The influence of calcium on ANF release in the isolated rat atrium

Capsaicin-sensitive neural pathway mediates atrial natriuretic factor (ANF) release in response to physiological stimuli

Increases in intravascular volume are detected by mechanoreceptors situated at the junctions of the great veins with the atria. We had previously shown that localized distension of the superior vena caval/right atrial junction, simulating increased cardiac preload, elicits release of ANF remotely from the atrial appendage. We proposed that ANF secretion is stimulated via intrinsic neural pathways running from the venoatrial junctions to the appendage. We developed a technique whereby non-adrenergic, non-cholinergic sensory nerves could be selectively destroyed in the heart of adult rats by instilling capsaicin into the pericardial space. Four days later, the animals were killed, and isolated perfused atria were prepared with small balloons positioned so that the superior vena caval/right atrial junction could be discretely stretched. Immunoreactive ANF secretion into the perfusate was measured. Although distension of the venoatrial junction increased ANF secretion from the control atria, there was no such response in the denervated atria. We conclude (A) that local application of capsaicin to the heart of adult rats induces selective functional neural deficits and (B) that information regarding distension of the junction of the great veins and the atria is normally transmitted across the atrium via these nerves to stimulate ANF secretion from peptide stores located in the atrial appendage. We propose that these pathways are crucial to ensure appropriate ANF secretion in response to an increase in circulating blood volume.

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.

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.

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

Atrial secretion of atrial natriuretic peptide (ANP) has been shown to be regulated by atrial workload. Although modulating factors for the secretion of ANP have been reported, the role for intracellular Ca(2+) on the secretion of ANP has been controversial. The purpose of the present study was to define roles for L- and T-type Ca(2+) channels in the regulation of ANP secretion in perfused beating rabbit atria. BAY K 8644 (BAY K) increased atrial stroke volume and pulse pressure. BAY K suppressed ANP secretion and ANP concentration in terms of extracellular fluid (ECF) translocation concomitantly with an increase in atrial dynamics. BAY K shifted the relationship between ANP secretion and ECF translocation downward and rightward. These results indicate that BAY K inhibits myocytic release of ANP. In the continuous presence of BAY K, diltiazem reversed the effects of BAY K. Diltiazem alone increased ANP secretion and ANP concentration along with a decrease in atrial dynamics. Diltiazem shifted relationships between ANP secretion and atrial stroke volume or ECF translocation leftward. The T-type Ca(2+) channel inhibitor mibefradil decreased atrial dynamics. Mibefradil inhibited ANP secretion and ANP concentration in contrast with the L-type Ca(2+) channel inhibitor. These results suggest that activation of L- and T-type Ca(2+) channels elicits opposite effects on atrial myocytic release of ANP.

Intra-atrial communication and control of atrial natriuretic factor (ANF) release

Atrial natriuretic factor (ANF) release was studied in isolated perfused atria prepared from rats. When the vein-atrial junction (VAJ) was distended with an inflatable balloon, ANF release into the perfusate was greater in intact atria than in appendectomized atria. It was concluded that distention of the VAJ causes ANF release from the atrial appendage. A cascade experiment was then prepared whereby buffer from one isolated atrium perfused a second atrium. Although the VAJ of the first atrium could be distended by balloon, the atrial appendage was ligated so ANF was not secreted into the perfusate. The second atrium was intact, but no balloon was inserted. Despite the fact that there were no changes in intraluminal pressure, ANF secretion from the second atrium increased when the VAJ of the first atrium was distended. This response was blocked by the endothelin (ET) A receptor antagonist BQ-123. However, no distention-induced changes in ET-1 levels could be found in the perfusate from the first atrium. It is proposed that, in response to changes in distention of the VAJ, ANF is released remotely from the atrial appendage. The mediator does not appear to be ET-1 itself, but rather some factor that stimulates ET-1-induced ANF release within the tissue of the atrial appendage.Key words: atrial natriuretic factor (ANF) secretion, endothelin, atrial distention, BQ-123, atrial appendectomy.

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.

The role of calcium in the response of cardiac muscle to stretch

This review focuses on the complex interactions between two major regulators of cardiac function; Ca2+ and stretch. Initial consideration is given to the effect of stretch on myocardial contractility and details the rapid and slow increases in contractility. These are shown to be related to two diverse changes in Ca2+ handling (enhanced myofilament Ca2+ sensitivity and increased intracellular Ca2+ transient, respectively). Interaction between stretch and Ca2+ is also demonstrated with respect to the rhythm of cardiac contraction. Stretch has been shown to alter action potential configuration, generate stretch-activated arrhythmias, and increase the rate of beating of the sino-atrial node. A variety of Ca(2+)-dependent mechanisms including attenuation of Ca2+ extrusion via Na+/Ca2+ exchange, Ca2+ entry through stretch-activated channels (SACs) and mobilisation of intracellular Ca2+ stores have been proposed to account for the effect of stretch on rhythm. Finally, the interaction between stretch and Ca2+ in the secretion of natriuretic peptides and onset of hypertrophy is discussed. Evidence is presented that Ca2+ (entering through L-type Ca2+ channels or SACs, or released from sarcoplasmic reticular stores) influences secretion of both atrial and B-type natriuretic peptide; there is data to support both positive and negative modulation by Ca2+. Ca2+ also appears to be important in the pathway that leads to expression of precursors of hypertrophic protein synthesis. In conclusion, two of the major regulators of cardiac muscle function, Ca2+ and stretch, interact to produce effects on the heart; in general these effects appear to be additive.

Adrenomedullin suppresses atrial natriuretic factor (ANF) secretion from isolated atrium

Atrial natriuretic factor (ANF) secretion was studied using isolated perfused right atria prepared from rats. Adrenomedullin (ADM), a recently identified 52-amino acid peptide whose biological activity has a striking resemblance to that of ANF, was added to the perfusate at a concentration of 1 microg/ml. The concentration of ANF secreted into the perfusate was measured by radioimmunoassay, under basal conditions (atrial wall unstressed), and during atrial distention (intraluminal pressure raised to 4 and to 6 cm water). It was found that basal secretion of ANF was not altered by ADM. However, when intraluminal pressure was raised, there was a significantly smaller increase in ANF secretion in the ADM-infused atria than in the control atria. It is concluded that ADM significantly reduces stretch-induced secretion of ANF, while having only minimal effects on basal secretion. Such an inhibitory mechanism would ensure the necessary negative feedback mechanism to counter the previously-reported stimulatory actions of ANF on ADM secretion. Moreover, these results support the hypothesis that ADM could be responsible for the reduction in stretch-induced ANF release observed during pregnancy.

Role of calcium in stretch-induced release and mRNA synthesis of natriuretic peptides in isolated rat atrium

To investigate the role of Ca2+ in stretch-induced synthesis and release of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) isolated superfused rat atria were stretched by raising intra-atrial pressure. The immunoreactive (ir-) ANP and BNP concentrations were analysed by radioimmunoassay and the corresponding mRNA levels were quantified by Northern blot and dot blot analyses. Stretch-induced ir-ANP release and a rise in BNP mRNA levels increased at high (3.0 mM) compared to low (0.5 mM) extracellular Ca2+ concentration ([Ca2+]o). Moreover, the adaptation of stretch-induced ir-ANP release was dependent on [Ca2+]o. Atrial BNP mRNA levels were increased by stretch also in non-paced, electrically silent atria, where voltage-activated Ca2+ channels are not activated. The stretch-induced rise in BNP mRNA was blocked by gadolinium (80 microM), but not by the L-type channel blocker diltiazem (3.0 microM). This study indicates that both the stretch-secretion coupling of ir-ANP release and the pressure-stimulated synthesis of BNP mRNA are Ca2+-dependent processes. Gadolinium inhibits the stretch-stimulated rise in BNP mRNA levels in contracting and non-contracting atria, which is similar to its ability to block stretch-activated ir-ANP release, suggesting the involvement of Ca2+-permeable stretch-activated channels.

Control of intravascular volume during pregnancy

1. We wished to determine whether, during pregnancy, there is reduced renal response to atrial distension, whether secretion of atrial natriuretic factor (ANF) is suppressed and whether neural input from the atrial volume receptors to the central nervous system is altered. 2. Conscious, chronically instrumented female rats were used. Atrial distension was achieved by implanting small balloons positioned at the superior vena caval/right atrial junction; inflation of the balloon did not impede blood flow through the heart. 3. In virgin rats, atrial stretch caused increased urine volume, urine sodium and potassium output and decreased free water clearance. These responses were abolished during pregnancy. 4. In response to atrial stretch, plasma ANF levels increased significantly in virgin rats. No such secretory response was observed in the pregnant animals. 5. Distension of isolated atria derived from unmated and 7 day pregnant rats resulted in an increase in secretion of ANF into the perfusate. Atria from 14 and 21 day pregnant rats were unresponsive to distension. 6. Pretreatment with oestradiol (50 micrograms daily for 10 days) caused plasma ANF levels and ANF secretion by isolated perfused atria to increase. By contrast, testosterone pretreatment (15 mg twice weekly for 2 weeks) abolished stretch induced secretion of ANF by isolated atria. 7. C-fos activity in the paraventricular nucleus of virgin rats increased in response to atrial distension. This response was reduced in the 7 day pregnant rats and abolished at 21 days. 8. We conclude that there is attenuation of both hormonal and neural responses to atrial distension in the pregnant animal. This allows blood volume to increase without eliciting homeostatic mechanisms to eliminate the extra fluid.

Block of stretch-activated atrial natriuretic peptide secretion by gadolinium in isolated rat atrium

1. Isolated superfused rat atrial preparations were used to study the mechanism of stretch-induced atrial natriuretic peptide (ANP) secretion. The stretch of the atrial myocytes was induced by raising the intra-atrial pressure. The secretion rates were analysed by measuring ANP concentrations from the superfusate fractions by radioimmunoassay. 2. The effect of gadolinium, a blocker of stretch-activated ion channels, on stretch-induced and basal ANP secretion was investigated by superfusing the atrial preparation with 5, 20 or 80 microM GdCl3. Gadolinium decreased stretch-induced ANP secretion in a dose-dependent manner, but did not affect basal secretion. 3. Because high concentrations of gadolinium may block voltage-gated calcium channels, we tested whether the selective blockers of L-type (diltiazem) and T-type (NiCl2) calcium channels affect the stretch-stimulated ANP release. Neither diltiazem at 3 microM nor NiCl2 at 50 microM affected stretch-induced ANP release in paced atrial preparation. 4. Gadolinium, but not diltiazem, also inhibited stretch-stimulated ANP secretion in non-paced, quiescent atria. 5. The findings that ANP release is inhibited by Gd3+, but not by diltiazem or NiCl2, and that the stretch-induced secretion in quiescent atria is also inhibited by Gd3+, suggest that stretch-activated ion channels are involved in the regulation of stretch-induced ANP release.

The influence of reproductive hormones on ANF release by rat atria

The effects of subcutaneous (sc) administration of estradiol (50 micrograms/100 microliters/day x 10 day) or testosterone (15 mg/100 microliters twice a week x 2 weeks) on ANF release were examined in isolated perfused rat atria. The concentration of ANF in the perfusate was measured by radioimmunoassay, under basal conditions (atrial wall unstressed), and during atrial distention (intraluminal pressure raised to 4, 6, and 10 cm water). In both male and female control (vehicle-injected) groups, increased atrial pressure resulting in distention of the atrium caused a significant increase in ANF release. Estradiol increased basal secretion of ANF but did not influence stretch-induced stimulation of ANF secretion. By contrast, although testosterone did not affect basal secretion, it completely abolished the stretch-induced increase in ANF secretion. Neither estradiol nor testosterone affected atrial compliance.