Effects of arginine vasopressin and extracellular osmolarity on atrial natriuretic peptide release by superfused rat atria

Scorpion toxin of Androctonus australis garzonii induces neuropeptide Y release via bradykinin stimulation in rat atria and kidneys

The ability of scorpion toxins to produce hemodynamic alterations is well documented but all mediators implied in cardiovascular disturbances are not known. In the present investigation we studied the effect of North African Androctonus australis garzonii scorpion toxin on neuropeptide Y (NPY) release from rat atria and kidneys by a perifusion system in vitro. To further understand the mechanisms of the scorpion toxin action on NPY release, the effects of icatibant (HOE 140, a selective bradykinin-B2 receptor antagonist), tetrodotoxin (TTX, a sodium channel antagonist) and diltiazem (a calcium channel antagonist), and the effect of the scorpion toxin on bradykinin (BK, a potent vasorelaxant peptide of the kinin group) release were studied in both tissues. We showed that the scorpion toxin (10(-6)M) increased the NPY release from both atria (35%) and kidneys (40%). This increase was significantly (p<0.001) inhibited by HOE 140 (10(-5)M). The scorpion toxin (10(-6)M) enhanced BK secretion in both atria (52%) and kidneys (55%). Diltiazem (10(-5)M) and TTX (10(-5)M) decreased by 45-75% NPY levels induced by scorpion toxin in both organs. The results show that A. australis garzonii scorpion toxin stimulates NPY release from both rat atria and kidneys, and suggest that the toxin induces NPY release via BK stimulation through B2 receptors. This effect appears to involve calcium and sodium channel activation.

Aging and photoperiod affect the daily rhythm pattern of atrial natriuretic peptide in the rat atrium

This study investigates the release characteristics of atrial natriuretic peptide (ANP) from young (10 weeks) and old (22 months) rat atrium. Levels of ANP release from samples of atrium were studied by organ perifusion. Rats were exposed to light:dark (LD) cycles of 12:12 or 18:6 and sacrificed at different zeitgeber time (ZT) points: ZT0, ZT6, ZT8, ZT12, ZT16, and ZT19 for LD 12:12 or ZT0, ZT9, ZT16, ZT18, ZT20, and ZT 21.5 for LD 18:6. The heart was collected, and the right atrium was removed, weighed, and perifused with Krebs-bicarbonate buffer for 100 min, including a period of 50 min for stabilization of secretion rate. ANP concentrations released by atrium did not differ between the two age groups either under LD 12:12 or under LD 18:6, except at the light:dark transition under LD 12:12 conditions where ANP levels were significantly (P<0.05) lower in young compared to old rats. ANP exhibited daily variations in concentrations under LD 12:12, with a peak during the beginning of photophase (ZT0) in young rats and a peak at the beginning of scotophase (ZT12) in old animals. These variations were strongly modified under LD 18:6, where the pattern of the release exhibited a peak during the light phase at ZT16 in both young and old rats. This strongly suggests that the atrial ANP rhythm is dependent on the environmental light:dark cycle. Moreover, the total ANP levels released by atria in old rats were significantly increased under LD 18:6 compared to standard LD 12:12. This observation strongly suggests that old animals are more sensitive to a photoperiodic change. In conclusion, our results show that ANP concentrations in the rat atrium exhibit daily variations which are significantly affected by the daylength (photoperiod) change in aged rats.

Neurohormonal activation in severe scorpion envenomation: correlation with hemodynamics and circulating toxin

We studied the effects of scorpion (Androctonus australis hector) venom on hemodynamics and on the release of catecholamines, neuropeptide Y (NPY), endothelin-1 (ET-1) and atrial natriuretic peptide (ANP) in dog model of severe scorpion envenomation. Nine mongrel anesthetized dogs were submitted to mechanical ventilation through intubation and were administered intravenously purified dried scorpion venom (Androctonus autstralis) 0.05 mg/kg. Measurements including pulmonary artery catheter derived parameters, serum toxin levels and humoral variables were performed at baseline (before venom injection) and 5, 15, 30 and 60 min after venom injection. Humoral variables included: serum lactate, epinephrine (EP), norepinephrine (NE), NPY, ET-1 and ANP plasma concentrations. Scorpion venom caused rapid and transient increase of mean arterial pressure (MAP) and PAOP associated with a marked and sustained decline in cardiac output (-55% at 60 min; P < 0.001). Hemodynamic changes were associated with a rapid and significant increase of all measured hormones. The highest increase was for NE (28-fold) and EP (25-fold). MAP was closely correlated with NE and less significantly correlated with toxin levels. Similarly, significant correlation was observed between PAPO and ANP plasma levels. These findings support the implication of excessive catecholamines release in hemodynamic disturbances of severe SE and suggest that NPY and ET-1 could be involved in this process. Serum toxin does not appear to consistently contribute to these effects. Through its correlation with PAOP, ANP could be a reliable and useful marker of cardiac dysfunction in SE.

Endocrinologic response to vasopressin infusion in advanced vasodilatory shock

OBJECTIVES

To evaluate the endocrinologic response to a combined arginine vasopressin and norepinephrine (AVP/NE) infusion in advanced vasodilatory shock, and to examine the relationship between baseline plasma AVP concentrations and the hemodynamic response to AVP.

DESIGN

Preliminary, prospective, randomized, controlled clinical study.

SETTING

Twenty-three-bed general and surgical intensive care unit.

PATIENTS

Thirty-eight patients with advanced vasodilatory shock. Hemodynamic and laboratory data of 34 patients have already been presented in a recently published prospective, randomized, controlled study.

INTERVENTIONS

Continuous AVP (4 units/hr) and NE infusion in study patients; NE infusion only in control patients.

MEASUREMENTS AND MAIN RESULTS

At baseline, 24 hrs, and 48 hrs after randomization, plasma concentrations of AVP, adrenocorticotropic hormone, cortisol, renin, angiotensin II, aldosterone, prolactin, endothelin I, and atrial natriuretic factor were determined. Hemodynamic variables were recorded at baseline and 1, 12, and 24 hrs after randomization. Linear mixed effects models were used to test for differences between groups. The relationship between AVP plasma concentrations and hemodynamic response to AVP was analyzed using linear regression analyses. AVP/NE patients exhibited significantly higher AVP (p <.001) and prolactin (p <.001) plasma concentrations during the study period; there were no significant differences in plasma concentrations of other hormones. No significant correlation was detected between plasma AVP concentrations and the increase in mean arterial pressure after 1 hr (Pearson's correlation coefficient =.134, p =.584) and after 24 hrs (Pearson's correlation coefficient = -.198, p =.417). There were further no correlations between AVP plasma concentrations and the 24-hr response to AVP therapy in heart rate (Pearson's correlation coefficient = -.065, p =.791), stroke volume index (Pearson's correlation coefficient = -.106, p =.687), and NE requirements (Pearson's correlation coefficient =.04, p =.869).

CONCLUSIONS

The preliminary results of this study indicate that a combined AVP and NE infusion increases prolactin plasma concentrations in advanced vasodilatory shock. Hemodynamic effects of AVP infusion are independent of baseline plasma AVP concentrations.

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.

Daily variation in the concentration of neuropeptide Y in the rat atrium: effects of age and photoperiodic conditions

This study investigates the release characteristics of neuropeptide Y (NPY) from young (10 weeks) and old (22 months) rat atrium. Levels of NPY release from samples of atrium were studied by organ perifusion. Rats were exposed to light:dark (LD) cycles of 12:12 or 18:6 and sacrificed at different circadian stages: 0, 4, 7, 12, 18, and 20 h after dark onset (HADO) for LD 12:12 or 0, 2, 3.5, 6, 15, and 22 HADO for LD 18:6. The heart was collected, and the right atrium was removed, weighed, and perifused with Krebs-bicarbonate buffer for 100 min, including a period of 50 min for stabilization of secretion rate. NPY concentrations released by atrium did not differ between the two age groups. NPY exhibited daily variations in concentrations in LD 12:12, with a peak during the end of scotophase, at 12 HADO, in both the young and old rats. These variations were strongly modified in LD 18:6, where the pattern of the release exhibited two peaks occurring during the two thirds of dark (3.5 HADO) and light (22 HADO) periods. This strongly suggests that the NPY rhythm is dependent on the environmental light:dark cycle. In this paper we show that NPY concentrations in the rat atrium exhibit daily variations, which are maintained with ageing. Moreover, photoperiod greatly influences NPY levels in the atrium.

A canine study of immunotherapy in scorpion envenomation

OBJECTIVE

To evaluate the effects of scorpion venom and antivenom in experimental envenomation.

DESIGN

Prospective, controlled animal study.

SETTING

University research laboratory

SUBJECTS

Twenty-nine anesthetized and ventilated dogs.

INTERVENTIONS

The first group of animals had venom alone (0.05 mg/kg). Animals from the second group had simultaneous administration of 10 ml of scorpion antivenom (SAV). In the third and fourth groups, 10 ml and 40 ml SAV, respectively, were injected 10 min following venom.

MEASUREMENTS AND RESULTS

Hemodynamic parameters using right heart catheter were recorded and dosage of catecholamines, neuropeptide Y (NPY), endothelin-1, and atrial natriuretic peptide (ANP) were performed at baseline and during 60 min following envenomation. In the control group, at 5 min, there was a sharp increase in pulmonary artery occluded pressure (PAOP, from 2 mmHg to 23 mmHg), mean arterial pressure (MAP, from 125 mmHg to 212 mmHg) and systemic vascular resistance (SVR, from 2450 dyn sec(-1 )m(5) to 5775 dyn sec(-1 )m(5), P<0.05 for all). Heart rate, cardiac output, and stroke volume decreased. There was a 40-fold increase in epinephrine and norepinephrine plasma concentrations. Circulating NPY and ANP dosages increased too. PAOP and MAP decreased thereafter to reach baseline levels. Simultaneous administration of SAV with venom totally offset the hallmarks of scorpion envenomation. Delayed administration of SAV at any dosage failed to alter the features of scorpion envenomation.

CONCLUSION

While simultaneous administration of SAV and scorpion venom is effective in preventing scorpion envenomation-related manifestations, delayed administration of SAV, either at standard or elevated dosages, failed to alter any of the scorpion envenomation features.

Effect of nitric oxide on basal and stretch-induced release of atrial natriuretic factor (ANF) from isolated perfused rat atria

We investigated the effect of the NO donor SNAP (6.7 nM) on basal and stretch-induced ANF release from isolated perfused rat atria. There was no significant difference in basal ANF secretion between the vehicle- and SNAP-infused atria (SNAP: 388+/-63 pg. 100 microl(-1), n = 13 vs. vehicle: 349+/-26 pg. 100 microl(-1), n = 5). Atrial distention caused an increase in ANF secretion in both the buffer- and SNAP-treated groups. SNAP greatly attenuated the stretch-induced increase in ANF (SNAP: 225+/-7 pg. 100 microl(-1), n = 5 vs. vehicle: 448+/-72 pg. 100 microl(-1), n = 13, P < 0.05). The compliance of atria treated with SNAP was lower than that of the vehicle-perfused atria (P < 0.05). Thus, although SNAP appeared to attenuate stretch-induced ANF secretion, there was in fact no significant difference in the ratio of Delta[ANF] to Deltaintraluminal volume (SNAP: 5.8+/-1.3 pg. 100 microl(-1). microl(-1) vs. vehicle: 8.2+/-1.4 pg. 100 microl(-1). microl(-1).). In conclusion, we found no evidence that NO alters the control of basal or stretch-induced ANF secretion. NO can however reduce ANF release by shifting the pressure-volume curve, so that a given increase in atrial pressure is associated with a smaller increase in intraluminal volume and reduced atrial distention.

Role of nitric oxide on atrial natriuretic peptide release induced by angiotensin II in superfused rat atrial tissue

The present study investigated the role of nitric oxide (NO) on atrial natriuretic peptide (ANP) release stimulated by angiotensin II (Ang II) (10(-7) M) in superfused sliced rat atrial tissue. The use of N(G)-nitro-L-arginine methyl ester (L-NAME) at 10(-4) M, an inhibitor of nitric oxide synthase did not modify basal ANP release. In presence of Ang II (10(-7) M), we observed that L-NAME enhanced ANP secretion induced by Ang II. Furthermore, cGMP levels increased significantly in the presence of Ang II and was attenuated by L-NAME. On the other hand, the perfusion of 8 bromo-cGMP (10(-5) M) with Ang II reduced the effect of this octapeptide on ANP secretion. Secondly, we evaluated the effect of authentic NO on ANP release and observed that perfusion of NO reduced significantly the effect of Ang II on ANP release. We propose that the effect of Ang II on ANP secretion was modulated by NO likely via cGMP pathway.

Release of neuropeptide Y and hemodynamic changes during surgical removal of human pheochromocytomas

This study investigates the release of Neuropeptide Y from eight human pheochromocytomas. Profil immunoreactive Neuropeptide Y (Ir-NPY) levels during the management of surgery were compared with these of norepinephrine (NE) while hemodynamics were monitored. Plasma IrNPY and NE levels increased during tumor manipulation and returned to near normal one hour after operation. However, Ir-NPY levels remained high just after tumor resection while NE levels were significantly decreased. At tumor manipulation and just after tumor resection, plasma Ir-NPY levels were correlated with the systemic vascular resistances (SVR) (r = 0.74; P<0.04 and r = 0.86; P<0.006 respectively). No correlation was found either between plasma Ir-NPY and NE levels or between plasma NE levels and SVR. The release of Ir-NPY from tumor tissue, studied by a superfusion method, exhibited a significant correlation with the plasma Ir-NPY concentrations at the time of corresponding tumor resection (r = 0.95; P<0.007). Chromatographic analysis showed that Ir-NPY in plasma and outflow migrate as human NPY (1-36). These results confirmed that in pheochromocytoma, plasma NPY mainly originates from the tumor and argue for an important role of NPY in pheochromocytoma hypertension as indicated by the correlation between the Ir-NPY levels and the SVR.

Regulation of natriuretic peptide secretion by the heart

Secreted by the heart, more specifically by atrial cardiomyocytes under normal conditions but also by ventricular myocytes during cardiac hypertrophy, natriuretic peptides are now considered important hormones in the control of blood pressure and salt and water excretion. Studies on natriuretic peptide secretagogues and their mechanisms of action have been complicated by hemodynamic changes and contractions to which the atria are constantly subjected. It now appears that atrial stretch through mechano-sensitive ion channels, adrenergic stimulation via alpha 1A-adrenergic receptors, and endothelin via its ETA receptor subtype are major triggering agents of natriuretic peptide release. With several other stimuli, such as angiotensin II and beta-adrenergic agents, modulation of natriuretic peptide release appears to be linked to local generation of prostaglandins. In all cases, intracellular calcium homeostasis, controlled by several ion channels, is considered a key element in the regulation of natriuretic peptide secretion.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Angiotensin II-induced phosphoinositide production and atrial natriuretic peptide release in rat atrial tissue

The effect of angiotensin II (Ang II) on inositol phosphate (IP) production and atrial natriuretic peptide (ANP) release was studied in sliced rat atrial tissue. The ability of Ang II (10(-7) M) to stimulate IP accumulation was detected after 1 min of incubation, and the maximal increase was observed at 5 min. In (2-3H) inositol-labeled atrial tissue, Ang II induced the formation of (2-3H) inositol monophosphate (IP1) in a dose-dependent manner. The effect of Ang II (10(-7) M) on IP1 was prevented by losartan (10(-7) M) but was not affected by PD123319 (10(-7) M). Similar effects were observed on Ang II-induced ANP release in the presence of these antagonists. The mechanism of ANP liberation induced by this peptide was independent of cyclic adenosine monophosphate (cAMP) and regulated by nitric oxide (NO). The role of Ca2+ in the effect of Ang II was tested by 1,2-bis (o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra (acetoxymethyl) ester (BAPTA-AM; 10(-5) M), a chelator of intracellular Ca2+ that prevented the release of ANP by Ang II stimulation. We concluded that Ang II induced IP production and ANP release through AT1 receptors. Stimulation of ANP release by Ang II was dependent on intracellular Ca2+.

Cellular mechanism of angiotensin II-induced atrial natriuretic peptide release in rat right atrial tissue

This study presents an investigation of the mechanism of angiotensin II (Ang II)-induced atrial natriuretic peptide (ANP) release in superfused sliced right atria of rats. Ang II (0.1 microM) enhanced ANP release by 49%. This phenomenon was significantly blocked by (Sara1-Ileu 8) Ang II (1 microM) and losartan (0.1 microM). The use of neomycin (100 microM), a phospholipase-C inhibitor completely suppressed the effect of Ang II on ANP increase. To elucidate the intracellular mechanism of ANP released by Ang II, the role of protein kinase C (PKC) was determined by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) and phorbol ester : 4-beta-phorbol 12-myristate-13-acetate (PMA). We observed that PMA (0.1 microM) stimulated ANP release whereas H-7 (10 microM), an inhibitor of PKC in the presence of Ang II, prevented ANP increase. The role of calcium was also evaluated with 8-(N-N-diethylamino)-ocytyl-3,4,5, trimethoxy-benzoate (TMB-8) (10 microM) and N-(6-aminohexyl)-5-chloro-1-naphtalene sulfonamide (W-7) (10 microM), which completely inhibited ANP release by Ang II. Pre-treatment with diltiazem (10 microM), an antagonist of the Ca++ channel, did not prevent ANP increase due to Ang II, but A23187 (5 microM) enhanced ANP release by Ang II. These results suggest that PKC and intracellular calcium play an important role in ANP release under the influence of Ang II in rat atrial tissue.

Requirement for prostaglandin synthesis in secretion of atrial natriuretic factor from isolated rat heart

Release of atrial natriuretic factor (ANF) from the heart is primarily affected by myocyte stretch. In addition, ANF release can be modulated by a variety of hormones and neurotransmitters, but the mechanisms involved in such modulation are not completely understood. In the present study, we investigated the effect of inhibition of cyclooxygenase activity on release of ANF from the isolated, spontaneously beating rat heart: (1) during basal conditions; and (2) in response to arginine vasopressin (AVP), acetylcholine (ACh) and angiotensin II (ANG II), in order to determine if cardiac prostaglandin synthesis is involved in modulation of basal and hormone-mediated ANF secretion. Basal secretion in the time controls remained stable for the duration of the experiment. AVP, ACh and ANG II reduced basal secretion significantly by 58 +/- 4%, 51 +/- 6% and 26 +/- 8%, respectively, independently of concomitant changes in coronary flow and heart rate. Inhibition of cyclooxygenase with indomethacin (1 x 10(-5) M) decreased basal ANF release by 38 +/- 6%, indicating that basal secretion requires prostaglandin production. The effects of AVP, ACh and ANG II were maintained during perfusion with indomethacin, suggesting a common mechanism of action which operates via inhibition of cyclooxygenase. Based on our previous findings that the effects of indomethacin, AVP and ACh are overcome by inhibition of NO/EDRF synthesis, we suggest a common mechanism of action by means of which NO/EDRF mediates the effects of these agents by inhibiting cyclooxygenase activity.

Mechanism of hyperosmolality stimulation of ANP secretion: its dependency on calcium and sodium

The calcium dependency of hyperosmolality stimulation of atrial natriuretic peptide (ANP) secretion was determined using isolated superfused nonbeating rat left atrium. Increasing osmolality by 65, 85, and 100 mosmol/kgH2O by superfusion with sucrose produced a peak rise in ANP secretion of 1.8-, 2.0-, and 2.7-fold. To determine whether calcium influx played a role in osmolality (osm)-stimulated ANP secretion, atria were superfused with 2 mM lanthanum, a calcium antagonist. Lanthanum inhibited by 85% the response to a 100 mosmol/kgH2O increase in osm. The voltage-dependent calcium channel blocker isradipine had no effect on osm-stimulated ANP secretion, suggesting that calcium influx via voltage-dependent calcium channels was not playing a significant role. Likewise, depleting sarcoplasmic reticulum calcium with 1 microM ryanodine did not block the response to osm, suggesting that calcium influx was not adequate to induce consequential release of calcium from the sarcoplasmic reticulum. To determine whether calcium influx was via Na(+)-Ca2+ exchange, we determined the sodium dependency of osm-stimulated ANP secretion. Replacement of sodium with lithium or choline blocked the secretory response to 100 mosmol/kgH2O. We conclude that osm-stimulated ANP secretion is calcium and sodium dependent. Calcium influx via Na(+)-Ca2+ exchange is highly implicated as the mechanism of cellular calcium entry.

Effect of extracellular calcium depletion on the two-step ANP secretion in perfused rabbit atria

Employing isolated perfused rabbit atrial model we have found that stretch-activated atrial natriuretic peptide (ANP) secretion takes place in two steps: release of ANP from myocytes into surrounding intercellular space, and then translocation of the released ANP into atrial lumen along with the extracellular fluid (ECF) translocated upon releasing the stretch. Ca2+, one of the most important factors regulating secretory processes, has been reported by many workers to have influence on the ANP secretion, but at large variance. In the present study, therefore, we undertook to clarify the influence of Ca2+ depletion on ANP secretion and further to define which of the two steps is affected by Ca2+ removal. Extracellular Ca2+ depletion resulted in increased basal secretion and in accentuated secretion of immunoreactive (ir) ANP in response to mechanical stimuli. The translocation of ECF increased in response to atrial stretch-and-release, but it was not affected by Ca2+ depletion. The irANP concentration in the extracellular space calculated as the amount of irANP secreted in relation to the ECF translocated significantly increased when extracellular Ca2+ was depleted. These results indicate that extracellular Ca2+ depletion accentuates the stretch-induced ANP secretion through the augmentation of ANP release into the interstitium without changes in the ECF translocation.

Protein kinase C-dependent prostaglandin production mediates angiotensin II-induced atrial-natriuretic peptide release

The respective roles of protein kinase C (PKC) and endogenous prostaglandin formation in angiotensin II (Ang II)-induced myocardial secretion of atrial natriuretic peptide (ANP) was studied in cultured, spontaneously beating, neonatal-rat cardiomyocytes. Incubation of cardiomyocytes with 0.1 microM Ang II led to a rapid but transient increase in particulate-bound PKC activity, a response accompanied by marked increases in cellular 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha) generation and ANP secretion. A role for PKC in Ang II-induced 6-oxo-PGF1 alpha formation and ANP secretion was apparent, insofar as both responses were suppressed in the presence of the PKC inhibitors staurosporine (1 microM) and CGP 41251 (1 microM), as well as in cells in which PKC had been previously down-regulated by pretreatment with phorbol diester. Furthermore, Ang II-induced 6-oxo-PGF1 alpha production was found to be strongly correlated with Ang II-induced ANP release (r = 0.87, P < 0.001, n = 6), indicating a role for prostacyclin (PGI2) in Ang II-induced ANP secretion in these cells. This hypothesis was confirmed by finding that both Ang II-induced 6-oxo-PGF1 alpha production and ANP release were abolished in the presence of the respective phospholipase A2 and cyclo-oxygenase inhibitors quinacrine (10 microM) and indomethacin (10 microM), whereas exogenously applied PGI2 (1 microM) and prostaglandin E2 (0.1 microM) mimicked Ang II-induced ANP secretion in this system. Taken together, these results suggest that Ang II induces ANP secretion in spontaneously beating rat cardiomyocytes via a PKC-dependent autocrine pathway involving a cyclo-oxygenase product and a yet-to-be-identified myocardial prostanoid receptor.