Plasma levels of atrial natriuretic peptides in conscious adult spontaneously hypertensive rats

Increased cyclic guanosine monophosphate production and overexpression of atrial natriuretic peptide A-receptor mRNA in spontaneously hypertensive rats

Atrial natriuretic peptide (ANP) specifically stimulates particulate guanylate cyclase, and cyclic guanosine monophosphate (cGMP) has been recognized as its second messenger. Spontaneously hypertensive rats (SHR) have elevated plasma ANP levels, but manifest an exaggerated natriuretic and diuretic response to exogenous ANP when compared to normotensive strains. In isolated glomeruli, the maximal cGMP response to ANP corresponds to a 12- to 14-fold increase over basal levels in normotensive strains (Wistar 13 +/- 2; Wistar-Kyoto 12 +/- 2; Sprague-Dawley 14 +/- 2) while a maximal 33 +/- 3-fold elevation occurs in SHR (P < 0.001). This hyperresponsiveness of cGMP is reproducible in intact glomeruli from SHR from various commercial sources. Furthermore, this abnormality develops early in life, even before hypertension is clearly established, and persists despite pharmacological modulation of blood pressure, indicating that it is a primary event in hypertension. In vitro studies have revealed a higher particulate guanylate cyclase activity in membranes from glomeruli and other tissues from SHR. This increase is not accounted for by different patterns of ANP binding to its receptor subtypes between normotensive and hypertensive strains, as assessed by competitive displacement with C-ANP102-121, an analog which selectively binds to one ANP receptor subtype. The hyperactivity of particulate guanylate cyclase in SHR and its behavior under basal, ligand (ANP), and detergent-enhanced conditions could be attributed either to increased expression or augmented sensitivity of the enzyme. Radiation-inactivation analysis does not evoke a disturbance in the size of regulatory elements normally repressing enzymatic activity, while the expression of particulate guanylate cyclase gene using mutated standard of A- and B-receptors partial cDNAs, quantified by polymerase chain reaction (PCR) transcript titration assay, manifests a selective increase of one guanylate cyclase subtype. Our data suggest that in hypertension, genetic overexpression of the ANP A-receptor subtype is related to the exaggerated biological response to ANP in this disease.

Atrial natriuretic factor: plasma concentration and specific binding to renal glomeruli during the development of genetic hypertension in rats of the Lyon strain

1. The pathophysiologic role of atrial natriuretic factor (ANF) in genetic hypertension was investigated in 5- and 21-week-old genetically hypertensive (LH), normotensive (LN) and low blood pressure (LL) rats of the Lyon strain. 2. Plasma ANF was significantly higher in 5-week-old LH than in age-matched LN and LL animals. It tended to be lower in 21-week-old LH than in LN rats. 3. Left and right atrial ANF content was also significantly lower at 21 weeks in the LH group. 4. Glomerular ANF receptors showed a significant decrease in LH in comparison to LL and LN animals.

Generation of atrial natriuretic peptide (ANP) in perfused lungs of spontaneously hypertensive rats (SHR). Comparison to Wistar-Kyoto (WKY) and Wistar (W) rat strains

1. Lungs can take up from the vasculature, circulating forms of atrial natriuretic peptide (Turrin and Gillis, 1986, 1987) and also to synthesize ANP. 2. The lung peptide directly delivered by lungs into the lung vasculature could play a role in the local water/electrolytic balance. 3. Using Spontaneously Hypertensive Rats (SHR), isogenic normotensive controls, the Wistar-Kyoto strain (WKY), and the regular Wistar strain as second control (W), and using a highly sensitive RIA, we measured the immunoreactive IR-ANP content of extracted plasma, lung homogenate and lung perfusate, since there are references of altered ANP levels in this kind of hypertension. 4. The IR-ANP measured in the lung vasculature effluent collected throughout 32 min of Krebs perfusion, was significantly different in all of the three analyzed strains (SHR > WKY > W). 5. The results support the idea of a local function for the peptide hormone directly delivered into the lung vasculature of SHR, which could represent a local adaptation to haemodynamics SHR characteristics besides a genetic characteristic distinguishing WKY from W strains.

Differential effects of antidepressant treatments on fenfluramine-induced increases in plasma prolactin and corticosterone in rats

Intravenous administration of 5-HT releasing agent, fenfluramine, to rats produced increases in plasma prolactin and corticosterone concentrations. Short-term or long-term treatment with either clorgyline or imipramine did not affect baseline levels of prolactin or corticosterone. On the other hand, short-term but not long-term lithium treatment significantly increased baseline levels of corticosterone but not of prolactin. Short-term treatment with lithium but not clorgyline or imipramine potentiated fenfluramine-induced increases in plasma prolactin but not corticosterone. On the other hand, long-term treatment with clorgyline but not imipramine or lithium attenuated fenfluramine's effect on plasma prolactin but not on corticosterone. These findings demonstrate differential effects of antidepressant treatments on fenfluramine-induced increases in plasma prolactin and corticosterone in rats and are consistent with several other clinical and animal studies demonstrating dissimilar actions of different antidepressant treatments on two different 5-HT-mediated neuroendocrine functions.

Atrial natriuretic factor and volume expansion-induced natriuresis in the spontaneously hypertensive rat

The implication of atrial natriuretic factor (ANF) during acute volume expansion in the spontaneously hypertensive rat (SHR) was evaluated. The effect of short-term afterload relief was also investigated. Fourteen- to 15-week-old SHR and Wistar-Kyoto (WKY) rats were treated with hydralazine for 5 days. The systolic blood pressure (BP) of SHR decreased to normotensive levels but cardiac hypertrophy was not reduced. Isotonic, iso-oncotic volume expansion (equivalent to 10% of total blood volume) was performed 3 times at 15-min intervals on conscious animals. The effect of volume expansion on central venous pressure was identical among the groups. Changes in left ventricular end-diastolic pressure (LVEDP) induced by volume expansion were greater in SHR than in WKY rats and were not affected by treatment. The increases in plasma N-terminal ANF (ANF-(1-98)) concentrations were larger in both treated and untreated SHR verses WKY rats. Despite enhanced ANF release in SHR, the overall magnitude of the diuretic and natriuretic responses to volume expansion was similar in all groups. The natriuretic response was strongly correlated with plasma ANF in WKY rats, this relationship was weak in control SHR, and restored by treatment. It is suggested that ANF release is not impaired in SHR at a 10% volume load; however, there seems to be a lower renal responsiveness to ANF in SHR.

Atrial natriuretic factor release during volume expansion in the spontaneously hypertensive rat--effect of long-term hydralazine treatment

We tested the ability of spontaneously hypertensive rats (SHR) to release atrial natriuretic factor (ANF) during acute volume loading and its relationship to right and left atrial pressures. The effect of decreasing cardiac afterload by hypotensive treatment was also investigated. Fourteen to 15-week-old SHR and Wistar-Kyoto (WKY) rats were treated with hydralazine (12 mg/kg.day p.o.) for a period of 4 weeks. Untreated rats served as controls. The systolic blood pressure (BP) of SHR decreased to normotensive levels and cardiac hypertrophy was also reduced. Isotonic, iso-oncotic volume expansion (VE) was performed 3 times as 10% increments and at 15-min intervals. Despite greater changes in left-ventricular end-diastolic pressures (LVEDP) and to a lesser extent in central venous pressure (CVP) in SHR controls, the increases in plasma ANF (N-terminal concentrations) induced by VE were of a similar magnitude in both SHR and WKY control rats. The LVEDP and ANF C-terminal elevations were of a lower magnitude in treated SHR. Auricular ANF concentrations, measured at the end of VE, were lower in the left and higher in the right atrium in SHR versus WKY. It is concluded that despite a lower distensibility of their left atrium, ANF release is not impaired in SHR upon a VE. This adequate ANF release is obtained through higher increases in atrial pressures.

Dietary sodium loading elevates blood pressure in baroreceptor denervated rats

We have examined the possibility that defective baroreflex function may contribute to the hypertensive effect of a high dietary sodium intake. In a preliminary study we found that, after interrupting the baroreflex by sino-aortic denervation (SAD), feeding a high-sodium diet to normotensive Wistar/Kyoto rats (WKY) caused their tail-cuff blood pressure to rise to hypertensive levels. In the present study, SAD and sham-operated WKY were fed diets with either a low or a high sodium content. An increase in the sensitivity of blood pressure to sodium after baroreceptor denervation was confirmed by direct measurement of mean arterial pressure (MAP) in conscious rats via indwelling aortic catheters. After 8 weeks the MAP in SAD rats on the high sodium diet was 35 mm Hg higher than in SAD rats on the low sodium diet. Ganglion blockade reduced MAP to a similar level in all treatment groups, but pressor responses to phenylephrine were greater in SAD rats on the high sodium diet, suggesting that the hypertensive effect of sodium in this group might have been due to increased sympathetic vasoconstriction.

Possible involvement of atrial natriuretic factor in the antihypertensive action of a high-calcium diet in spontaneously hypertensive and Wistar-Kyoto rats

The present study was designed to determine the possible involvement of atrial natriuretic factor (ANF) in the hypotensive action of a high-calcium diet. The effects of increased dietary calcium (2.9% calcium, HCa) on blood pressure, urinary sodium excretion, and ANF were examined in 30 spontaneously hypertensive rats (SHR) and 30 Wistar-Kyoto rats (WKY). Control groups of 30 SHR and 30 WKY were fed normal calcium lab chow (0.4% calcium, NCa). The HCa diet reduced blood pressure and serum phosphorus concentration and increased urinary excretion of sodium and calcium in SHR and WKY. The HCa diet also caused a sustained increase in plasma ANF concentration and, finally, a decrease in atrial ANF concentration in both groups. A significant inverse correlation was observed between ANF concentrations in plasma and atria of the four experimental groups. Plasma ANF concentration was positively correlated with daily calcium consumption, and blood pressure was inversely correlated with daily calcium consumption in HCa- and NCa-SHR groups and in HCa- and NCa-WKY groups, respectively. Furthermore, a significant inverse correlation between blood pressure and plasma ANF concentration was observed in SHR groups and in WKY groups, respectively. The observed sustained increment in endogenous plasma ANF concentration, which is probably caused by increased secretion from the atrium, may contribute, in part, to the blood-pressure-lowering effects of the HCa diet.

Haemodynamic and renal tubular responses to low-dose infusion or bolus injection of the peptide ANF in anaesthetized rats

1. Renal haemodynamic and tubular transport responses to low-dose infusions (0.1-5.0 ng min-1) and injections (50-200 ng) of atrial natriuretic factor (ANF) were studied in anaesthetized rats (average body weight, 300 g). 2. The lowest infusion dose (average, 0.3 ng kg-1 min-1) was above threshold for significant hypotension (-11 mmHg), increased glomerular filtration (20%), urine flow (104%) and sodium output (191%). 3. Compared with the lowest rate of infusion of ANF, the smallest injection dose (50 ng) was less effective in reducing blood pressure, caused no change in glomerular filtration rate but induced similar diuresis and natriuresis. Based on total dose administered, sensitivity to injection was only one-tenth that of infusion. 4. Absolute proximal reabsorption, derived from lithium clearance measurements, did not increase in parallel with filtration rate during infusions, and fractional proximal reabsorption was markedly depressed. Proximal glomerulo-tubular balance was only 50% effective at 0.1 ng min-1, falling to 16% at 5 ng min-1. 5. Despite complete recovery of filtration rate and filtration fraction 30 min after terminating infusions, diuresis, natriuresis and depressed fractional proximal reabsorption persisted. Time-control animals displayed no changes in renal function. 6. It is proposed that disruption of glomerulo-tubular balance occurred in these experiments from inhibition of endogenous angiotensin II-stimulated proximal sodium reabsorption by ANF. Persistence of this action may be due to accumulation of intracellular messengers or to sequestration of biologically active ANF bound to 'clearance receptors'.

Long-term imipramine treatment potentiates m-chlorophenylpiperazine-induced changes in prolactin but not corticosterone or growth hormone levels in rats

Intravenous administration of m-chlorophenylpiperazine (m-CPP, a selective 5-HT agonist) to rats produced increases in plasma prolactin and corticosterone and a decrease in plasma growth hormone concentrations. Long-term but not short-term imipramine treatment potentiated m-CPP's effect on plasma prolactin, but not its effects on corticosterone or growth hormone. Short-term or long-term imipramine treatment did not produce significant changes in baseline levels of prolactin, corticosterone or growth hormone. These findings are compatible with development of functional supersensitivity of 5-HT receptors mediating prolactin release. Lack of potentiation of m-CPP's effects on corticosterone and growth hormone following long-term imipramine treatment suggests either differential regulation of these hormones by serotonergic and possibly other mechanisms, or different 5-HT receptor subtypes mediating the release of these hormones. Alternatively, adaptive changes in other aminergic neurotransmitter mechanisms such as the noradrenergic system may account for the differential effect of long-term imipramine treatment on m-CPP-induced neuroendocrine changes.

Brain binding sites for atrial natriuretic factor (ANF): alterations in prehypertensive Dahl salt-sensitive (S/JR) rats

The binding of radioiodinated atrial natriuretic factor (125I-ANF-28) to discrete areas of brain in 7 week old, inbred Dahl salt-sensitive (S/JR) and salt-resistant (R/JR) rats was studied utilizing quantitative film autoradiography. At this age, S/JR rats exhibit systolic blood pressures that are prehypertensive and tend to be slightly higher than systolic blood pressures of age-matched R/JR rats. Scatchard analysis of 125I-ANF-28 binding in forebrain revealed that S/JR rats have a significantly increased number of binding sites for 125I-ANF-28 in the subfornical organ as compared to R/JR controls. In contrast, values for 125I-ANF-28 binding capacity in the choroid plexus and area postrema were similar for both strains, and binding affinity constants for 125I-ANF-28 binding revealed no strain differences in any brain area examined. The elevation in the number of binding sites for atrial natriuretic factor may serve as a compensatory mechanism acting in part to lower fluid volume and sodium levels prior to the precipitous increase in blood pressure which occurs in S/JR rats by 10 weeks of age.

Binding sites for atrial natriuretic factor (ANF) in kidneys and adrenal glands of spontaneously hypertensive (SHR) rats

Binding sites for atrial natriuretic factor (ANF) were studied in kidneys and adrenal glands of 17 week old male spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats by quantitative autoradiography using 125I-ANF-28. In kidney, 125I-ANF-28 binding sites were found in high concentrations in glomeruli and in much lower concentrations in the renal papilla. In adrenal gland, 125I-ANF-28 binding sites were highly localized to the zona glomerulosa and were of moderate density in the inner cortical regions. ANF binding sites did not occur in the adrenal medulla. The maximum binding capacity (Bmax) of 125I-ANF-28 was reduced by 50% in the kidney glomeruli of SHRs compared to WKY controls. In contrast, the affinity constant (Ka) for 125I-ANF-28 was elevated by 100% in kidney glomeruli of SHRs. There were no significant strain differences in values for Bmax or Ka for 125I-ANF-28 binding in the adrenal zona glomerulosa. These findings suggest that the natriuretic and diuretic actions of ANF within kidney glomeruli may be compromised in adult SHR rats and these alterations may contribute to the development and maintenance of hypertension in rats of this strain.

Anterior pituitary gland modulates the release of atrial natriuretic peptides from cardiac atria

Myocytes in the atria contain a prohormone that gives rise to atrial natriuretic peptides (ANP), which have intrinsic hemodynamic regulatory activity. Little is known about mechanisms regulating ANP release. In rats with indwelling catheters, acute blood volume expansion with 5% (wt/vol) dextrose increases the amount of circulating immunoreactive ANP by a factor of 2-3, as determined by radioimmunoassay. Pithing, which both removes neurogenic influences and interrupts humoral influences of the brain and pituitary gland on the heart, completely blocked stimulus-induced release of ANP. Because our studies using pharmacological blockade of the autonomic nervous system had suggested that neurogenic mechanisms do not play a major role in ANP release, we sought a humoral mechanism involved in ANP secretion. Basal and stimulated release of ANP were significantly blunted in hypophysectomized rats (8 days after operation) but were completely restored when the resected anterior pituitary was reimplanted under the kidney capsule. This suggests that hormones of anterior pituitary origin are required for ANP secretion in response to acute volume loading.