Release Of Immunoreactive Atrial Natriuretic Factor Research Articles

Stretch-induced atrial natriuretic factor release utilizes a rapidly depleting pool of newly synthesized hormone

Atrial muscle stretch is widely believed to be the main stimulus for atrial natriuretic factor (ANF) release. However, a few reports have shown that although stretch induces an immediate increase in ANF output, this release rapidly decays even though hormone stores are not significantly depleted. In the present work, this phenomenon was studied in an isolated rat atria preparation using double isotope labeling. The tissue was labeled with [14C]leucine for 3 h followed by a 1-h chase, and then with [3H]leucine for 1 h. A final 1-h chase period was conducted with the tissue under basal (0.2-g load) or stretched (5-g load) conditions. During this final chase period, the [14C]ANF represented older, stored ANF and the [3H]ANF represented the newly synthesized peptide. After both the [14C]- and [3H]leucine pulses, immunoprecipitable isotope incorporated into ANF appeared in the chase medium within the first 10 min and stabilized to lower levels after 20 min of chase. Stretch resulted in an immediate significant increase in immunoreactive ANF release and a decrease in the medium [14C]ANF specific activity (SA). However, no change was observed in the medium [3H]ANF SA but the tissue SA tended to decrease. It is concluded that a portion of ANF is immediately and preferentially released upon synthesis, while the remainder is taken up into tissue stores and released from them at a lower rate. The secretory response to stretch was demonstrated to consist of a rapid, short-lived burst of newly synthesized ANF, suggesting an increased translocation of newly synthesized hormone into a stretch-sensitive, rapidly depletable pool. Given the nature of this pool, additional factors yet to be characterized likely come into play to maintain chronically elevated circulating levels of ANF.

Stretch-induced atrial natriuretic factor release utilizes a rapidly depleting pool of newly synthesized hormone.

Atrial muscle stretch is widely believed to be the main stimulus for atrial natriuretic factor (ANF) release. However, a few reports have shown that although stretch induces an immediate increase in ANF output, this release rapidly decays even though hormone stores are not significantly depleted. In the present work, this phenomenon was studied in an isolated rat atria preparation using double isotope labeling. The tissue was labeled with [14C]leucine for 3 h followed by a 1-h chase, and then with [3H]leucine for 1 h. A final 1-h chase period was conducted with the tissue under basal (0.2-g load) or stretched (5-g load) conditions. During this final chase period, the [14C]ANF represented older, stored ANF and the [3H]ANF represented the newly synthesized peptide. After both the [14C]- and [3H]leucine pulses, immunoprecipitable isotope incorporated into ANF appeared in the chase medium within the first 10 min and stabilized to lower levels after 20 min of chase. Stretch resulted in an immediate significant increase in immunoreactive ANF release and a decrease in the medium [14C]ANF specific activity (SA). However, no change was observed in the medium [3H]ANF SA but the tissue SA tended to decrease. It is concluded that a portion of ANF is immediately and preferentially released upon synthesis, while the remainder is taken up into tissue stores and released from them at a lower rate. The secretory response to stretch was demonstrated to consist of a rapid, short-lived burst of newly synthesized ANF, suggesting an increased translocation of newly synthesized hormone into a stretch-sensitive, rapidly depletable pool. Given the nature of this pool, additional factors yet to be characterized likely come into play to maintain chronically elevated circulating levels of ANF.

The effect of isoproterenol on right and left atrial dynamics and plasma atrial natriuretic factor in anesthetized rabbits

The effect of isoproterenol on mean right and left atrial pressures (RAP, LAP) and dimensions (RAD, LAD), and plasma immunoreactive atrial natriuretic factor (IR-ANF) was investigated in anesthetized rabbits. Infusion of isoproterenol (10 micrograms.kg-1.min-1 for 10 min) significantly increased plasma IR-ANF and heart rate. There were no significant changes in mean RAP or LAP following isoproterenol. Neither mean RAD, systolic RAD and diastolic RAD nor mean LAD, systolic LAD or diastolic LAD changed significantly. Systolic right and left atrial wall stress and diastolic right and left atrial wall stress did not change significantly during the infusion of isoproterenol. Since atrial dimensions did not increase, it is unlikely that the release of IR-ANF in response to isoproterenol is mediated by atrial stretch. These results suggest that the release of IR-ANF in response to this dose of isoproterenol is mediated by factors other than stretch or changes in atrial dynamics.

Requirement for Solutions with Oncotic Pressure to Release ANF During Blood Volume Expansion in Conscious and Anesthetized Rats

We examined the effect of a 30% blood volume expansion with either blood, isotonic saline, or isotonic saline plus albumin on the release of immunoreactiveatrial natriuretic factor (IR-ANF) in conscious and barbiturate-anesthetized rats. Immediately prior to volume expansion, resting plasma IR-ANF levels were significantly ( p < 0.05) higher in the conscious animals (61 ± 5 pg mL , n = 19) compared to the anesthetized animals (41 ± 3 pg mL , n = 19) . Volume expansion with blood from donor rats significantly ( p < 0.05) elevated IR-ANF in both groups, but the increase in the conscious rats (+492 ± 32 pg mL , n = 6) was significantly ( p < 0.05) greater than that of the anesthetized rats (74 ± 25 pg mL , n = 6) . Similarly, volume expansion with isotonic saline containing 38 mg mL albumin significantly ( p < 0.05) elevated IR-ANF levels in both groups, but the increase in the conscious group (379 ± 102 pg mL , n = 7) was significantly ( p < 0.05) greater than that of the anesthetized group (94 ± 32 pg mL , n = 7) . These elevations in IR-ANF levels were not significantly different between either of the conscious groups or either of the anesthetized groups. Volume expansion with isotonic saline alone did not have any significant effect upon resting IR-ANF levels in either conscious (+8 ± 14 pg mL , n = 6) or anesthetized (−7 ± 6 pg mL , n = 6) animals. From these results, it would appear that a) isotonic saline alone does not stimulate IR-ANF release when used to expand blood volume, and b) barbiturate anesthesia significantly lowers resting plasma IR-ANF levels and attenuates the release of IR-ANF in response to volume expansion.

Chronotropic stimulation: a primary effector for release of atrial natriuretic factor.

Release of atrial natriuretic factor (ANF) following an elevation in heart rate is thought to be mediated primarily by a change in atrial stretch. To evaluate the direct effect of chronotropic stimulation on ANF release, isolated rat left atria were electrically paced (1-9 Hz) at constant resting tension (0.5-4 g), and the amount of immunoreactive ANF (IRANF) released at each frequency and tension was quantitated with a sensitive radioimmunoassay. Our results show that at controlled resting tensions greater than 1 g, chronotropic stimulation increased IRANF secretion in a manner dependent on the pacing frequency; rapid atrial rates (e.g., 8 and 9 Hz) were necessary to release ANF at tensions of 1 g or less. Resting tension influenced the magnitude of the secretory response to electrical stimulation. Release of IRANF with contraction frequency was transient in nature and, at high frequencies, was associated with a decrease in developed (systolic) tension in accordance with the negative force-frequency relation inherent in the rat heart. When evaluated at a single diastolic tension and pacing frequency, IRANF release was positively correlated with systolic tension. ANF released under in vitro conditions was approximately 3,000 Da, in agreement with the size of the physiologically circulating form. In atria from reserpinized rats, evidence for involvement of catecholamines in chronotropic-stimulated ANF release was suggested. The presence of lidocaine (5 x 10(-4) M) had no effect on rate-induced ANF secretion. Therefore, chronotropic stimulation releases ANF independently of changes in atrial stretch. The magnitude of this response depends on a combination of pacing frequency and diastolic tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcium and ouabain on the release of atrial natriuretic factor.

In order to examine the role of calcium in the secretory process of atrial natriuretic factor (ANF), we studied the effects of hypercalcaemia and ouabain on the plasma concentration of immunoreactive ANF, and the effect of calcium on immunoreactive ANF release from isolated rat atria. Anaesthetized dogs were treated with CaCl2 infusion, ouabain or phenylephrine injection. With CaCl2 infusion, serum calcium and plasma immunoreactive ANF respectively increased to three and four times their basal levels. Ouabain increased plasma immunoreactive ANF to two and a half times the initial level. Neither CaCl2 nor ouabain produced any effect on right atrial pressure and heart rate, but they both significantly increased arterial pressure. Phenylephrine caused a greater increase in arterial pressure than both CaCl2 and ouabain. However, there was no significant increase in plasma immunoreactive ANF. Moreover, calcium stimulated the release of immunoreactive ANF from isolated rat atria. These results suggest that the calcium may play a key role in the secretory process of ANF.

Effects of volume change on circulating immunoreactive atrial natriuretic factor in rats.

The mammalian atrial hormone atrial natriuretic factor (ANF) has been shown to have potent natriuretic and diuretic actions as well as vasodilator effects when released into the circulation. To investigate how the levels of the circulating form of this peptide change with alteration of intravascular fluid volume, we measured immunoreactive ANF in the plasma of Wistar rats after acute saline load, acute furosemide treatment, and chronic water restriction. Circulating levels of immunoreactive ANF increased significantly (p less than 0.001) 1 minute after acute saline load and returned to normal levels within 5 minutes. Volume contraction induced by furosemide treatment of chronic water restriction significantly reduced the circulating immunoreactive ANF. These data indicate that acute volume expansion causes an immediate release of immunoreactive ANF into the general circulation and acute volume contraction results in a decline of circulating levels of immunoreactive ANF, which is maintained during chronic volume contraction. These results suggest that the atria detect alterations in intravascular fluid volume and respond by changing the levels of ANF acutely as well as chronically and thereby participate in the regulation of body fluids and, perhaps, of blood pressure.

Release of atrial natriuretic factor. Effects of repetitive stretch and temperature.

The effect of continuous and repetitive mechanical stretch on release of immunoreactive atrial natriuretic factor (IR-ANF) was studied in isolated rat atria distended by inflation of miniature balloon catheters. After 10 min incubation, the concentration of IR-ANF in the medium increased in proportion to the degree of continuous atrial stretch. Repetitive atrial expansion induced a rate dependent increase in release of IR-ANF which at the highest rate tested (89 inflations/min) was sixfold greater than that observed with continuous maximal expansion. Comparison of the amount of IR-ANF released from the right or left atrium separately revealed that during continuous and repetitive stretch, both atria contribute equally to the total amount of release IR-ANF. A reduction of bath temperature (37 degrees to 21.5 degrees C) decreased both basal- and stretch-induced IR-ANF release approximately 85%. It was concluded that release of atrial natriuretic factor is temperature-dependent and induced by both the extent and rate of atrial stretch.

Plasma levels of immunoreactive atrial natriuretic factor increase during supraventricular tachycardia

A significant diuretic and natriuretic response occurs during paroxysmal supraventricular tachycardia (SVT). Although the diuresis may be secondary to suppression of vasopressin secretion, the etiology of the natriuresis remains unexplained. To determine if atrial natriuretic factor (ANF) could contribute to the polyuric response during SVT, 10 patients were studied: five during spontaneous SVT and five during simulated SVT produced by rapid simultaneous atrial and ventricular pacing. Plasma immunoreactive ANF (IR-ANF) levels measured by radioimmunoassay were obtained at baseline (before and/or 24 to 48 hours after SVT) and after at least 15 minutes of SVT in all patients. During spontaneous and simulated SVT, IR-ANF was significantly elevated (mean ± SE; 275 ± 68 pmol/L) compared to baseline (28 ± 7 pmol/L; p = 0.0036). Similar increases in IR-ANF were noted during both simulated and spontaneous SVT. To determine if this IR-ANF release was related to the increase in heart rate or the rise in right atrial pressure during SVT, IR-ANF levels were also measured in five patients with sinus tachycardia and in six patients with congestive heart failure, IR-ANF was significantly related to right atrial pressure ( r = 0.93; p = 0.0009) but not to heart rate ( r = 0.46). Thus, IR-ANF is elevated during SVT and may contribute to the natriuretic response. The stimulus to IR-ANF secretion during SVT appears to be related to the rise in right atrial pressure rather than to the increase in heart rate.