atrial ANF) included as an internal standard. A23187 treatment of cells exhibiting primarily reticular ir-ANF caused a change to a pattern of more distinct, peripherally localized granules. This change occurred within 1 h after A23187 treatment and was dependent on the presence of Ca2+ in the medium. In cultures containing primarily ir-ANF granules, A23187 (0.5 micrograms/ml) induced a peripheral translocation of the granules at 30 min and a complete degranulation by 7 h. Enzyme-linked immunoadsorbent assay (EIA) confirmed a dose-dependent effect of A23187 on ir-ANF release into the medium. These results suggest that some of the effects of Ca2+ in the thyroid could be ascribed to its mobilization and release of ir-ANF, which in turn may have autocrine effects on thyroid follicular cells.

Stretch-dependent regulation of atrial peptide synthesis and secretion in cultured atrial cardiocytes

We have developed a novel system to study stretch-dependent secretion of atrial natriuretic peptide (ANP) using cultured neonatal rat atriocytes in vitro. Application of tension (i.e., 2 sequential stretches) to cells grown on a flexible culture surface effected a dose-dependent increase in immunoreactive (ir) ANP release into the medium. Analysis of atriocyte cytoplasmic RNA 24 h poststretch revealed an increase in ANP mRNA levels of about ninefold relative to the unstretched controls. Medium ATP levels, measured as an index of cellular damage, were similar in control and stretched cells. Furthermore, cooling the cultures to 0 degrees C suppressed both basal as well as stretch-stimulated release. These findings argue against cellular damage and nonspecific release of irANP as an explanation for the increase in medium immunoreactivity. Stretch was incapable of amplifying the secretory response to prostaglandin F2 alpha, suggesting possible overlap in the pathways whereby these stimuli effect release of the peptide. The calcium channel blocker verapamil had no effect on stretch-dependent irANP release, whereas calmidzolium, a calmodulin inhibitor, suppressed basal as well as stretch-dependent secretion, implying a potentially important relationship between intracellular calcium metabolism and irANP release.

Passive mechanical stretch releases atrial natriuretic peptide from rat ventricular myocardium

Ventricular hypertrophy is characterized by augmentation of synthesis, storage, and release of atrial natriuretic peptide (ANP) from ventricular tissue, but the physiological stimulus for ANP release from ventricles is not known. We determined the effect of graded, passive myocardial stretch on ANP release in isolated, arrested, perfused heart preparations after removal of the atria in 13-20-month-old Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). By this age, ANP gene expression was increased in the hypertrophic ventricular cells of SHR, as reflected by elevated levels of immunoreactive ANP and ANP mRNA and the increased ANP secretion (SHR, 93 +/- 14 pg/ml, n = 22; WKY rats, 22 +/- 2 pg/ml, n = 20; p less than 0.001) from perfused ventricles after removal of the atria. The release of ANP from ventricles was examined at two levels of left ventricular pressure by increasing the volume of the intraventricular balloon for 10 minutes. Stretching of the ventricles produced a rapid but transient increase in ANP secretion. As left ventricular pressure rose from 0 to 14 and 26 mm Hg in WKY rats and from 0 to 13 and 27 mm Hg in SHR, increases in ANP release into the perfusate of 1.4 +/- 0.1-fold and 1.5 +/- 0.2-fold (p less than 0.05) in WKY rats and 1.1 +/- 0.1-fold and 1.6 +/- 0.2-fold (p less than 0.05) in SHR, respectively, were observed. There was a highly significant correlation between the left ventricular pressure level and the maximal concentration of ANP in the perfusate during stretching (p less than 0.001, r = 0.59, n = 42), as well as between the maximal ANP concentrations in perfusate during stretching and the ventricular weight/body weight ratios of the corresponding animals (r = 0.38, p less than 0.05, n = 42). High performance liquid chromatographic analysis revealed that the ventricles both before and during stretch primarily released the processed, active, 28-amino acid ANP-like peptide into the perfusate. These results indicate that stretching is a direct stimulus for ventricular ANP release and show that ANP is also a ventricular hormone.

Gene expression and atrial natriuretic factor processing and secretion in cultured AT-1 cardiac myocytes

BACKGROUND

Studies were carried out to characterize several biochemical features of cultured AT-1 cells.

METHODS AND RESULTS

These cells were obtained from a transplantable atrial cardiomyocyte tumor lineage. Reverse transcriptase-polymerase chain reaction-based analyses demonstrated that the pattern of gene expression of cultured AT-1 cells was similar to that of adult atrial myocytes. AT-1 cells expressed atrial natriuretic factor (ANF), alpha-cardiac myosin heavy chain, alpha-cardiac actin, and connexin43. Radioimmunoassays verified that the cells synthesized, stored, and secreted ANF. Through size-exclusion, reversed-phase, and carboxymethyl-ion-exchange high-performance liquid chromatography, it was shown that cultured AT-1 cells stored ANF as pro-ANF (ANF-[1-126]), which was cosecretionally processed quantitatively to ANF-(1-98) and the bioactive 28-amino-acid ANF-(99-126). In addition, cultured AT-1 cells secreted ANF at almost a sixfold greater rate in response to endothelin-1, a potent secretagogue of ANF. KCl, metenkephalinamide, isoproterenol, phenylephrine, and 12-O-tetradecanoyl-phorbol-13-acetate also stimulated ANF release.

CONCLUSIONS

These studies, in combination with previous findings, demonstrated that cultured AT-1 cells, while maintaining the ability to proliferate, have retained functional, biochemical, and ultrastructural features that are characteristic of adult atrial myocytes.

Release of atrial natriuretic factor during selective cardiac alpha- and beta-adrenergic stimulation, intracoronary Ca2+ infusion, and aortic constriction in pigs

The effects of alpha- and beta-adrenergic stimulation on release of atrial natriuretic factor (ANF) were examined in seven anesthetized, open-chest pigs. The alpha-adrenergic agonist phenylephrine (28.0 micrograms/min) and the beta-adrenergic agonist isoproterenol (0.3 micrograms/min) were infused into the proximal part of the circumflex coronary artery to stimulate the left atrial adrenoceptors without concomitant changes in left and right atrial filling pressures (v wave). Isoproterenol reduced plasma immunoreactive ANF (irANF) by 15 +/- 7 pg/ml (20%) from 76 +/- 10 pg/ml despite a rise in left atrial systolic pressure (a wave). A comparable rise in left atrial systolic pressure, induced by intracoronary infusion of calcium chloride (8.0 mg/min), increased plasma irANF by 33 +/- 10 pg/ml (53%) from 62 +/- 7 pg/ml. Phenylephrine increased plasma irANF by 9 +/- 4 pg/ml (14%) from 66 +/- 10 pg/ml without altering right and left atrial pressures. A rise in left atrial filling pressure of 3.2 +/- 0.5 mm Hg, induced by constricting the ascending aorta, increased plasma irANF by 83 +/- 35 pg/ml (141%) from 59 +/- 11 pg/ml. This increase was nine times that during phenylephrine infusion. In conclusion, alpha-adrenergic stimulation increases and beta-adrenergic stimulation inhibits ANF release by a direct action on the atrial myocytes. The direct effects of alpha- and beta-adrenergic stimulation on ANF release in vivo are small compared with the effect of a moderate rise in atrial filling pressure.

Effect of phorbol ester on the release of atrial natriuretic peptide from the hypertrophied rat myocardium

1. To determine the cellular mechanisms of atrial natriuretic peptide (ANP) release from ventricular cardiomyocytes, the secretory and the cardiac effects of a phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), known to stimulate protein kinase C activity in heart cells, were studied in isolated, perfused heart preparations from 2- and 21-month-old Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. TPA was added to the perfusion fluid for 30 min at a concentration of 46 nM after removal of atrial tissue. Additionally, atrial and ventricular levels of immunoreactive ANP (IR-ANP) and ANP mRNA, the distribution of ANP within ventricles as well as the relative contribution of atria and ventricles in the release of ANP were studied. 2. Ventricular hypertrophy that gradually developed in hypertensive rats resulted in remarkable augmentation of ANP gene expression, as reflected by elevated levels of immunoreactive ANP and ANP mRNA. The total amount of IR-ANP in the ventricles of the SHR rats increased 41 fold and ANP mRNA levels 12.9 fold from the age of 2 to 21 months. At the age of 21 months, levels of IR-ANP and ANP mRNA in the ventricles of SHR rats were 5.4 fold and 3.7 fold higher, respectively, than in the normotensive WKY rats. Immunohistochemical studies demonstrated ANP granules within the hypertrophic ventricles of the old SHR rats, but not within normal ventricular tissue. 3. In isolated perfused heart preparations, the severely hypertrophied ventricular tissue of SHR rats after atrialectomy secreted more ANP into the perfusate than did the control hearts. Interestingly, the ANP release from the hypertrophied ventricles of the old SHR rats increased considerably (from 413 + 30 to the maximum of 623 + 75 pgml-1, F = 10.8, P < 0.001, two-way analysis of variance), whereas only a small increase was seen in old WKY rats and no effect was observed in young animals of either strain. When intact rat hearts (without atrialectomy) were used, infusion of phorbol ester also increased the ANP secretion into the perfusate in young animals. 4. Our present results indicate that the phorbol ester TPA increases the release of ANP from the hypertrophied, but not from normal rat myocardium. Thus, hypertrophied rat ventricular myocytes appear to possess the cellular mechanisms necessary to secrete ANP by a regulated pathway. The results further suggest that protein kinase C activity may be involved in the the regulation of ANP secretion from ventricular cells, as has been shown earlier for atrial myocytes.

Cytosolic Ca2+ during atrial natriuretic peptide secretion from cultured neonatal cardiomyocytes

Mechanisms of atrial natriuretic peptide (ANP) release were studied in neonatal rat heart atrial and ventricular myocytes cultured on Cytodex 3 microcarriers. For simultaneous observations of cytosolic free calcium concentration ([Ca2+]f) and ANP secretion, the culture was packed in a chromatography column, inserted into the cell holder of a spectrofluorometer was perifused with a buffer solution. [Ca2+]f was measured by the fluorescent calcium indicator Fura-2 and ANP in the effluent perfusate by radioimmunoassay. No cell damage was observed and the basal ANP secretion rate and [Ca2+]f were comparable with values obtained by other methods. K(+)-induced depolarization raised [Ca2+]f by 50%, but it rapidly declined again to a steady level 10-20% above the baseline. The calcium channel agonist Bay k8644 elicited a similar temporal pattern of [Ca2+]f changes and 1 microM ionomycin induced a 100-fold increase in [Ca2+]f with a slow re-establishment of the original baseline. None of these stimuli increased the ANP secretion rate of the atrial or ventricular myocytes. Protein kinase C activation by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulated ANP secretion from the atrial myocytes, while the ventricular myocytes were unresponsive to TPA. It is concluded that Ca2+ is not the main mediator in the regulation of ANP release in cultured neonatal heart cells.

Extracellular calcium regulates expression of the gene for atrial natriuretic factor

Primary cultures of neonatal rat cardiocytes were exposed for 24 hours to culture media containing 0-2.0 mM extracellular calcium. Both atrial natriuretic factor (ANF) messenger RNA (mRNA) and ANF secretion were increased approximately threefold in the presence of 2.0 mM CaCl2 (vs. Ca2(+)-free medium). When cardiocytes were treated with the calcium channel blockers diltiazem, nifedipine, or verapamil, both ANF synthesis and secretion fell to 25-40% of control values. The choice of transcription start site on the ANF gene was not altered by the calcium channel blockers. When exogenous calcium was added to cardiocytes treated with verapamil, secretion of ANF was partially restored to control levels. High-performance liquid chromatographic analysis of medium from cardiocytes exposed to varying extracellular calcium concentrations or treated with calcium channel blockers for 24 hours revealed that the majority of secreted immunoreactivity (60-70%) migrated with pro-ANF (17 kDa) and that none of the various experimental manipulations significantly changed the ratio of pro-ANF to ANF in the media. These results were confirmed by immunoprecipitation analysis of the culture medium from the individual treatment groups. Treatment of cardiocytes for 24 hours with either the calcium ionophore A23187 or the phorbol ester 12-O-tetradecanoylphorbol 13-acetate increased ANF secretion. The combined use of these agents resulted in stimulation of both ANF secretion and ANF mRNA accumulation.