Possible involvement of presynaptic alpha 1-adrenoceptors in the effects of idazoxan and prazosin on 3H-noradrenaline release from tail arteries of SHR

Purinergic modulation of vascular sympathetic neurotransmission

It is generally agreed that the release of norepinephrine (NE) is inhibited by activation of prejunctional purinoceptor. We examined the pharmacological properties of purinoceptors on vascular sympathetic nerve terminals and the source of endogenous adenyl purines. Electrically (1 Hz) evoked NE-release was inhibited by not only P1-agonists but also P2-agonists. Although the inhibition induced by P2-agonists was blocked by P1-antagonists, P2-agonists-induced inhibition was not due to the breakdown to adenosine. Therefore, there may be a new class of purinoceptor that is activated by both P1- and P2-agonists and antagonized by P1-antagonists. Electrical stimulation at 8 Hz but not at 1 Hz evoked the release of adenyl purines such as ATP, ADP, AMP and adenosine, in addition to NE; and the purines-release was blocked by an alpha1-antagonist. Methoxamine, an alpha1-agonist, also evoked the release of purines. Electrically (1 Hz)-evoked NE-release was inhibited by methoxamine, and this inhibition was blocked by not only an alpha1-antagonist but also a P1-antagonist. Therefore, the activation of alpha1-adrenoceptor appeared to release purines, which in turn inhibited NE-release via prejunctional purinoceptors. From these results, it is suggested that the unique purinoceptor and the endogenous purines released from alpha1-adrenoceptor-sensitive sources participate in the antidromic transsynaptic modulation of vascular sympathetic neurotransmission.

Transient expression of alpha-1B adrenoceptor messenger ribonucleic acids in the rat superior cervical ganglion during postnatal development

We have examined the developmental profile of the alpha-1 and alpha-2 adrenergic receptor messenger ribonucleic acids expression in the rat superior cervical ganglion. The expression of the six messenger ribonucleic acids was studied using reverse transcription-polymerase chain reaction. At four weeks, the dominant messenger ribonucleic acids transcripts in this sympathetic ganglion were alpha-1C, alpha-2A, alpha-2B and alpha-2C. The expression of alpha-1 genes in the superior cervical ganglion appears to be regulated during postnatal development in that two alpha-1 (alpha-1B, alpha-1C) genes were expressed at birth, three, seven and 14 days postnatal but no amplified product for alpha-1B was detected at 28 days and in the aged animals, while the alpha-1C transcript continued to be expressed. No amplified product for alpha-1D was detected in superior cervical ganglion at any of the ages studied. While all three alpha-2 genes were expressed in the superior cervical ganglion at four weeks the dominant alpha-2 messenger ribonucleic acids transcript expressed in the superior cervical ganglion was alpha-2A. This pattern of alpha-2 adrenoceptor gene expression was maintained from birth, throughout development and into old age. These results suggest that the expression of alpha-1 adrenergic receptors in the superior cervical ganglion is regulated developmentally while the expression of alpha-2 genes remains unchanged.

Involvement of pertussis toxin-sensitive and -insensitive mechanisms in alpha-adrenoceptor modulation of noradrenaline release from rat sympathetic neurones in tissue culture

1. Sympathetic neurones derived from superior cervical ganglia of neonatal rats and maintained in tissue culture were used to investigate the modulation of neurotransmitter release by presynaptic receptors. Three week old cultures of neurones were loaded with [3H]-noradrenaline to label endogenous neurotransmitter stores. Release of noradrenaline was evoked by depolarization with raised extracellular K+ in the presence of desipramine and corticosterone to prevent uptake of released catecholamine. 2. Potassium (55 mmol l-1) depolarization for 30 s caused more than a four fold increase in 3H overflow from basal levels but this increase was reduced by up to 40% in the presence of exogenous noradrenaline (1 mumol l-1). The inhibition by noradrenaline of depolarization-evoked overflow was blocked by the alpha 1/alpha 2-adrenoceptor antagonist, phentolamine. Phentolamine alone did not increase K(+)-evoked 3H overflow. 3. The alpha 2-adrenoceptor antagonist, yohimbine, produced a concentration-dependent block of the inhibition by noradrenaline of K(+)-evoked overflow, while the alpha 1-adrenoceptor antagonist, prazosin, was without effect at concentrations up to 0.1 mumol l-1. 4. The beta-adrenoceptor antagonist, propranolol, neither reduced K(+)-evoked overflow nor increased the degree of inhibition caused by the addition of 1 mumol l-1 noradrenaline. 5. The alpha 2-adrenoceptor agonist, clonidine (1 mumol l-1) was less effective than noradrenaline at inhibiting K(+)-evoked overflow, while the alpha 1-adrenoceptor agonist, phenylephrine (1 mumol l-1) had no significant effect. 6. The L-channel calcium blocker, nicardipine (1 mumol l-1) significantly inhibited 3H overflow evoked by K+. In the presence of L-channel block, however, noradrenaline still inhibited residual evoked overflow.7. In the presence or absence of nicardipine, pertussis toxin pretreatment (1 nmol 1-1) reduced, but did not prevent, the effect of noradrenaline (1 micromol 1-1). Pertussis toxin alone caused a significant enhancement of K+-evoked 3H overflow.8. The data indicate that on postganglionic neurones of cultured rat sympathetic ganglia there are alpha 2-adrenoceptors that modulate neurotransmitter release, but no functional beta-adrenoceptors that mediate an enhancement of transmitter release. The data suggest further that in this preparation the mechanism of alpha2-adrenoceptor modulation may involve pertussis toxin sensitive and insensitive G-proteins and effects on calcium channels other than L-type.

Methoxamine enhances the release of endogenous noradrenaline from rabbit ear artery: possible involvement of ATP

The effect of methoxamine, an alpha 1-adrenoceptor agonist, on the electrically-evoked release of endogenous noradrenaline was examined in the isolated rabbit ear artery. Noradrenaline was quantified by high performance liquid chromatography-electrochemical detection. The release of adenine nucleotides and nucleosides by methoxamine was examined using high performance liquid chromatography-fluorescence detection. The release of noradrenaline evoked by electrical field stimulation (EFS) at 4 Hz was reduced by tetrodotoxin 0.3 mumol/l and clonidine 1 mumol/l by approximately 80% and 50%, respectively. On the other hand, methoxamine at 10 but not 1 mumol/l enhanced the release of noradrenaline to approximately twice the control, and the enhancement was prevented by prazosin 1 mumol/l. The facilitatory action of methoxamine was also abolished after desensitization of P2-purinoceptors by alpha,beta-methylene ATP 30 mumol/l as well as by the presumed P2-purinoceptor antagonist suramin given at 10 mumol/l. Exogenous ATP 10 mumol/l significantly enhanced the EFS-evoked release of noradrenaline, and the enhancement was abolished by alpha,beta-methylene ATP and suramin. None of the drugs changed the spontaneous outflow of noradrenaline. These results indicate that endogenous ATP, acting at prejunctional purinoceptors, may participate in the facilitatory effect of methoxamine. Indeed, methoxamine 10 mumol/l significantly enhanced the spontaneous outflow of ATP and, less so, ADP. The methoxamine evoked release of ATP and ADP was antagonized by prazosin 1 mumol/l. It is concluded that methoxamine releases endogenous ATP from postjunctional sites which then, via prejunctional purinoceptors, facilitates action potential-evoked release of noradrenaline in rabbit ear artery.

Methoxamine inhibits noradrenaline release through activation of alpha 1- and alpha 2-adrenoceptors in rat isolated kidney: involvement of purines and prostaglandins

The effects of the alpha 1-adrenoceptor agonist methoxamine and the alpha 2-adrenoceptor agonist bromoxidine (UK 14034) on the stimulation induced (S-I) outflow of radioactivity at 100 Hz/6 pulses from rat isolated kidney preincubated with 3H-noradrenaline were investigated. Methoxamine (0.3-30 mumol/l) inhibited S-I outflow of radioactivity to a maximum of 83% with a pEC50 of 5.85 (5.71-5.94). UK 14304 (0.0003-0.3 mumol/l) inhibited S-I outflow of radioactivity to a maximum of 99% with a pEC50 of 8.35 (8.26-8.47). alpha-Adrenoceptor antagonist affinities (pKD) against methoxamine and UK 14304 at prejunctional alpha-adrenoceptors were determined. The concentration response curve of methoxamine was shifted to the right by the alpha 1/alpha 2B-adrenoceptor antagonist prazosin (0.1 mumol/l) with a pKD of 7.41 and that of UK 14304 by prazosin (0.3 mumol/l) with a pKD of 6.24. The alpha 2-adrenoceptor antagonist rauwolscine (0.1 mumol/l) shifted the concentration response curve of UK 14304 potently to the right with a pKD of 8.34. The concentration response curve of methoxamine was shifted also to the right by rauwolscine (0.1 mumol/l) and the alpha 2-adrenoceptor antagonist idazoxan (0.1 mumol/l), however, both antagonists suppressed the maximum response of methoxamine to 46% and 56%, respectively. A ten times lower concentration of rauwolscine (0.01 mumol/l) did not shift the concentration response curve of methoxamine but the inhibitory effect of methoxamine still reached only a maximum of 59%.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological and electrochemical analysis of the secretion of ATP and noradrenaline from the sympathetic nerves in rat tail artery: effects of ?2-adrenoceptor agonists and antagonists and noradrenaline reuptake blockers

The aim of this study was to investigate whether or not nerve impulses release ATP and noradrenaline in parallel from the sympathetic nerve terminals of the rat tail artery. The extracellularly recorded excitatory junction current (EJC) was used to study, pulse by pulse, the release of ATP. An electrochemical method was used to study online the nerve stimulation-induced rise in the extracellular concentration of endogenous noradrenaline at the probe, a carbon fibre electrode (CF). This parameter, which does not directly represent noradrenaline release, but reflects release minus clearance, has been termed delta[NA]CF. The effects of a number of pharmacological agents on the EJCs were examined both at 0.1 and 2 Hz, and the effects on the EJC response to 100 pulses at 2 Hz compared with that on the delta[NA]CF response. Clonidine and xylazine were used as alpha 2-agonists, yohimbine and idazoxan as alpha 2-antagonists and desipramine and cocaine as blockers of noradrenaline reuptake. Most of these agents had unwanted side effects, especially at higher concentrations. However, clonidine and xylazine depressed at lower concentrations the EJC and delta[NA]CF responses to about the same extent; these effects were partially or completely reversed by yohimbine. Yohimbine or idazoxan did not affect the EJCs at 0.1 Hz but enhanced the EJC and delta[NA]CF responses to 100 pulses at 2 Hz to the same extent. All effects of desipramine (1 microM) seemed explainable as a result of block of noradrenaline reuptake, while cocaine (10 microM) in addition exerted an 'unspecific' depressant (probably local anesthetic) effect. Under control conditions, both agents depressed the EJC but dramatically enhanced the delta[NA]CF response to 100 pulses at 2 Hz. Addition of yohimbine prevented the depressant effect of desipramine on the EJCs completely and reduced that of cocaine, but increased their effects on the delta[NA]CF response. These results are compatible with the view that ATP and noradrenaline are released in parallel from the sympathetic nerve terminals of this tissue. The different, and under some conditions even opposite, effects of desipramine or cocaine on the EJC and delta[NA]CF responses are explainable in terms of the known post-secretory effects of these agents.

A comparison of guanfacine, bunazosin, atenolol and nadolol on blood pressure and plasma noradrenaline responses to cold pressor testing

1. The role of the presynaptic adrenoceptor subtypes in man was investigated based on observation of the changes in blood pressure (delta BP) and plasma noradrenaline concentration (delta NA) with the cold pressor test (CPT). 2. The CPT was well reproducible for BP and NA when performed at a 2 week interval in patients with mild hypertension. 3. After administration for 4 weeks, guanfacine (Gf; alpha 2-adrenoceptor agonist) decreased the delta NA response to CPT. 4. After administration for 2 or 4 weeks, bunazosin (Bu; alpha 1-adrenoceptor antagonist) atenolol (At; beta 1-adrenoceptor antagonist) and nadolol (Nd; non-selective beta-adrenoceptor antagonist) did not affect the delta NA response to CPT. 5. Both Gf and Bu decreased the systolic blood pressure response (delta SBP) to CPT after 4 weeks of the administration. Neither At nor Nd significantly changed the delta SBP response to CPT. 6. It is likely that Gf stimulated the presynaptic alpha 2 adrenoceptors at the sympathetic nerve endings as well as the central alpha 2 adrenoceptors, inhibiting the release of noradrenaline. It is unlikely that Bu, At and Nd exerted any clearly defined action on the presynaptic adrenoceptors in human hypertensive subjects.

Activation of α1- and α2-adrenoceptors inhibits noradrenaline release in rabbit renal arteries: effects of pertussis toxin and N-ethylmaleimide

1. The effects of selective alpha 1- and alpha 2-adrenoceptor agonists and antagonists on the stimulation-induced (S-I) outflow of radioactivity at 2 Hz were investigated in superfused rabbit renal arteries incubated with [3H]-noradrenaline. 2. The alpha 1-adrenoceptor agonist methoxamine (10 microM) inhibited S-I outflow of radioactivity and this effect was abolished by the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) but not by the alpha 2-adrenoceptor antagonist rauwolscine (1 microM). Neither the prostaglandin synthesis inhibitor indomethacin (10 microM) nor the adenosine receptor antagonist 8-phenyl-theophylline (1 microM) prevented the inhibitory effect of methoxamine. 3. The alpha 2-adrenoceptor agonists clonidine (0.1 microM) and UK 14304 (0.1 microM) both inhibited S-I outflow of radioactivity. The inhibitory effect of clonidine was blocked by rauwolscine but not by prazosin. The inhibitory effect of UK 14304 was markedly reduced by rauwolscine. 4. Prazosin (0.1 microM) alone did not enhance the S-I outflow of radioactivity at 2 Hz and slightly enhanced S-I outflow at 4 Hz. Rauwolscine (1 microM) alone markedly enhanced S-I outflow of radioactivity at 2 and 4 Hz. 5. Pretreatment of the arteries with pertussis toxin (1 microgram ml-1) did not significantly alter the inhibitory effects of methoxamine or UK 14304 or the potentiation by rauwolscine. However, pretreatment of the arteries with a higher concentration of pertussis toxin (5 micrograms ml-1) prevented the inhibitory effect of methoxamine but still did not affect the responses to UK 14304 and rauwolscine. 6. Pretreatment of the arteries with N-ethylmaleimide (NEM, 10 microM) for 30 min did not alter the inhibitory effect of methoxamine but markedly attenuated the inhibitory effect of UK 14304 and the facilitatory effect of rauwolscine. 7. The results suggest that both alpha 1- and alpha 2-adrenoceptors take part in the modulation of noradrenaline release from sympathetic nerves in rabbit renal arteries. Alpha 1-adrenoceptor mediated inhibition may be coupled to G-proteins which are pertussis toxin sensitive and alpha 2-adrenoceptor mediated inhibition to G-proteins which are NEM-sensitive.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Participation by purines in the modulation of norepinephrine release by methoxamine

The alpha 1-receptor agonist methoxamine reduced, by a prazosin sensitive mechanism, the nerve stimulation evoked release of norepinephrine in the rat caudal artery. The effect of methoxamine was also antagonized by the purinoceptor antagonist 8-(p-sulfophenyl)theophylline suggesting an involvement of endogenous purines in this process. Indeed, methoxamine caused the release of adenine nucleotides and adenosine, an action which was blocked by prazosin. These results suggest that methoxamine releases ATP or a related purine which in turn decreases transmitter release by acting on prejunctional purinoceptors.

Prejunctional alpha 1-adrenoceptors modify release of [3H]noradrenaline in the guinea-pig vas deferens

1. Several alpha 1- and alpha 2-adrenoceptor agonists and antagonists were examined for effects on spontaneous and stimulus-evoked release of [3H]noradrenaline from sympathetic nerves in guinea-pig vas deferens. 2. Prazosin (0.1 and 1 microM), phentolamine (30 microM) and yohimbine (10 microM) each enhanced the stimulus-evoked release of [3H]noradrenaline. 3. Prazosin and phentolamine increased the spontaneous outflow of [3H]noradrenaline, whereas yohimbine was without effect. 4. Methoxamine (10 microM) and clonidine (0.1 microM) inhibited the stimulus-evoked release of [3H]noradrenaline, whereas only methoxamine (1 microM) decreased the spontaneous outflow of [3H]noradrenaline. 5. The identity of prejunctional alpha-adrenoceptors in the guinea-pig vas deferens is discussed.

Modulation of noradrenaline release in slices of rat kidney cortex through alpha 1- and alpha 2-adrenoceptors

Slices of rat kidney cortex were incubated in [3H]noradrenaline, then placed in a flow cell and subjected to electrical field stimulation. At a stimulation frequency of 5 Hz, both the alpha 2-adrenoceptor antagonist idazoxan (0.1 microM) and the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) significantly enhanced the stimulation-induced (S-I) outflow of radioactivity from the slice. However, neither clonidine (0.1 microM) nor methoxamine (10 microM), alpha 2- and alpha 1-agonists respectively, affected the S-I outflow of radioactivity at this stimulation frequency. At a lower stimulation frequency (1 Hz), the S-I outflow of radioactivity was not affected by idazoxan or prazosin, but was inhibited by both clonidine and methoxamine. The effect of clonidine was prevented by idazoxan (0.1 microM), but not by prazosin (0.1 microM). The effect of methoxamine was abolished by prazosin (0.1 microM), but not by idazoxan (0.1 microM). The inhibitory effect of methoxamine was not prevented by the prostaglandin synthesis inhibitor indomethacin (10 microM) or the adenosine receptor antagonist 8-phenyltheophylline (1 microM) and thus was not mediated by either prostaglandins or adenosine. The results suggest that both prejunctional alpha 1- and alpha 2-adrenoceptors are directly involved in modulation of noradrenaline release from the renal sympathetic nerves of the rat.

The differential effects of prazosin and hydralazine on sympathoadrenal activity in conscious rats

The ability of the vasodilator hydralazine and the alpha 1-adrenoceptor antagonist prazosin to increase sympathoadrenal outflow was compared by measuring plasma norepinephrine and epinephrine concentrations, norepinephrine clearance and norepinephrine spillover rate into plasma in conscious Sprague-Dawley rats and spontaneously hypertensive rats (SHR). Even though the vasodepressor effect of 1 mg/kg (i.p.) of prazosin (-23 mm Hg) was significantly less than that caused by 1 mg/kg (i.p.) of hydralazine (-31 mm Hg) in normotensive rats, the increases in plasma norepinephrine concentration and norepinephrine spillover rate were significantly larger in prazosin-treated rats. In conscious SHR, 0.5 mg/kg (i.p.) of prazosin and 0.3 mg/kg (i.p.) of hydralazine lowered blood pressure to the same extent (-22 mm Hg), but prazosin again produced significantly larger increases in plasma norepinephrine concentration and norepinephrine spillover rate. Neither prazosin nor hydralazine affected norepinephrine clearance, and only prazosin elicited a significant rise in plasma epinephrine concentration. This differential effect of prazosin and hydralazine on sympathoadrenal activity is best explained by the differing effects of these drugs on venous return and thus the afferent activity of the cardiopulmonary baroreceptors.

Yohimbine-induced alterations of monoamine metabolism in the spontaneously hypertensive rat of the Okamoto strain (SHR). II. The central nervous system (CNS)

Steady state levels of monoamine neurotransmitters were examined in SHR, a genetic model of hypertension and compared to its normotensive control (WKY). SHR and WKY were also challenged with alpha 2-adrenergic antagonists, (yohimbine, YOH, idazoxan) or an alpha 1-antagonist (prazosin) and alterations in CNS monoamine metabolism evaluated. SHR were found to have elevated levels of NE and 5-HT in a number of brain regions involved in cardiovascular control when compared to WKY. DA levels and metabolism were also altered in the SHR. Blockade of alpha 2-adrenoceptors and other direct and indirect actions of YOH exacerbated the abnormalities in central monoaminergic neurotransmission in SHR. Significant decreases in NE content were produced by YOH or idazoxan treatment in both SHR and WKY, presumably the result of the inhibition of alpha 2-adrenoceptor medicated presynaptic control of NE release. YOH treatment abolished the differences in steady state levels of NE between SHR and WKY, however, idazoxan did not. YOH administration resulted in significant increases in DA and 5-HT in a number of brain regions of both SHR and WKY. Idazoxan or prazosin produced few changes in DA and 5-HT metabolism except for increases in DA content in the spinal cord and brainstem of SHR given idazoxan. The YOH-induced increases in DA and 5-HT content of SHR were of a greater magnitude than the WKY in several brain regions. DOPAC levels were significantly elevated by YOH in both WKY and SHR, reflecting the antidopaminergic properties of YOH. 5-HIAA content was significantly reduced by YOH in a number of brain regions in both SHR and WKY, however, this effect was attenuated in several brain regions in SHR. The results of the present study demonstrate the multifarious nature of the alterations in CNS monoamine metabolism in SHR.

Yohimbine-induced alterations of monoamine metabolism in the spontaneously hypertensive rat. I. The peripheral nervous system

The effects of alpha 2 adrenoreceptor blockade with YOH on blood pressure, plasma catecholamines and norepinephrine (NE) stores in kidney, adrenal and spleen of spontaneously hypertensive rats of the Okamoto strain (SHR) and Wistar-Kyoto (WKY) control animals were examined. YOH administration resulted in a significant (p less than 0.001) reduction in arterial pressure in both SHR and WKY. Plasma NE and EPI were significantly (p less than 0.05) elevated by YOH treatment in both SHR and WKY, but SHR exhibited a significantly (p less than 0.05) greater percent increase in plasma NE than WKY. YOH produced significant decreases in splenic NE content in both SHR and WKY but reduced renal NE content in the SHR only. SHR had significantly higher basal renal NE and DA content and fewer NE uptake (3H-desmethylimipramine binding) sites (p less than 0.05) than WKY. Treatment of SHR or WKY with either the alpha 2-adrenergic antagonist, idazoxan, or the alpha 1-antagonist, prazosin, failed to significantly alter renal NE levels from those found after saline injection. The enhanced YOH-induced renal NE depletion in SHR suggests an alteration in the presynaptic control of NE release in the genetically hypertensive rat, however, the effects of YOH in the SHR may be mediated by mechanisms unrelated to alpha 2-adrenergic receptors.