Renal sympathetic nerve activation in relation to reserpine-induced depletion of neuropeptide Y in the kidney of the rat

Galanin/GMAP- and NPY-like immunoreactivities in locus coeruleus and noradrenergic nerve terminals in the hippocampal formation and cortex with notes on the galanin-R1 and -R2 receptors

By using immunofluorescence methodology, extensive galanin (GAL) and GAL message-associated peptide (GMAP)-positive terminal networks were observed in the hippocampal formation. The majority of the GAL/GMAP fibers were dopamine beta-hydroxylase- (DBH) positive, that is, they were noradrenergic. This finding was established with GAL/GMAP-DBH double-staining and with 6-hydroxy-dopamine treatment, which totally abolished all fibers in which GAL/GMAP and DBH coexisted. Also, reserpine treatment caused a marked depletion of GAL. No evidence for GAL/GMAP coexistence with 5-hydroxytryptamine was obtained. In the ventral hippocampus, GAL/GMAP-, DBH-negative fibers were seen in the stratum oriens, the anterior stratum radiatum, along the granule cell layer and in the strata oriens and alveus. In the locus coeruleus (LC), around 80% of the GMAP-positive neurons contained neuropeptide tyrosine (NPY), and about 40% of the NPY-positive neurons expressed GMAP. GAL-R1 receptor mRNA was expressed in Barrington's nucleus (close to the LC), but was not detected in the hippocampal formation/dorsal cortical areas. GAL-R2 receptor mRNA was found in the granule cell layer in the dentate gyrus. The present results show that most, but not all, immunohistochemically detectable GAL/GMAP in the hippocampal formation/dorsal cortex is present in noradrenergic nerve terminals originating in the LC, which has a robust GAL/GMAP synthesis. The functional role of GAL may be related to noradrenaline, possibly by a presynaptic action. However, the presence of GAL in other systems and of GAL-R2 receptor mRNA in granule cells also indicates other targets.

SR 120107A antagonizes neuropeptide Y Y1 receptor mediated sympathetic vasoconstriction in pigs in vivo

The effects of the neuropeptide Y Y1 receptor antagonist SR 120107A (1-[2-[2-(2-naphtylsulfamoyl)-3-phenylpropionamido]-3-[4-[N- [4- (dimethylaminomethyl)-cis-cyclohexylmethyl]amidino]phenyl]propiony l] pyrrolidine, (S,R) stereoisomer) on sympathetic non-adrenergic vasoconstriction in a variety, of vascular beds were studied in reserpinized anesthetized pigs in vivo. The rapid vasoconstrictor response evoked by single impulse stimulation, in hind limb and nasal mucosa, was not affected by SR 120107A (1.5 mg kg-1 i.v.). In contrast, SR 120107A potently inhibited the long-lasting phase of vasoconstriction evoked by high frequency (60 impulses at 20 Hz) sympathetic nerve stimulation, in the main and deep femoral, the saphenous and the internal maxillary arteries, leaving merely the initial rapid peak of vasoconstriction in these vessels. Furthermore, the vasoconstrictor response was nearly abolished in the kidney and was attenuated in the spleen and main femoral artery, despite maintained neuropeptide Y overflow. The vasoconstrictor response evoked in the kidney by peptide YY, a neuropeptide Y Y1 and Y2 receptor agonist, was also nearly abolished in the presence of SR 120107A. This inhibitory effect on the response to exogenous agonist correlated well with the long-lasting inhibition of the response to nerve stimulation in the same tissue. The peptide YY-evoked vasoconstriction in the spleen was not altered by SR 120107A, in accordance with the view that the neuropeptide Y receptor population in this organ consists mainly of neuropeptide Y Y2 receptors. SR 120107A did not influence the vasoconstrictor effects of alpha, beta-methylene ATP (mATP) or phenylephrine in any of the tissues studied. We conclude that SR 120107A is a potent neuropeptide Y Y1 receptor antagonist with long duration of action in vivo. Endogenous neuropeptide Y acting on the neuropeptide Y Y1 receptor is likely to account for the long-lasting component of the reserpine-resistant sympathetic vasoconstriction upon high frequency stimulation in hind limb and nasal mucosa. Furthermore, the peak vasoconstriction in kidney, and to some extent in spleen, is also neuropeptide Y Y1 receptor mediated.

Prejunctional regulation of reserpine-resistant sympathetic vasoconstriction and release of neuropeptide Y in the pig

The prejunctional regulation of non-adrenergic sympathetic vasoconstriction and release of neuropeptide Y (NPY) was investigated in vivo. In reserpinized pigs (with depleted noradrenaline (NA)), it was demonstrated that brief sympathetic nerve stimulation (2 pulses of 20 Hz) of the spleen, kidney and hind limb in the presence of the alpha 2-adrenoceptor agonist UK 14,304 (1 micrograms/kg per min i.v.) evoked reproducible vasoconstrictor responses which were reduced by 40-80% in comparison to that in the absence of UK 14,304. In addition, the splenic overflow of NPY-like immunoreactivity (-LI) was reduced. After cessation of the UK 14,304 infusion all these effects were reversed by addition of the alpha 2-adrenoceptor antagonist yohimbine (0.2 mg/kg i.v.). Also the Y2 receptor agonist NPY(13-36) reduced the splenic overflow of NPY-LI. Splenic vasoconstriction per se was evoked by another Y2 receptor agonist N-acetyl[Leu28Leu31]NPY(24-36), while no vascular effects in the kidney or hind limb were observed. Both Y2 agonists displaced [125I]NPY binding to splenic membranes with higher potency than the Y1-receptor agonist [Leu31Pro34]NPY(1-36). No evidence was obtained for angiotensin II mechanisms being important for the enhanced NPY release after reserpine in spite of elevated renin release. The present results show that in the absence of NA, repetition of brief sympathetic nerve stimulation evokes vascular effects and NPY-LI release which are repeatable and these effects are efficiently modulated via alpha 2-adrenoceptors. Furthermore, the Y2 receptors may mediate both prejunctional inhibition of NPY release, as well as postjunctional vasoconstrictor effects in the pig spleen.

Inhibition of sympathetic vasoconstriction in pigs in vivo by the neuropeptide Y-Y1 receptor antagonist BIBP 3226

1. Recently, a potent non-peptide antagonist of neuropeptide Y (NPY)-Y1 receptors has been developed. In this study, the selectivity of this compound, BIBP 3226, as a functional Y1 receptor antagonist, and the possible role of endogenous NPY in sympathetic vasoconstriction in different vascular beds have been investigated in anaesthetized pigs. 2. BIBP 3226 specifically displaced [125I]-NPY binding with an IC50 value of 7 nM in membranes of pig renal arteries, which also were responsive to a Y1 receptor agonist, but had only minor effects in the pig spleen (IC50 55 microM), where instead [125I]-NPY binding was markedly inhibited by a Y2 receptor agonist. IC50 values in the same nM range for BIBP 3226 were also observed in rat and bovine cortex and dog spleen. 3. In anaesthetized control pigs in vivo BIBP 3226 (1 and 3 mg kg-1) markedly inhibited the vasoconstrictor effects of the Y1 receptor agonist [Leu31, Pro34] NPY(1-36), without influencing the responses to the Y2 receptor agonist N-acetyl [Leu28, Leu31] NPY(24-36), or to noradrenaline, phenylephrine, alpha,beta-methylene adenosine triphosphate or angiotensin II. 4. High frequency stimulation of the sympathetic trunk in control pigs caused a biphasic vasoconstrictor response in nasal mucosa, hind limb and skin: there was an immediate, peak response, followed by a long-lasting vasoconstriction. BIBP 3226 (1 and 3 mg kg-1) reduced the second phase by about 50% but had no effect on the peak response. In the spleen, kidney and mesenteric circulation (which lack the protracted response) BIBP 3226 was likewise without effect on the maximal vasoconstriction, and did not influence noradrenaline overflow from spleen and kidney. 5. The corresponding S-enantiomer BIBP 3435 had only marginal influence on [125I]-NPY binding (microM range) and did not inhibit the vasoconstrictor effects of any of the agonists used, including the Y1 receptor peptide agonist. Furthermore, BIBP 3435 did not affect the response to sympathetic nerve stimulation. Both BIBP 3435 and BIBP 3226 caused a slight transient decrease in mean arterial blood pressure (by about 5 and 15 mmHg at 1 mg kg-1 and 3 mg kg-1, respectively), accompanied by splenic and mesenteric vasodilatation, suggesting that this effect was unrelated to Y1 receptor blockade. 6. The peptide YY (PYY)- and NPY-evoked vasoconstriction in the kidney of reserpine-treated pigs was markedly reduced (by 95%) by BIBP 3226 while the vasoconstrictor effect in the spleen was attenuated by only 20%. BIBP 3226 did not influence stimulation-evoked NPY release. The vasoconstrictor response in reserpine-treated pigs to single impulse stimulation, which is observed only in nasal mucosa and hind limb, was unchanged regarding maximal amplitude and the integrated effect was only moderately reduced (by about 25%) in the presence of BIBP 3226 (1 mg kg-1). BIBP 3226 (1 mg kg-1) markedly reduced (by 55-70%) the long-lasting vascular response (total integrated blood flow reduction) evoked by sympathetic nerve stimulation at high frequency (40 impulses at 20 Hz) in spleen, kidney, nasal mucosa and hind limb. Furthermore, the maximal amplitude of the vasoconstriction was reduced mainly in the kidney (by 60%) and also in the spleen (by 40%). 7. It is concluded that BIBP 3226 can act as a selective Y1 receptor antagonist in the pig. Endogenous NPY via Y1 receptor activation may play a role in evoking the long-lasting vasoconstriction seen in nasal mucosa, hind limb and skin after high frequency stimulation of sympathetic nerves in control pigs. Furthermore, NPY via Y1 receptor mechanisms seems to be of major importance for the long-lasting component of the reserpine resistant sympathetic vasoconstriction in many vascular beds, and for the maximal vasoconstrictor response in the kidney. Circulating NPY and PYY induce splenic vasoconstriction via Y2-receptors in contrast to neuronally released NPY which mainly activates Y1 receptors.

Repeated renal and splenic sympathetic nerve stimulation in anaesthetized pigs: maintained overflow of neuropeptide Y in controls but not after reserpine

The overflow and the arterial vascular effects of neuropeptide Y (NPY) in response to repeated sympathetic nerve stimulation of kidney and spleen were investigated in anaesthetized pigs. The responses under control conditions were compared to those evoked in pigs with tissue stores of noradrenaline (NA) selectively depleted by reserpine pretreatment combined with sympathetic nerve transection. The renal and splenic sympathetic nerves were repeatedly stimulated at 1 h intervals with one 5 Hz stimulation for 48 s and transmitter overflow determined. Between these stimulations, 5 min stimulations with bursts of 20 Hz (for 1 s every 10 s) were given in order to induce a depletion of nerve transmitter. In the control group, overflow of NPY and NA and vasoconstrictor responses were almost identical for the 5 consecutive stimulations in the kidney, whereas in the spleen the parameters showed a slight tendency to be reduced. In the reserpine-treated group, the initial evoked overflow of NPY was increased 8-fold and 3-fold in the kidney and spleen, respectively, compared to the control group. Upon each subsequent stimulation the overflow decreased gradually, in parallel with the evoked vasoconstrictor response. After a 2 h recovery period no change in evoked overflow of NPY compared to the amount released by the previous stimulation was observed. The present study illustrates, the high capacity of maintenance of not only NA but also NPY overflow and vascular responses in control conditions, whereas the enhanced release of NPY in the absence of NA cannot be maintained. It is therefore possible that the NA-mediated prejunctional feedback mechanism is important for the maintenance of a constant NPY release in situations of high sympathetic activation.

Effects of vinblastine on neuropeptide Y levels in the sympathoadrenal system, bone marrow and thrombocytes of the rat

The dose-related and time-related effects of vinblastine on tissue, platelet and plasma content of neuropeptide Y (NPY) were investigated in the rat and compared to the effects on catecholamine (CA) content. CA was quantified by HPLC with electrochemical detection and NPY-like immunoreactivity (LI) was analyzed by radioimmunoassay (RIA). Vinblastine (3.0 mg/kg, i.v.) decreased levels of both NPY-LI and CA after 48 h in the kidney, vas deferens and adrenal gland, whereas in the coeliac ganglion and bone marrow vinblastine induced an increase of NPY-LI which occurred already at a dose of 0.3 mg/kg. Also the content of NPY-LI in platelet-poor plasma and platelets as well as the decapitation-induced increase of plasma levels of noradrenaline (NA) and adrenaline (A) were attenuated by vinblastine (3.0 mg/kg). The elevation of NPY-LI content in the kidney, coeliac ganglion and bone marrow as well as the reduced levels of NPY-LI in platelets and platelet-poor plasma was observed already after 24 hours, whereas the reduction of NPY-LI and CA in the kidney and adrenal gland was present after 2 days. Vinblastine caused a biphasic effect on the content of NPY-LI in the sympathetic nerves of the kidney with an initial increase (by 120% at 24 h) followed by a decrease (by 79% at 4 days). The effect on NA-levels, however, was only a decrease. The axonal transport of NPY-LI as revealed by accumulation above a ligation of the sciatic nerve was reduced by 27% 2 days after vinblastine 3 mg/kg. The vinblastine-evoked depletion of NPY-LI and catecholamines in the kidney as well as in the adrenal was largely prevented by chlorisondamine, a nicotinic ganglionic blocking agent, suggesting that preganglionic neuronal activity was a key factor for this effect, in contrast to the influence on the coeliac ganglion cells and the megakaryocytes in the bone marrow. Furthermore, the delayed vinblastine depletion of NPY-LI in the kidney resembled the influence of surgical axotomy while reserpine caused a more rapid and complete depletion of both NPY-LI and NA. It is concluded that the multiple effects of vinblastine on sympathetic nerves, adrenal gland and megakaryocytes/thrombocytes can be monitored by analysis of NPY and be related to interference with microtubuli function and/or neuronal activation.

Rapid decrease in neuropeptide Y gene expression in rat adrenal gland induced by the alpha 2-adrenoceptor agonist, clonidine

1 The mechanism of regulation of the neuropeptide Y (NPY) gene by pharmacological treatment with the alpha 2-adrenenoceptor agonist, clonidine, was investigated by quantitative Northern blot analysis of the effects of this drug on the NPY mRNA levels in rat adrenal gland and medulla oblongata/pons. 2 In the adrenal gland, clonidine-treatment (50 microgram kg-1, s.c., once daily) resulted in decrease in the amount of NPY mRNA to 44 +/- 10% of the control level in 24 h and then its increase to 162 +/- 16% of the control level after 5 days. Concomitant changes in putative NPY pre-mRNA species (7.0 and 3.3 kb) were observed, probably due to changes at the level of NPY gene transcription. 3 The short-term (24 h) effect of clonidine was blocked by yohimbine (5 mg kg-1, i.p., once daily). Yohimbine alone tended to increase the NPY mRNA level after 24h. 4 The recovery/increase in the NPY mRNA level in the adrenal gland after 5 days treatment with clonidine was similar to its increase after treatment with reserpine (0.5 mg kg-1, i.p., once daily). 5 NPY gene expression in the medulla oblongata/pons was not changed by short- or long-term treatment with clonidine. 6 These results suggest that clonidine suppresses NPY gene expression in the adrenal gland, probably at the level of transcription, by activation of the alpha 2-adrenoceptor.

Neuropeptide Y and catecholamine synthesizing enzymes and their mRNAs in rat sympathetic neurons and adrenal glands: studies on expression, synthesis and axonal transport after pharmacological and experimental manipulations using hybridization techniques and radioimmunoassay

The effects of reserpine treatment (10 mg/kg, i.p.) on the content of neuropeptide Y-like immunoreactivity and catecholamines were compared with the levels of mRNA coding for neuropeptide Y, tyrosine hydroxylase and phenylethanolamine N-methyltransferase in rat sympathetic neurons and adrenal gland. A reversible depletion of neuropeptide Y-like immunoreactivity was observed in the right atrium of the heart, kidney and masseter muscle, while the immunoreactive neuropeptide Y content in the stellate and lumbar sympathetic ganglia and its axonal transport in the sciatic nerve increased following reserpine. The increase in the stellate ganglion was maximal at 48 h and absent 9 days after reserpine treatment. The expression of neuropeptide Y mRNA and tyrosine hydroxylase mRNA in both the stellate and the superior cervical ganglion increased earlier than the neuropeptide Y content, with a clear cut two-fold elevation at 24 h after reserpine. The increase in both mRNAs in the superior cervical ganglion and the depletion of neuropeptide Y, but not of noradrenaline, in terminal areas was prevented after pretreatment both with a nicotinic receptor antagonist (chlorisondamine) and with surgical preganglionic denervation. A marked (75-90%) depletion of neuropeptide Y-like immunoreactivity and adrenaline in the adrenal gland, concomitant with 3-4-fold increases in neuropeptide Y mRNA and tyrosine hydroxylase mRNA expression, was present at 24 h after reserpine treatment. Also in the adrenal gland, there was a reversal of the reserpine-induced increase in neuropeptide Y mRNA and tyrosine hydroxylase mRNA and depletion of neuropeptide Y and adrenaline following splanchnic denervation. Pharmacological, ganglionic blockade prevented the depletion of neuropeptide Y and the increased expression of neuropeptide Y mRNA, but not fully, the tyrosine hydroxylase mRNA elevation. In addition, a marked decrease in phenylethanolamine N-methyltransferase mRNA levels was noted after reserpine. This decrease was reversed by denervation and by ganglionic blockade. Denervation alone led to a small but significant decrease in all mRNAs examined both in the superior cervical ganglion and the adrenal medulla. The present data suggest that the depletion of neuropeptide Y-like immunoreactivity in sympathetic nerves and in the adrenal gland after reserpine is associated with a compensatory increase in neuropeptide Y synthesis and axonal transport, most likely due to increased nicotinic receptor stimulation. Whereas the reserpine depletion of neuropeptide Y in both sympathetic nerves and adrenal gland is related to neuronal activation, adrenal but not nerve terminal depletion of catecholamines can be prevented by the ganglionic blocker chlorisondamine.4+e difference in effect of pharmacological ganglionic

Effect of reserpine and colchicine on neuropeptide mRNA levels in the rat hypothalamic paraventricular nucleus

Using in situ hybridization and immunohistochemistry, we have studied mRNA and peptide levels in the hypothalamic paraventricular nucleus (PVN) 24 h after a single large dose of reserpine (10 mg/kg, i.p.) and 24 h after an intraventricular (i.c.v.) injection of colchicine (120 microliters/20 microliters saline). Sections of the PVN were hybridized using synthetic oligonucleotide probes complementary to mRNA for corticotropin-releasing hormone (CRH), neurotensin (NT), enkephalin (ENK), vasoactive intestinal polypeptide (VIP) and thyrotropin-releasing hormone (TRH). For immunohistochemistry rabbit antisera to CRH, NT, ENK, VIP and TRH were used. In situ hybridization showed a clear increase in CRH mRNA as compared to control rats after both treatments. Also NT and VIP mRNA could be seen in parvocellular neurons in reserpine and in colchicine-treated rats, whereas we so far have not been able to demonstrate these mRNAs in untreated rats. No changes in TRH mRNA could be detected after reserpine of colchicine. These results provide final evidence that subpopulations of parvocellular PVN neurons can synthesize not only CRH and ENK, but also NT and VIP, in agreement with earlier immunohistochemical results. With immunochemistry, after reserpine, many CRH-, but no NT- or VIP- positive neurons could be observed in the parvoecellular part of the PVN. The present results demonstrate that treatment with two drugs, the monoamine depleting drug reserpine and the mitosis inhibitor colchicine, causes increased levels of mRNA for several peptides in neurons of the PVN, located almost exclusively in its parvocellular part and being part of the hypothalamo-pituitary adrenal axis.

Decrease of catecholamine and neuropeptide Y-like immunoreactivity in the glycerol-induced acute renal failure of rats

Changes of catecholamine and neuropeptide Y (NPY) were investigated in experimental acute renal failure (ARF) of rats. Concentrations of noradrenaline (NA) and dopamine (DA) were determined by chromatographic analysis using electrochemical detection. Renal content of NPY, identified by radioimmunoassay, was expressed as NPY-like immunoreactivity (NPY-LI). All animals with a plasma urea value higher than 200 mg/dl induced by injection of glycerol were employed as ARF subjects for the experiment. Formation of ARF was also confirmed by histological findings showing diffused necrosis of tubular epithelia. In ARF rats, renal contents of NA and DA decreased markedly (P less than 0.001), NA (ng/g wet tissue) decreased from 186.3 +/- 19.6 to 2.81 +/- 0.67 (n = 8), and DA (ng/g wet tissue) decreased from 14.69 +/- 4.97 to 4.05 +/- 2.66 (n = 8). Similarly, NPY-LI (pg/g wet tissue) in ARF was reduced significantly (P less than 0.001) from 435.23 +/- 35.82 to 4.61 +/- 0.52 (n = 8). The decrease of NA in ARF was obtained parallel to the change of NPY-LI; degeneration of adrenergic nerve fibers was confirmed by immunohistochemical observation. Results obtained suggest damage to the adrenergic and the dopaminergic innervation in the kidneys during ARF.

Neuropeptide Y and sympathetic neurotransmission

The coexistence of neuropeptide Y (NPY) with noradrenaline (NA) in perivascular nerves as well as in sympathetic nerves to muscle in the heart, spleen and vas deferens suggests a role for NPY in autonomic transmission. Sympathetic nerve stimulation or reflexogenic activation in experimental animals or man are associated with NPY release as revealed by overflow mainly upon strong activation. This difference between NPY and NA secretion may be related to the partly separate subcellular storage whereby NPY seems to be exclusively present in the large dense-cored vesicles. The NPY secretion is likely to be regulated by the local biophase concentrations of NA acting on prejunctional alpha-2-adrenoceptors since alpha-2 agonists inhibit and antagonists enhance NPY overflow, respectively. Furthermore, after NA has been depleted by reserpine, the nerve stimulation-evoked release of NPY is enhanced leading to a progressive depletion of tissue content of NPY. Exogenous NPY binds to both pre- and postjunctional receptors, inhibits NA and NPY release, enhances NA-evoked vasoconstriction and induces vasoconstriction per se. The prejunctional action of NPY which is especially noticeable in the vas deferens may serve to reduce transmitter secretion upon excessive stimulation. The long-lasting vasoconstriction evoked by sympathetic stimulation in several tissues including skeletal muscle, nasal mucosa and spleen, which remains in animals pretreated with reserpine (to deplete NA) combined with preganglionic denervation (to prevent the concomitant excessive NPY release and depletion), is mimicked by NPY and highly correlated to NPY release. Under these circumstances the NPY content in the local venous effluent reaches levels at which exogenous NPY evokes vasoconstriction.

Pharmacology of noradrenaline and neuropeptide tyrosine (NPY)-mediated sympathetic cotransmission

Pharmacological and physiological aspects for neuropeptide Y (NPY) and noradrenaline (NA) cotransmission have been studied in the peripheral sympathetic nervous control of blood vessels, heart, spleen and vas deferens. NPY coexists with NA in large dense cored vesicles and is released compared to NA mainly upon high frequency stimulation or strong reflex sympathetic activation. NPY release is inhibited via prejunctional alpha-2 adrenoceptors and adenosine receptors but facilitated by angiotensin II or beta-receptor activation. NPY exerts prejunctional inhibitory actions on both NA and NPY release, enhances the vasoconstrictor effect of NA and evokes potent, long-lasting vasoconstriction. Specific receptor mechanisms for NPY exist at both the pre- and postjunctional levels; a large amidated C-terminal portion of NPY is necessary for receptor binding, inhibition of cyclic AMP formation and vasoconstrictor effects. Denervation results in supersensitivity for both NA and NPY-evoked vasoconstriction. Reserpine pretreatment is associated with depletion of NA as well as NPY; the effect on NPY is entirely dependent on an intact nerve activity. Reserpine treatment combined with preganglionic denervation depletes NA by 99% while NPY levels are maintained intact. The characteristic appearance of the nerve stimulation evoked vasoconstrictor response with a high correlation to NPY outflow after reserpine treatment, suggests that NPY may be involved as a transmitter in a variety of vascular beds. NPY-synthesis in ganglia seems to be regulated by nicotinic receptor activity; secondary stimulation by eg reserpine stimulates and nicotine antagonists decrease NPY-synthesis. Many classical pharmacological agents including guanethidine, clonidine, yohimbine, angiotensin II, nicotine and desipramine influence NPY release. A complex interplay therefore seems to occur at both the pre- and postjunctional levels of transmission for the classical transmitter NA and the coexisting peptide NPY, creating a great diversity of chemical signalling potential.

Release and vasoconstrictor effects of neuropeptide Y in relation to non-adrenergic sympathetic control of renal blood flow in the pig

The possible involvement of neuropeptide Y (NPY) in sympathetic control of renal blood flow was investigated in the pig in vivo. Exogenous NPY caused renal vasoconstriction with a threshold effect at an arterial plasma concentration of 164 pmol 6(-1). Stimulation of the renal nerves (0.59, 2 and 10 Hz) in control animals evoked rapid and frequency-dependent reduction in renal blood flow and overflow of NPY-like immunoreactivity (NPY-LI) and noradrenaline (NA) from the kidney, suggesting co-release from sympathetic nerves. Following the administration of the alpha- and beta-adrenoceptor antagonists phenoxybenzamine and propranolol, the vasoconstrictor response to exogenous NA was reduced by 98%, whereas that of NPY was unaltered. The response to nerve stimulation with 0.59 Hz was abolished, whereas relatively slowly developing reductions in renal blood flow by 7 and 28% were obtained upon stimulation with 2 and 10 Hz respectively. The nerve stimulation-evoked overflow of NA at 0.59 and 2 Hz, but not at 10 Hz and not that of NPY-LI, was enhanced after adrenoceptor blockade. Twenty-four hours after reserpine treatment (1 mg kg-1 i.v.) the contents of NPY-LI and NA in the renal cortex were reduced by 80 and 98% respectively. Sectioning of the renal nerves largely prevented the reserpine-induced depletion of NPY-LI, but not that of NA. Nerve stimulation of the denervated kidney with 2 and 10 Hz 24 h after reserpine treatment evoked slowly developing and long-lasting reductions in renal blood flow by 6 and 52% respectively. These responses were associated with overflow of NPY-LI, which was similar to and threefold higher than that observed in controls at 2 and 10 Hz respectively, while no detectable overflow of NA occurred. Repeated stimulation with 10 Hz resulted in a progressive fatigue of the vasoconstrictor response and the associated overflow of NPY-LI, giving a high correlation (r = 0.86, P less than 0.001) between the two parameters. It is concluded that NPY is a potent constrictor of the renal vascular bed. Furthermore, although NA is the likely transmitter mediating most of the responses to low to moderate nerve activation under control conditions, the data suggest that NPY may mediate the non-adrenergic reductions in renal blood flow evoked by high-frequency sympathetic nerve stimulation after reserpine treatment.