Chronic salt loading and central adrenergic mechanisms in the spontaneously hypertensive rat

Dietary salt exacerbates isoproterenol-induced cardiomyopathy in rats

Spontaneously hypertensive heart failure rats (SHHFs) take longer to develop compensated heart failure (HF) and congestive decompensation than common surgical models of HF. Isoproterenol (ISO) infusion can accelerate cardiomyopathy in young SHHFs, while dietary salt loading in hypertensive rats induces cardiac fibrosis, hypertrophy, and--in a minority-congestive HF. By combining ISO with dietary salt loading in young SHHFs, the authors sought a nonsurgical model that is more time--and resource-efficient than any of these factors alone. The authors hypothesized that salt loading would enhance ISO-accelerated cardiomyopathy, promoting fibrosis, hypertrophy, and biochemical characteristics of HF. SHHFs (lean male, 90d) were infused for 4 wk with ISO (2.5 mg/kg/day) or saline. After 2 wk of infusion, a 6-wk high-salt diet (4%, 6%, or 8% NaCl) was initiated. Eight percent salt increased heart weight, HF markers (plasma B-type natriuretic peptide, IL-6), lung lymphocytes, and indicators of lung injury and edema (albumin and protein) relative to control diet, while increasing urine pro-atrial natriuretic peptide relative to ISO-only. High salt also exacerbated ISO-cardiomyopathy and fibrosis. Thus, combining ISO infusion with dietary salt loading in SHHFs holds promise for a new rat HF model that may help researchers to elucidate HF mechanisms and unearth effective treatments.

Merits of non-invasive rat models of left ventricular heart failure

Heart failure (HF) is characterized as a limitation to cardiac output that prevents the heart from supplying tissues with adequate oxygen and predisposes individuals to pulmonary edema. Impaired cardiac function is secondary to either decreased contractility reducing ejection (systolic failure), diminished ventricular compliance preventing filling (diastolic failure), or both. To study HF etiology, many different techniques have been developed to elicit this condition in experimental animals, with varying degrees of success. Among rats, surgically induced HF models are the most prevalent, but they bear several shortcomings, including high mortality rates and limited recapitulation of the pathophysiology, etiology, and progression of human HF. Alternatively, a number of non-invasive HF induction methods avoid many of these pitfalls, and their merits in technical simplicity, reliability, survivability, and comparability to the pathophysiologic and pathogenic characteristics of HF are reviewed herein. In particular, this review focuses on the primary pathogenic mechanisms common to genetic strains (spontaneously hypertensive and spontaneously hypertensive heart failure), pharmacological models of toxic cardiomyopathy (doxorubicin and isoproterenol), and dietary salt models, all of which have been shown to induce left ventricular HF in the rat. Additional non-invasive techniques that may potentially enable the development of new HF models are also discussed.

Impaired effect of salt loading on nitric oxide-mediated relaxation in aortas from stroke-prone spontaneously hypertensive rats

1. The present study was designed to characterize the effects of salt on vasorelaxation via the nitric oxide (NO)/cGMP pathway in stroke-prone spontaneously hypertensive rats (SHRSP), which are highly salt sensitive. 2. Male 8-week-old SHRSP were given 1% NaCl solution as drinking water for 4 weeks, whereas control animals were given water only. 3. In aortic rings from salt-loaded SHRSP, relaxations in response to acetylcholine and sodium nitroprusside were significantly impaired compared with those in the control. In the presence of zaprinast, a cGMP-specific cyclic nucleotide phosphodiesterase (PDE)-5 inhibitor, the cGMP levels induced by these drugs were significantly reduced by salt loading, but remained unchanged in the absence of zaprinast. The protein levels of endothelial NO synthase, soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase (PKG) remained unchanged with salt loading, but those of PDE-5 decreased significantly and those of phosphorylated PKG tended to decrease, although the change was not statistically significant. Salt loading significantly impaired the relaxation in response to 8-bromo-cGMP. 4. These results indicate that, in aortas from SHRSP, salt loading causes impairment of relaxation in response to NO, which may be due to a decrease in cGMP production by sGC and impairment of the relaxation pathway downstream of cGMP, which, in turn, probably causes a decrease in PKG activity. Reduced PDE-5 protein expression may act, in part, as a compensatory response to impairment of cGMP-mediated relaxation.

Chronic nitric oxide inhibition with L-NAME: effects on autonomic control of the cardiovascular system

To determine whether increased sympathetic activity contributes to the hypertension induced by chronic exposure to moderate nitric oxide synthase (NOS) inhibition, various indexes of autonomic function were measured in rats given the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 10 mg/100 ml, approximately equal to 16 mg.kg-1.day-1) in the drinking water. One week of treatment raised blood pressure (139 +/- 3 vs. 106 +/- 1 mmHg; P < 0.01) and lowered heart rate (319 +/- 4 vs. 379 +/- 6 beats/min, P < 0.01). L-NAME had no effect on cardiac sympathetic tone, but elevated cardiac parasympathetic tone (-73 +/- 4 vs. -56 +/- 7 beats/min; P < 0.05). Depressor responses to ganglionic blockade were greater in L-NAME-treated rats (-50 +/- 5 vs. -34 +/- 5 mmHg; P < 0.05), whereas resting plasma, renal, and adrenal catecholamine values did not differ between groups. Treated rats also showed evidence of reduced baroreflex sympathetic stimulation of heart rate during hypotension and reduced parasympathetic activation during hypertension. Together, these data provide only very limited, indirect evidence that sympathetic stimulation contributes to the hypertension associated with moderate NOS inhibition.

Effects of dietary sodium and calcium on blood pressure reactivity in the SHR and WKY

Salt-sensitive spontaneously hypertensive rats (SHR) and their normotensive control strain, Wistar-Kyoto rats (WKY) were fed four diets varying in sodium chloride and calcium content in order to assess the effects of diet on learned blood pressure responses. The animals were exposed to a classical conditioning paradigm in which one tone was always followed by a brief electric shock and a second tone was never followed by shock. Sodium chloride loading raised baseline blood pressure in both strains, while supplemental calcium attenuated blood pressure. Sodium chloride potentiated blood pressure orienting responses to initial presentations of the tones among calcium deficient, but not calcium replete SHR. Increased sodium chloride intake also potentiated the learned pressor responses to the tone paired with shock in the SHR, but not the WKY. Calcium intake had no apparent effect on the learned blood pressure responses to the two tones.

Chronic salt loading and adrenergic mechanisms in the Sprague-Dawley rat

The effect of chronic salt loading on adrenergic mechanisms was evaluated in Sprague-Dawley rats (and NMRI mice) maintained on a high sodium (8%) or normal sodium (0.3%) regime for 4 weeks. The basal blood pressure (carotid artery) was not influenced by the high salt diet but the heart rate and blood pressure increases to mental stress (jet air) were larger in the salt loaded rats. There were indications of an increased sympathetic tone in rats on the high salt diet since in these rats sympathoinhibitory treatment with ganglionic blockade or clonidine induced larger falls in blood pressure and heart rate than in the controls. Central catecholamines (brain stem, striatum, hemispheres) were determined spectrofluorimetrically after cation exchange chromatography. The high salt diet influenced neither the endogenous levels of noradrenaline nor central noradrenaline turnover (disappearance of noradrenaline after synthesis inhibition by alpha-methyltyrosine and accumulation of dihydroxyphenylalanine after decarboxylase inhibition by 3-hydroxybenzylhydrazine). There were no changes in central alpha 2-adrenoceptor responsiveness when assessed as clonidine-induced deceleration of noradrenaline turnover in the brain and in central alpha 1-adrenoceptor responsiveness (clonidine-induced increase of flexor reflex in spinalized rats and clonidine-induced increase of motor activity in reserpinized mice). Peripheral sympathetic function was assessed in pithed rats. The pressor responses to intravenously administered noradrenaline (0.01-10 micrograms/kg) and electrical stimulation of the spinal sympathetic nerves (SNS, 0.25-2 Hz) were similar in the two groups, suggesting that salt did not influence vascular alpha-adrenoceptor responsiveness or transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)

A low dietary sodium intake reduces neuronal noradrenaline release and the blood pressure in spontaneously hypertensive rats

Young male spontaneously hypertensive rats were placed on a low (0.5 mmol/100 g), normal (13 mmol/100 g) or high (120 mmol/100 g) sodium diet for 6 weeks. Subsequent to the assessment of the basal blood pressure and heart rate in freely moving animals, the rats were pithed. In some of the pithed rats dose-response curves were constructed to exogenously administered noradrenaline (NA) and sympathetic nerve stimulation (SNS) through the pithing rod before and after the administration of an inhibitor of the neuronal uptake mechanism (desipramine, DMI, 0.5 mg/kg). In other pithed rats the SNS-induced (2 Hz) increase in plasma NA levels was assessed before and after the administration of clonidine (30 micrograms/kg), an agonist at the prejunctional alpha 2-adrenoceptors. We found that following the dietary intervention period the basal blood pressure and heart rate were higher in the high sodium group and lower in the low sodium group compared to values obtained in the control group. The neurogenic pressor responses clearly differed between the various diet groups. These differences could not be explained by differences in postjunctional responsiveness to exogenous NA. Rather, they were probably due to differences in the amount of transmitter at the synapse since the low sodium group was associated with decreased levels of plasma NA during SNS. Pretreatment with DMI potentiated the pressor responses, but largely to the same degree in the various diet groups, suggesting that there were no major differences in the function of the neuronal uptake mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)