Catecholamines in kidneys of normotensive and genetically hypertensive rats. Effects of salt load

Aldosterone augments Na+-induced reduction of cardiac norepinephrine reuptake

Impairment of the cardiac norepinephrine (NE) reuptake by the neuronal NE transporter contributes to enhanced cardiac NE net release in congestive heart failure. Elevated plasma levels of aldosterone (AL) promote sympathetic overstimulation in failing hearts by unclear mechanisms. Our aim was to evaluate if elevated AL and/or alterations in Na(+) intake regulate cardiac NE reuptake. To test the effects of AL and Na(+) on cardiac NE reuptake, Wistar rats were fed a normal-salt (NS) diet (0.2% NaCl), a low-salt (LS) diet (0.015% NaCl), or a high-salt (HS) diet (8% NaCl). Another group of animals received AL infusion alone (0.75 μg/h) or AL infusion plus HS diet. Specific cardiac [(3)H]NE uptake via the NE transporter in a Langendorff preparation and AL plasma levels were measured at different time points between 5 and 42 days of treatment. To compare these findings from healthy animals with a disease model, Dahl salt-sensitive rats were investigated as a model of congestive heart failure with endogenously elevated AL. In summary, neither exogenous nor endogenous elevations of AL alone were sufficient to reduce cardiac NE reuptake. Only the HS diet induced a reduction of NE reuptake by 26%; additional infusion of AL augmented this effect to a further reduction of NE reuptake by 36%. In concordance, Dahl salt-sensitive rats treated with a HS diet displayed elevated AL and a marked reduction of NE reuptake. We conclude that exogenous or endogenous AL elevations alone do not reduce cardiac NE reuptake, but AL serves as an additional factor that negatively regulates cardiac NE reuptake in concert with HS intake.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Ganglionic tyrosine hydroxylase and norepinephrine transporter are decreased by increased sodium chloride in vivo and in vitro

The present study tested the hypothesis that, in normal male rats, chronic changes in salt intake alter the levels of tyrosine hydroxylase and the norepinephrine transporter in sympathetic ganglia. Increasing dietary salt (from 0.02% to 1%, 4% or 8% NaCl in rat chow) decreased (p<0.05) the mRNA levels of tyrosine hydroxylase and the norepinephrine transporter in the adrenal gland, superior cervical ganglia and celiac ganglia. In addition, tyrosine hydroxylase and norepinephrine transporter protein levels were decreased (p<0.05) in the adrenal gland. To test the hypothesis that NaCl acts directly on postganglionic neurons to suppress the expression of these proteins, it was determined if increases in NaCl concentrations, of a magnitude achieved during increases in dietary salt in vivo, suppress expression of tyrosine hydroxylase and the norepinephrine transporter in cultured sympathetic neurons in vitro. Increased dietary salt increased plasma NaCl concentrations each by up to 4-6 mEq l(-1) (p<0.05), with the greatest increases occurring at night when the rats consume most of their food. In addition, NaCl added to cultured neurons decreased tyrosine hydroxylase and norepinephrine transporter protein and mRNA levels, and norepinephrine uptake; however, the NaCl concentration increases required were 15-30 mEq l(-1). These data suggest that increased dietary salt can influence the activity of the sympathetic nervous system by suppressing the levels of tyrosine hydroxylase and the norepinephrine transporter. While increased NaCl levels can act directly on neurons to suppress these proteins, this action may occur in vivo only in severe pathophysiological states, but not during increases in dietary salt without the synergistic effect of other factors.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Transgenic mice to study the role of dopamine receptors in cardiovascular function

Dopamine, an intrarenal regulator of sodium transport, is important in the pathogenesis of hypertension. The transduction of D1-like receptors in renal proximal tubules is defective in animal models of genetic hypertension. The defect is associated with an impaired regulation of proximal tubular sodium transport and cosegregates with hypertension in rats. Moreover, mice lacking one or both D1A receptor alleles develop hypertension. Extrasynaptic D3 receptors in renal tubules and juxtaglomerular cells may also regulate renal sodium transport and renin secretion while presynaptic D3 receptors may act as autoreceptors to inhibit neural norepinephrine release. Mice lacking one or both D3 alleles have elevated systolic blood pressure and developed diastolic hypertension. Although basal urine flow, sodium excretion, and glomerular filtration rate are similar, mice homozygous to the D3 receptor have an impaired ability to excrete an acute saline load compared to heterozygous and wild type mice. These studies suggest that abnormalities in dopamine receptor genes or their regulation may lead to the development of hypertension via different pathogenetic mechanisms.

Transitory noradrenergic and peptidergic nerves in the cat kidney

Indirect immunohistochemical methods were used to visualize nerves immunoreactive for tyrosine hydroxylase (TH), dopamine beta hydroxylase (DBH), neuropeptide Y, (NPY) and calcitonin gene-related peptide (CGRP) in sections of the kidneys of cats of different ages. Nerve terminals immunoreactive for TH, DBH and NPY innervated interlobar veins and the renal arterial tree including medullary vascular bundles of cats of each age studied. Most nerve terminals immunoreactive for CGRP innervated interlobar arteries. In kidneys of cats 2 to 10 weeks old, TH- and DBH-immunoreactive axons formed elaborate plexuses that were distributed throughout much of the outer two thirds of the inner medulla. Inner medullary NPY-immunoreactive nerve terminals formed sparse plexuses by comparison, thus suggesting a large population of TH-immunoreactive nerve terminals not immunoreactive for NPY. Plexuses immunoreactive for CGRP also innervated the inner medullae of young cats. Some inner medullary axons appeared degenerate in 8 and 10 week old cats, and no inner medullary nerve terminal plexuses were visualized in 12 week old or adult cats. Cell death or paring of axons resulting from mechanisms intrinsic to the neuronal population or from a change in trophic factors secreted or expressed by cells in the medulla may effect the loss of inner medullary nerve terminals in the kidneys of young cats.

The influence of gender in the evaluation of platelet and plasma catecholamines

The platelet and plasma levels of catecholamines (CA) were simultaneously studied in a group of normal subjects in order to find possible sex-related changes in the distribution of CA in these two compartments. No significant differences between males and females were observed, but a marked platelet noradrenaline increase was found in the luteal phase as compared to the follicular phase. Furthermore, the platelet and plasma CA levels were strongly correlated in the male group but not in the female group. These results, while confirming the existence of a menstrual-related variability in noradrenergic activity, suggest a sex-related difference in the dynamic balance between platelet and plasma CA levels. The simultaneous assay of platelet and plasma CA enabled this phenomenon to be revealed, whereas the separate evaluation of platelet or plasma CA levels would not have done so.

Implications of a non-lamellar lipid phase for the tight junction stability. Part II: Reversible modulation of transepithelial resistance in high and low resistance MDCK-cells by basic amino acids, Ca2+, protamine and protons

The transepithelial resistance of confluent epithelial cell monolayers was monitored to investigate the influence of basic amino acids, Ca2+, protamine and protons on tight junction electrical resistance. In an accompanying paper we investigated the effect of these substances on the lamellar/hexagonal II phase transition in reconstituted phospholipid membranes containing phosphatidylserine and phosphatidylethanolamine. We conclude that the permeability of tight junctions may be described by a lipid phase equilibrium where the lamellar phase corresponds to an open state and the hexagonal lipid phase to the closed state of the cell contact. This dynamic lipid model is well suited to describe the morphological as well as functional properties of the tight junctions.

The role of the renal nerves in renin synthesis

1. Renin synthesis and secretion were studied in Balb/c mice with a denervated left kidney. 2. Denervation inhibited renin secretion. 3. Denervation reduced the renal renin content. 4. Denervation reduced renal renin mRNA. 5. Renal denervation inhibits renin secretion by blocking the synthetic system prior to mRNA formation.

A study of catecholamine concentrations in selected renal segments of cats of different ages

Renal concentrations of the catecholamine neurotransmitters norepinephrine (NE) and dopamine (DA) in 19 cats were measured by reverse-phase high pressure liquid chromatography (HPLC) with electrochemical detection. Animals of five different age groups (2-4, 6-8, 10-12 and 16-20 weeks and adult animals one year and older) were anesthetized and the kidneys were excised, sectioned into cortical, outer medullary and inner medullary segments, and processed for HPLC. There were no statistical differences in cortical NE concentrations between the 2-4 week and adult age groups, suggesting that peak noradrenergic nerve terminal density in the cortex is achieved at or before 2-4 weeks of age. NE concentration in the outer medulla and DA concentration in the cortex and outer medulla increased from the 2-4 to the 10-12 week age group but did not change significantly in the older animals. There were significant decreases in inner medullary NE and DA concentrations from the 10-12-week to the 16-20 week age group. The decrease in concentration of inner medullary catecholamines in the early postnatal period is consistent with the reported total loss of tyrosine hydroxylase- and dopamine beta hydroxylase-immunoreactive inner medullary nerve terminals at approximately 12 weeks of age.

Renal α2-adrenoceptors in New Zealand genetically hypertensive rats

1. Renal alpha 1- and alpha 2-adrenoceptors were characterized in the New Zealand strain of genetically hypertensive (GH) rat and the Otago random-bred albino normotensive (NT) control rat at 4 and 12 weeks of age with [3H]-prazosin and [3H]-rauwolscine. 2. At 4 weeks of age, the density of alpha-adrenoceptors in NT and GH rats was similar to both the alpha 1- (193 +/- 11 vs 163 +/- 14 fmol mg-1 protein) and alpha 2- (347 +/- 34 vs 319 +/- 41 fmol mg-1 protein) adrenoceptor. At 12 weeks of age, GH rats had a greater density of renal alpha 1- (152 +/- 27 vs 238 +/- 17 fmol mg-1 protein) and alpha 2- (175 +/- 42 vs 350 +/- 23 fmol mg-1 protein) adrenoceptors compared to the NT rats. 3. Pre-incubation of kidneys from GH rats (12 weeks of age) with 1 and 10 microM clonidine decreased the density of receptors identified by unlabelled clonidine displacement of [3H]-rauwolscine to 77% and 56% of control. Pre-incubation with adrenaline, 2,6 dimethylclonidine or phenylephrine failed to alter binding. 4. Pre-incubation of kidneys from NT rats (12 weeks) or young GH rats (4 weeks) with 10 microM clonidine failed to alter displacement of [3H]-rauwolscine by unlabelled clonidine. 5. These studies demonstrate that in another strain of hypertensive rat, the GH rat, alpha 2-adrenoceptor density is increased as compared to the normotensive control at 12 but not 4 weeks of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Free plasma catecholamine levels in healthy subjects: a basal and dynamic study. The influence of age

We studied the levels of free plasma noradrenaline (norepinephrine, NE), adrenaline (epinephrine, E), and dopamine (DA) in 143 normal subjects, either basally or, in a part of them, in response to four tests stimulating the sympathoadrenomedullary activity: the tilt-table test (TTT), the cold-pressor test (CPT), the mental-arithmetic test (MAT), and the insulin tolerance test (ITT). In both cases we evaluated the influence of age, which is considered the main physiological variable affecting the peripheral sympathetic activity. A normal distribution of values was observed, in the basal study, only for NE, while E and particularly, DA levels were distributed in an non-homogeneous fashion; a direct correlation was observed between age of subjects and NE levels, while neither E, nor DA levels showed any correlation with the aging process; sex did not influence any of the catecholamines. In the dynamic study, CPT, ITT and, above all, TTT elicited a significant rise in NE levels, while the E release was greatly stimulated by the insulin-induced hypoglycaemia. Neither NE, nor E levels rose after the MAT stimulation. Also in this case, the elderly showed an increased noradrenergic responsiveness when compared with the young subjects, confirming the existence of an 'up-regulation' of the peripheral sympathetic tone in old people.

Quantitation of tyrosine hydroxylase and neuropeptide Y immunoreactivity in single rat sympathetic neurons: effects of preganglionic nerve activity

Using computerised densitometry to measure immunocytochemical reaction product in a model system, we established conditions that produced a linear relationship between the logarithm of antigen concentration and the measured intensity of staining. We then applied the densitometric technique to assess the changes in tyrosine hydroxylase (TH) and neuropeptide tyrosine (NPY) within sympathetic neurons of rat superior cervical ganglion following chronic decentralization and following reserpine treatment. One week after surgical or pharmacological decentralization, there was appreciable reduction of neuronal levels of both TH and NPY. However, there remained considerable variation in the immunoreactivities of individual cells. Three days of treatment with reserpine elevated TH levels but substantially reduced NPY. Both these effects were prevented by prior decentralization of the ganglia. No differences were seen between normotensive and the Otago strain of genetically hypertensive rats, either in basal TH or NPY immunoreactivities or in responses to the maneuvers performed. Comparison of our findings with previous biochemical data indicate that densitometric immunocytochemistry provides an accurate index of neuronally localised antigen concentrations but also allows analysis of interneuronal differences that are not otherwise apparent.

Neuropeptide Y in rat sympathetic neurons is altered by genetic hypertension and by age

We have used immunocytochemistry to quantitate neuronal neuropeptide Y in superior cervical ganglia of a strain of normotensive Wistar-Otago rats and a related genetically hypertensive strain over the age range 1-60 weeks. The numbers of neuropeptide Y-immunoreactive cells and total ganglionic cell numbers were both greater in ganglia of young normotensive than in those of hypertensive rats. Between 10 and 60 weeks of age, peptide immunoreactivity and total cell numbers both fell in normotensive rat ganglia but remained constant in ganglia from hypertensive rats. Densitometric analysis showed that the concentrations of neuropeptide Y were similar in neurons of age-matched individuals of both strains, but during aging there was a substantial decline in neuronal peptide content that was similar in both strains and that was not accompanied by any decline in neuronal immunoreactivity for tyrosine hydroxylase. Our results suggest that there is a developmental abnormality of neuropeptide Y in sympathetic neurons of this strain of genetically hypertensive rat and that, furthermore, the aging process is accompanied by a selective loss of neuronal neuropeptide Y that is independent of blood pressure status.

Effects of 6-hydroxydopamine in hypertensive adult miniature swine

To validate a miniature swine model of sympathectomy, six swine that had chronic high blood pressures for unknown reasons and five DOCA hypertensive swine were treated with a single dose of the neurotoxin 6-hydroxydopamine (6-OHDA) (50 mg/kg i.v.). One week after 6-OHDA, conscious mean arterial pressure (MAP) had fallen by 47-49 mmHg and the pressor response to tyramine was attenuated in both groups. Norepinephrine content was significantly decreased in the kidneys (greater than 85%) and left ventricle (greater than 94%) in both 6-OHDA treated groups. These results indicate that 6-OHDA can be used to effectively sympathectomize adult swine. Chemical sympathectomy of swine with either unexplained high blood pressure or experimentally induced DOCA hypertension resulted in an equivalent fall in MAP in both of these populations. Further studies using 6-OHDA in miniature swine may help to elucidate the mechanisms involved in maintaining hypertension in this animal model.

Effects of renal denervation and atrial natriuretic factor on tubular reabsorption in anaesthetized rats

1. The effects of acute unilateral renal denervation were examined in 17 anaesthetized rats. Renal haemodynamic changes were monitored using standard clearance techniques. Lithium clearance was used to assess fractional proximal sodium and water reabsorption. 2. Denervation resulted in ipsilateral renal vasodilatation with marked natriuresis and diuresis, a small increase (15%, P less than 0.05) in glomerular filtration rate (GFR) and a consequent reduction in filtration fraction. Fractional lithium reabsorption decreased (67.3 +/- 2.9% to 54.5 +/- 4.0%, P less than 0.01) and absolute proximal reabsorption did not change, indicating impairment of proximal glomerulotubular balance (GTB). No similar changes in haemodynamic or transport parameters were observed in the contralateral, innervated kidney, although vascular resistance increased. 3. In 9 experiments following denervation of the left kidney, systemic low dose infusion (10 ng/min) of atrial natriuretic factor (ANF) resulted in a fall in mean arterial blood pressure from 116 +/- 3 mmHg to 107 +/- 3 mmHg (P less than 0.05). In the denervated kidney ANF increased urine flow rate and sodium excretion to rates above those established following denervation alone. However, in the right kidney, despite the increased filtered load (35%, P less than 0.01), the natriuretic and diuretic responses to ANF were abolished. 4. In the denervated kidney, ANF further reduced the fractional reabsorption of lithium from 53.6 +/- 2.3% to 45.6 +/- 3.8% (P less than 0.05). GFR increased by 32% (a total of 49% higher than during pretreatment) but absolute proximal reabsorption (APR) did not change. However, in the right, innervated kidney ANF infusion produced a 35% increase in GFR accompanied by a 53% rise in APR. 5. It is concluded that the natriuresis induced by unilateral denervation is due predominantly to impaired proximal GTB. The natriuretic action of ANF was associated with further impairment of proximal GTB, not dependent upon decreasing activity of renal sympathetic nerves, but was abolished when filtration fraction and renal sympathetic tone were elevated.

Rat renal epinephrine synthesis

Rats that underwent adrenal demedullation had a 93% decrease in plasma epinephrine (E) levels, but did not decrease their renal E. Even further treatment with 6-hydroxydopamine and reserpine failed to lower renal E levels. Similarly, urine E levels failed to decrease after adrenal demedullation and renal denervation. There is a renal E-synthesizing enzyme that differs from adrenal phenylethanolamine-N-methyltransferase (PNMT) in that it is only weakly inhibited by SKF 29661 and can synthesize epinine from dopamine, while adrenal PNMT does so poorly. When an adrenalectomized rat received intravenous [3H]methionine, its urine contained radioactivity that appeared to be [3H]E, with small amounts of [3H]epinine. However, after [3H]methionine was infused in the renal artery, the major product in urine appeared to be [3H]epinine, with a small amount of [3H]E. Adrenal demedullation induced renal E synthesis, but denervation returned the rate of renal E synthesis to control values. The combination of adrenal demedullation, 6-hydroxydopamine, and reserpine treatments increased renal E-forming activity to 350% of control. We conclude that appreciable portions of renal and urinary E are synthesized in the kidney by an enzyme distinct from PNMT. The enzyme is induced by some treatments that lower E and NE levels.

Substance P immunoreactivity in the superior cervical ganglia of normotensive and genetically hypertensive rats

Substance P-like immunoreactivity (SPI) was investigated in the superior cervical ganglion of normotensive and genetically hypertensive (GH) Otago Wistar rats aged 1, 2, 8-10 and 50-60 weeks, by used of an indirect immunoperoxidase method. SPI was not seen in neuronal cell bodies but a subpopulation of ganglion cells was supplied by SP-positive terminals which closely invested the cell surface. This subpopulation showed no particular topographical distribution. The number of SP-positive terminal varicosities per unit area was several times higher in GH rats than in normotensive rats at all ages over 2-60 weeks. The proportion of neurons supplied by SP-positive terminals (sampled in 8-10 week-old rats) was also greater in GH than in normotensive rats. Decentralization of the ganglion or chronic capsaicin treatment removed all SP-immunoreactive terminals around the cell bodies, indicating that the SP-positive terminals are collaterals of thoracic sensory afferents. As SP has been reported to have an excitatory effect in sympathetic ganglia, intraganglionic release of SP might contribute to the development of hypertension in the GH strain.

Adrenoceptors in the kidney: localization and pharmacology

The kidney plays a key role in the regulation of blood pressure. The sympathetic nervous system can influence many aspects of kidney function in relation to blood pressure control, e.g. renal vascular tone, intrarenal renin release and tubular reabsorption of electrolytes and fluid. The intrarenal distribution of adrenoceptors has now been studied on the basis of modern receptor ligand binding techniques combined with microscopic studies. The preferential localization of each adrenoceptor subtype within the kidney is reviewed. Furthermore, an attempt is made to describe the functional correlation of the presence of different adrenoceptor subtypes. Finally, the possible role of renal adrenoceptor abnormalities in the pathogenesis of hypertension is discussed.

Localization of sympathetic and sensory neurons innervating the rat kidney

Following injection of horseradish peroxidase (HRP) into the hilar region of the left kidney of the rat, 66% of labeled sympathetic neurons were located in the ipsilateral paravertebral ganglia, with most cells in T13 and L1, and 14% were located in equivalent segments of the contralateral chain. A similar distribution of sympathetic neurons projected to the right kidney, with most cells in T12 and T13 paravertebral ganglia. Only 20% of the total sympathetic supply to either kidney arose from the prevertebral ganglia. The renal sensory innervation was also bilateral in origin, with about 80% of the neurons arising from ipsilateral dorsal root ganglia. Injection of HRP into the caudal and rostral poles of the left kidney labeled paravertebral neurons which were concentrated in ganglia L1 and T13, respectively, but did not label any sensory neurons. We conclude that most of the renal sympathetic innervation is paravertebral in origin, and that a substantial bilateral component exists for both sympathetic and sensory supplies. Neurons arising from the contralateral side have their cell bodies in segments that provide the main ipsilateral innervation to the same kidney. The majority of sensory axons appear to be restricted to subcortical areas.

Neuronal and nonneuronal contributions to renal catecholamine content in the dog

Endogenous noradrenaline and 3,4-dihydroxyphenylethylamine (dopamine) levels were measured in different zones of the dog kidney following chronic unilateral renal denervation. In outer and inner renal cortex, and in outer medulla, greater than 95% of the tissue content of both catecholamines was contributed by renal nerves, whereas in inner medulla only nonneuronal catecholamines were found. The amounts of neuronal dopamine present in outer renal cortex were greater than would be expected for a population of solely noradrenergic nerves.