Plasma catecholamines and their major metabolites in spontaneously hypertensive rats

Sympathetic activation in adipose tissue and skeletal muscle of hypertensive rats

The activation of the sympathetic nervous system is a common feature of arterial hypertension and other cardiovascular diseases. This activation might be dependent on an altered baroreflex control of vascular resistance of which the inhibitory response on sympathetic activity appears impaired. The aim of the study was to monitor during the natural course of arterial hypertension in spontaneously hypertensive (SHR) and age-matched Wistar Kyoto (WKY) rats (5, 16, 30, and 54 weeks of age) the peripheral sympathetic activity expressed as interstitial norepinephrine (NE) release and as tyrosine hydroxylase (TH) activity, the rate-limiting enzyme of NE synthesis, in the differently baroreflex-controlled subcutaneous adipose tissues and skeletal muscles. Blood pressure and plasma NE in SHR were similar to WKY at 5 weeks of age but increased at all other ages. Body weight was similar in both 5-week-old rats but reduced in SHR at all other ages. The interstitial NE levels were greater in both SHR tissues at all ages as compared with WKY. In adipose tissue of SHR, TH activity was higher at all ages as compared with WKY, whereas TH activity in skeletal muscle was higher only after the development of hypertension. These data show that in both SHR tissues, an increase of interstitial NE release is always present during its lifespan. This suggests that increased sympathetic activation in the SHR model is not specific to baroreflex-controlled tissues such as skeletal muscle but involves also subcutaneous adipose tissue, the sympathetic efferents of which are independent from baroreflexes.

Norepinephrine reuptake is impaired in skeletal muscle of hypertensive rats in vivo

Certain forms of experimental hypertension are characterized by organ-specific alterations of catecholaminergic pathways. The purpose of this study was to evaluate, in the same awake and freely moving normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) before and after the development of arterial hypertension, the norepinephrine (NE) turnover and, in particular, the neuronal NE reuptake activity that ends its effects once released from nerve terminals, in subcutaneous adipose tissue and in skeletal muscle, whose sympathetic efferents are respectively independent or dependent from baroreflexes. Plasma and tissue interstitial NE and 3,4-dihydroxyphenylethylene glycol (DHPG), its major deaminated metabolite, were measured before and after blockade of NE reuptake by tissue perfusion of desipramine through microdialysis probes. Arterial pressure and plasma NE in SHR were similar to those in WKY at 5 weeks of age but increased at 16 weeks of age. In contrast, plasma DHPG was already higher in young SHR. Basal interstitial NE and DHPG were increased in both tissues of young and old SHR compared with age-matched WKY. Desipramine induced a higher rise of interstitial NE in SHR of both ages, with a lesser increase in the skeletal muscle of old compared with young SHR. These results indicate an increased NE turnover in prehypertensive and hypertensive SHR in both baroreflex-dependent and -independent tissues, not shown by plasma NE levels in young SHR. In the skeletal muscle, where sympathetic efferents are baroreflex dependent, the reduced interstitial NE reuptake contributes to the higher availability of interstitial NE for postsynaptic effects in old SHR.

Plasma norepinephrine and dihydroxyphenylglycol in essential hypertension

The aim of the present study was to examine whether essential hypertension is associated with altered plasma concentrations of dihydroxyphenylglycol, the principal presynaptic metabolite of norepinephrine. Forearm venous plasma dihydroxyphenylglycol and norepinephrine were determined at rest and during graded orthostasis in 47 normotensive control subjects and 58 outpatients with essential hypertension. There was no group difference in age. At supine rest as well as during sitting and standing, hypertensive subjects had plasma norepinephrine concentrations similar to those in normotensive control subjects, but plasma dihydroxyphenylglycol concentrations were higher than those in normotensive control subjects. Both groups showed a linear relation between plasma dihydroxyphenylglycol (ordinate) and plasma norepinephrine (abscissa). The resulting regression line was steeper (p less than 0.02) and its ordinate intercept higher (p less than 0.01) in hypertensive than in control subjects. Eleven normotensive and 14 hypertensive subjects were also tested 3 hours after desipramine (1.5 mg/kg orally) was administered to inhibit neuronal norepinephrine reuptake. The drug did not alter plasma norepinephrine, but did reduce plasma dihydroxyphenylglycol and did abolish plasma dihydroxyphenylglycol responses to upright posture in both groups of subjects. The mean plasma dihydroxyphenylglycol concentration observed in the presence of desipramine again was higher in the hypertensive than in the control group (p less than 0.01) and closely agreed, in both groups, with the dihydroxyphenylglycol concentration given by the ordinate intercept of the dihydroxyphenylglycol versus norepinephrine regression line in the absence of desipramine.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical aspects on presynaptic noradrenaline metabolism

In healthy subjects, similar absolute increments in plasma noradrenaline (NA) and dihydroxyphenylglycol (DOPEG) were observed in response to upright posture or isoprenaline infusion. Blockade of neuronal uptake by desipramine abolished these plasma DOPEG responses and reduced plasma DOPEG per se. In essential hypertensives we found higher than normal plasma DOPEG levels at any given plasma NA. Evidence is provided that both the desipramine-sensitive and -resistant pool of plasma DOPEG contribute to this hypertensive-normotensive difference.

The measurement of norepinephrine clearance and spillover rate into plasma in conscious spontaneously hypertensive rats

The clearance of norepinephrine from plasma and the spillover rate of norepinephrine into plasma were determined in conscious unrestrained spontaneously hypertensive rats by measuring the concentrations of 3H-norepinephrine and norepinephrine in arterial plasma after 90 min of i.v. infusion with 3H-norepinephrine. In 50 conscious spontaneously hypertensive rats treated with saline (control animals), the following basal values were obtained: plasma norepinephrine concentration = 149 +/- 5 pg/ml; plasma epinephrine concentration = 61 +/- 4 pg/ml; norepinephrine clearance = 188 +/- 4 ml min-1 kg-1; and norepinephrine spillover rate = 27.5 +/- 0.8 ng min-1 kg-1. A significant portion of infused 3H-norepinephrine appeared to be cleared from the plasma by the uptake1 process, since desipramine decreased norepinephrine clearance by 32%. The vasodilating agents hydralazine and minoxidil produced dose-related increases in norepinephrine spillover rate and plasma norepinephrine concentration, but the percent increases in norepinephrine spillover rate exceeded the percent increases in plasma norepinephrine concentration because of concomitant increases in norepinephrine clearance, particularly after treatment with minoxidil. The increase in norepinephrine clearance caused by hydralazine and minoxidil probably resulted from the increase in cardiac output and resultant increase in hepatic and/or pulmonary blood flow. Adrenal secretion of norepinephrine did not appear to contribute to the elevation in norepinephrine spillover rate elicited by hydralazine and minoxidil. Chlorisondamine suppressed norepinephrine spillover rate by 77%, in association with a 70% decline in plasma epinephrine concentration, whereas bretylium lowered norepinephrine spillover rate by only 41%, with no change in plasma epinephrine concentration. The decrements in norepinephrine clearance caused by chlorisondamine (-23%) and bretylium (-15%) were more or less proportional to the magnitude of the vasodepression caused by these drugs. Both norepinephrine spillover rate and clearance fell in a dose-related fashion after treatment with clonidine. After treatment with the sympathoinhibitory agents chlorisondamine, bretylium and clonidine, the percent decreases in norepinephrine spillover rate always exceeded the percent decreases in plasma norepinephrine concentration. Based on these observations, we conclude that norepinephrine spillover rate provides a more accurate measurement of the activity of the peripheral sympathetic nervous system than does plasma norepinephrine concentration in conscious spontaneously hypertensive rats.

Determinations of catechols in small volumes of plasma using ion-pair reversed phase liquid chromatography/electrochemistry

A method with improved sensitivity for detection of catechols (CA) in small volumes of plasma using an ion-pair reversed phase HPLC system with electromechemical detection is presented. Fast isocratic separations were obtained by using 7.5 cm x 4.6 mm (i.d.) reversed phase columns with 3C18 3 micron silica particles. The CA:s L-DOPA, Noradrenaline (NA), Adrenaline (A), Dihydroxybenzylamine (DHBA, i.s.), DOPAC and Dopamine (DA) were separated in less than 4 min. The performance of three different electrochemical cells was compared with respect to hydrodynamic voltammogram, band broadening effect, linearity and detection limit. The sample preparation procedure using alumina extraction of CA:s, was modified to improve recoveries and decrease dilution factors. A modified carbon paste cell (CP-O) gave a response 4-8 times higher than what is previously reported for GC cells. Detection limits were: L-DOPA 80, NA 1.25, A 1.25, DHBA 0.4, DOPAC 1.25 and DA 0.6 pg/injection. Application to plasma from rat and fish (cod) under rest, exercise and stress is reported. The method allows determination of CA:s in small volumes of plasma (less than 500 microliter) obtained several times a day from the same animal even if it is small (less than 1/2 kg), is under rest and parts of the plasma sample are to be used for analysis of other parameters than CA:s.

Sympathoadrenomedullary hyper-responsiveness to yohimbine in juvenile spontaneously hypertensive rats

We examined responses of arterial plasma levels of the sympathetic neurotransmitter, norepinephrine (NE), of the adrenomedullary hormone, epinephrine (E), and of the intraneuronal NE metabolite, dihydroxyphenylglycol (DHPG), after intravenous administration of the alpha-2 adrenoceptor antagonist, yohimbine, in conscious, freely-moving juvenile (4-week old) or mature (12-week old) rats with spontaneous hypertension (SHRs) and their normotensive Wistar-Kyoto (WKY) controls. Mature SHRs and WKY rats had similar levels of plasma catechols at rest, whereas juvenile SHRs had significantly higher levels of NE (400 +/- 109 (SD) vs 233 +/- 62 pg/ml), E (371 +/- 168 vs 148 +/- 67 pg/ml), and DHPG (800 +/- 147 vs 589 +/- 54 pg/ml). After yohimbine, average responses of NE in the juvenile SHRs were more than 5 times, of E more than 7 times, and of DHPG more than 11 times those of the juvenile WKY rats. The responses of plasma catechols to yohimbine were not excessive in mature 12-week old SHRs. The results demonstrate increased sympathoadrenomedullary activity at rest and markedly enhanced sympathoadrenomedullary responsiveness to yohimbine in juvenile but not mature SHRs and are consistent with the hypothesis that early in the development of hypertension in this laboratory animal model there is an abnormal dependence on central neural alpha-2 adrenoceptors as part of an incompletely successful compensatory mechanism for limiting sympathetic outflow.

Sympathetic activity and cardiac adrenergic receptors in one-kidney, one clip hypertension in rats

The activity of the sympathetic nervous system, as measured by levels of plasma and cardiac catecholamines and catecholamine metabolites and the function of cardiac alpha- and beta-adrenergic receptors, was evaluated at 3 days and 4 weeks after induction of one-kidney, one clip hypertension (1K1C) in the rat. At 3 days, the plasma level of norepinephrine (NE) was lower in the 1K1C group than the control group (p less than 0.01), whereas epinephrine (E) and the metabolites dihydroxymandelic acid (DOMA), dihydroxyphenylglycol (DOPEG), and normetanephrine (NMN) were similar in both groups. In addition, cardiac content of catecholamines, their metabolites, and adrenergic receptors were similar in both groups. At 4 weeks, plasma levels of NE and DOPEG were lower (p less than 0.01), whereas levels of DOMA and NMN were higher (p less than 0.02 and p less than 0.001, respectively) in the 1K1C group than the control group. Cardiac content of NE (p less than 0.01), and DOPEG (p less than 0.05) was significantly lower, whereas DOMA and NMN were significantly higher (p less than 0.01) in the 1K1C group as compared to controls. In addition, cardiac density of both alpha- and beta-adrenergic receptors was reduced in the 1K1C group, whereas receptor affinities were unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal deamination of endogenous and exogenous noradrenaline in the mesenteric artery of the spontaneously hypertensive rat

The noradrenaline (NA) content of the mesenteric arteries from spontaneously-hypertensive rats (SHR) are greater than those in arteries from normotensive Kyoto Wistar rats (WKY). The possibility that impaired neuronal monoamine oxidase (MAO) activity in mesenteric arteries from SHR rats was responsible for the differences in NA content was explored. The in-vitro formation of dihydroxyphenylethylene glycol (DOPEG) by intact segments of mesenteric arteries was used as an index of neuronal MAO activity. There were no differences in the production of DOPEG from endogenous NA by arteries from normotensive and hypertensive rats. Moreover, the formation of DOPEG from exogenous NA was similar in arteries from SHR and WKY rats. The neuronal uptake of NA was indistinguishable between mesenteric arteries from SHR and WKY rats. The results argue against an impairment of neuronal MAO in contributing to the enhanced content of NA in the mesenteric artery of the SHR rat.

Hypertension reduces the number of beta-adrenergic receptors in rat brain microvessels

Beta-adrenergic receptor function was measured in cerebral microvessels of spontaneously and DOCA-salt hypertensive rats using 125I-iodohydroxybenzylpindolol (IHYP). Both in genetic and in experimental hypertension, a significant decrease in the number of beta-receptor sites was observed, without receptor affinity changes. These results suggest that alterations of central adrenergic regulation of small vessels may participate in the pathogenetic mechanisms leading to the development of the central hypertensive disease.