Enhanced skeletal muscle arteriolar reactivity to ANG II after recovery from ischemic acute renal failure

In addition to the long-term renal complications, previous studies suggested that after acute renal failure (ARF), rats manifest an increased pressor response to an overnight infusion of ANG II. The present study tested whether recovery from ARF results in alterations in sensitivity to the peripheral vasculature. ARF was induced in Sprague-Dawley rats by 45 min of bilateral renal ischemia and reperfusion. Animals were allowed to recover renal structure and function for 5-8 wk, after which the acute pressor responses to ANG II were evaluated either in vivo in in situ skeletal muscle arterioles or in isolated gracilis muscle arteries in vitro. Baseline arterial pressure was not different in ARF rats vs. sham-operated controls, although ARF rats exhibited an enhanced pressor response to bolus ANG II infusion compared with control rats. Steady-state plasma ANG II concentration and plasma renin activity were similar between ARF and control rats. Constrictor reactivity of in situ cremasteric arterioles from ARF rats was enhanced in response to increasing concentrations of ANG II; however, no difference was observed in arteriolar responses to elevated PO2, norepinephrine, acetylcholine, or sodium nitroprusside. Isolated gracilis muscle arteries from ARF rats also showed increased vasoconstriction in response to ANG II but not norepinephrine. In conclusion, recovery from ischemic ARF is not associated with hypertension but is associated with increased arteriolar constrictor reactivity to ANG II. Although the mechanisms of this altered responsiveness are unclear, such changes may relate, in part, to cardiovascular complications in patients with ARF and/or after renal transplant.     

Responsiveness vs. basal activity of plasma ANG II as a determinant of arterial pressure salt sensitivity

Infusion of angiotensin II (ANG II) causes salt-sensitive hypertension. It is unclear whether this is due to the body's inability to suppress ANG II during increased salt intake or, rather, an elevated basal level of plasma ANG II itself. To distinguish between these mechanisms, Sprague-Dawley rats were instrumented with arterial and venous catheters for measurement of arterial pressure and infusion of drugs, respectively. The sensitivity of arterial pressure to salt was measured in four groups with the following treatments: 1) saline control (Con, n = 12); 2) administration of the angiotensin-converting enzyme inhibitor enalapril to block endogenous ANG II (ANG-Lo, n = 10); 3) administration of enalapril and 5 ng.kg(-1).min(-1) ANG II to clamp plasma ANG II at normal levels (ANG-Norm, n = 10); and 4) administration of enalapril and 20 ng.kg(-1).min(-1) ANG II to clamp ANG II at high levels (ANG-Hi, n = 10). Rats ingested a 0.4% NaCl diet for 3 days and then a 4.0% NaCl diet for 11 days. Arterial pressure of rats fed the 0.4% NaCl diet was lower in ANG-Lo (84 +/- 2 mmHg) compared with Con (101 +/- 3 mmHg) and ANG-Norm (98 +/- 4 mmHg) groups, whereas ANG-Hi rats were hypertensive (145 +/- 4 mmHg). Salt sensitivity was expressed as the change in arterial pressure divided by the change in sodium intake on the last day of the 4.0% NaCl diet. Salt sensitivity (in mmHg/meq Na) was lowest in Con rats (0.0 +/- 0.1) and progressed from ANG-Lo (0.8 +/- 0.2) to ANG-Norm (1.5 +/- 0.5) to ANG-Hi (3.5 +/- 0.5) rats. We conclude that the major determinant of salt sensitivity of arterial pressure is the basal level of plasma ANG II rather than the responsiveness of the renin-angiotensin system.     

High dietary salt reduces the contribution of 20-HETE to arteriolar oxygen responsiveness in skeletal muscle

The coupling of tissue blood flow to cellular metabolic demand involves oxygen-dependent adjustments in arteriolar tone, and arteriolar responses to oxygen can be mediated, in part, by changes in local production of 20-HETE. In this study, we examined the long-term effect of dietary salt on arteriolar oxygen responsiveness in the exteriorized, superfused rat spinotrapezius muscle and the role of 20-HETE in this responsiveness. Rats were fed either a normal-salt (NS, 0.45%) or high-salt (HS, 4%) diet for 4-5 wk. There was no difference in steady-state tissue Po(2) between NS and HS rats, and elevation of superfusate oxygen content from 0% to 10% caused tissue Po(2) to increase by the same amount in both groups. However, the resulting reductions in arteriolar diameter and blood flow were less in HS rats than NS rats. Inhibition of 20-HETE formation with N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) or 17-octadecynoic acid (17-ODYA) attenuated oxygen-induced constriction in NS rats but not HS rats. Exogenous 20-HETE elicited arteriolar constriction that was greatly reduced by the large-conductance Ca(2+)-activated potassium (K(Ca)) channel inhibitors tetraethylammonium chloride (TEA) and iberiotoxin (IbTx) in NS rats and a smaller constriction that was less sensitive to TEA or IbTx in HS rats. Arteriolar responses to exogenous angiotensin II were similar in both groups but more sensitive to inhibition with DDMS in NS rats. Norepinephrine-induced arteriolar constriction was similar and insensitive to DDMS in both groups. We conclude that 20-HETE contributes to oxygen-induced constriction of skeletal muscle arterioles via inhibition of K(Ca) channels and that a high-salt diet impairs arteriolar responses to increased oxygen availability due to a reduction in vascular smooth muscle responsiveness to 20-HETE.     

ANG II-induced hypertension and the role of the area postrema during normal and increased dietary salt

It has been shown that the area postrema (AP) plays a role in the development of certain types of chronic angiotensin II (ANG II)-induced hypertension in the rat but is not of great importance in the salt sensitivity of arterial pressure. It has recently been proposed, however, that elevated sodium levels may exacerbate the hypertensive effects of ANG II, which by itself dramatically affects salt sensitivity, by acting at sodium-sensing neurons in certain circumventricular organs of the brain. Thus the interactions of ANG II, sodium, and the central nervous system remain to be fully understood. The purpose of this study was to examine the role of the AP in ANG II-induced hypertension during periods of normal and elevated dietary salt. We hypothesized that an intact AP was necessary for the full development of hypertension under chronic ANG II infusion and that its role would be pronounced during periods of increased dietary sodium. To test this, male Sprague-Dawley rats underwent ablation of the area postrema (APx, n = 6) or sham operation (sham, n = 6). After 3 wk of recovery, rats were instrumented with radiotelemetry transducers for constant blood pressure and heart rate monitoring and venous catheters for vehicle infusion. After a 3-day control period of 0.9% saline infusion (7 ml/day) and 0.4% dietary sodium, a 10-day period of ANG II infusion (10 ng.kg(-1).min(-1)) was begun, immediately followed by a second 10-day period during which rats were fed a 4.0% sodium diet. By day 6 of ANG II infusion, mean arterial pressure (MAP) in APx rats had increased to 139 +/- 4 mmHg, whereas MAP in sham rats had increased to 126 +/- 3 mmHg. This difference was found to be significant and continued through day 1 of the high-salt period, after which MAP of the two groups had risen to similar levels. On day 9 of high salt, MAP was again observed to be significantly higher (162 +/- 1 mmHg) in APx rats when compared with sham rats (147 +/- 4 mmHg.) These results do not support the hypothesis that the AP is necessary for the full development of ANG II-induced hypertension at normal or elevated levels of dietary sodium.     

Increased dietary sodium alters neural control of blood pressure during intravenous ANG II infusion

Increased dietary sodium enhances both excitatory and inhibitory blood pressure responses to stimulation of the central sympathetic nervous system (SNS) centers. In addition, long-term (hours to days) administration of ANG II increases blood pressure by activation of the SNS. These studies investigated the effects of increased dietary sodium on SNS control of blood pressure during 0- to 24-h infusion of ANG II in conscious, male rats consuming either tap water or isotonic saline (Iso) for 2 to 3 wk. The SNS component (evaluated by ganglionic blockade with trimetaphan) of both control blood pressure and the pressor response to intravenous ANG II was reduced in Iso animals. Furthermore, although the pressor response to intravenous ANG II infusion was similar between groups, the baroreflex-induced bradycardia during the initial 6 h of ANG II infusion was significantly greater, whereas the tachycardia accompanying longer infusion periods was significantly attenuated in Iso animals. These data suggest that in normal rats increased dietary sodium enhances sympathoinhibitory responses during intravenous ANG II.     

Elevated salt intake impairs dilation of rat skeletal muscle resistance arteries via ANG II suppression

Vasodilator responses were assessed in resistance arteries (100-200 microm) isolated from the gracilis muscle of normotensive rats after changes in dietary salt intake. Sprague-Dawley rats were maintained on either a high-salt (HS) diet (4.0% NaCl) or a low-salt (LS) diet (0.4% NaCl) for 4-8 wk (chronic) or 3 days (short-term) with water ad libitum. One group of short-term HS rats received a continuous intravenous infusion of a low dose (5 ng x kg(-1) x min(-1)) of ANG II to prevent the ANG II suppression that occurs with HS diet. Short-term and chronic HS diet eliminated arterial dilation in response to ACh and reduced PO(2) (30-40 mmHg) and the stable prostacyclin analog iloprost. ANG II infusion preserved the response to these vasodilator stimuli in short-term HS animals. Dilator responses to sodium nitroprusside and forskolin were unaffected by HS diet. These findings suggest that ANG II suppression during HS diet impairs vascular relaxation mechanisms upstream from the cAMP and cGMP second messenger systems.     

Hypersensitivity to acute ANG II in female growth-restricted offspring is exacerbated by ovariectomy

Female growth-restricted offspring are normotensive in adulthood. However, ovariectomy induces a marked increase in mean arterial pressure (MAP) that is abolished by renin angiotensin system (RAS) blockade, suggesting RAS involvement in the etiology of hypertension induced by ovariectomy in adult female growth-restricted offspring. Blockade of the RAS also abolishes hypertension in adult male growth-restricted offspring. Moreover, sensitivity to acute ANG II is enhanced in male growth-restricted offspring. Thus, we hypothesized that an enhanced sensitivity to acute ANG II may contribute to hypertension induced by ovariectomy in female growth-restricted offspring. Female offspring were subjected to ovariectomy (OVX) or sham ovariectomy (intact) at 10 wk of age. Cardio-renal hemodynamic parameters were determined before and after an acute infusion of ANG II (100 ng·kg(-1)·min(-1) for 30 min) at 16 wk of age in female offspring pretreated with enalapril (40 mg·kg(-1)·day(-1) for 7 days). Acute ANG II induced a significant increase in MAP in intact growth-restricted offspring (155 ± 2 mmHg, P < 0.05) relative to intact control (145 ± 4 mmHg). Ovariectomy augmented the pressor response to ANG II in growth-restricted offspring (163 ± 2 mmHg, P < 0.05), with no effect in control (142 ± 2 mmHg). Acute pressor responses to phenylephrine did not differ in growth-restricted offspring relative to control, intact, or ovariectomized. Furthermore, renal hemodynamic responses to acute ANG II were significantly enhanced only in ovariectomized female growth-restricted offspring. Thus, these data suggest that enhanced responsiveness to acute ANG II is programmed by intrauterine growth restriction and that sensitivity to acute ANG II is modulated by ovarian hormones in female growth-restricted offspring.     

The susceptibility of glycogen phosphorylase to inactivation by endogenous and exogenous proteases

Phosphorylases a and b were inactivated very rapidly by a neutral, trypsin-like protease from rat intestinal muscle. With 32P-phosphorylase a as substrate, it was shown that the initial event in the inactivation was the release of a small, phosphopeptide from the N-terminus of the enzyme, leaving the original 100,000 subunit form virtually unchanged. Subsequent proteolysis was very limited, producing 85, 70 and 65,000 mol. wt. derivatives. The effects of several allosteric modulators of phosphorylase on the rates of inactivation of the two enzymes were studied. Removal of the pyridoxal phosphate cofactor from phosphorylase increased the susceptibility of the b form by three fold while the a form was unaffected. By comparison of these effects with those obtained from digestion with trypsin and chymotrypsin, it is concluded that the intestinal muscle protease has a markedly enhanced ability for inactivating enzymes in their native conformation. Assuming that this property is reflected in vivo, a possible role such neutral proteases in initiating protein degradation is advanced.     

Inhibition of calcium signaling in descending vasa recta endothelia by ANG II

The intracellular calcium ([Ca(2+)](i)) response of outer medullary descending vasa recta (OMDVR) endothelia to ANG II was examined in fura 2-loaded vessels. Abluminal ANG II (10(-8) M) caused [Ca(2+)](i) to fall in proportion to the resting [Ca(2+)](i) (r = 0. 82) of the endothelium. ANG II (10(-8) M) also inhibited both phases of the [Ca(2+)](i) response generated by bradykinin (BK, 10(-7) M), 835 +/- 201 versus 159 +/- 30 nM (peak phase) and 169 +/- 26 versus 103 +/- 14 nM (plateau phase) (means +/- SE). Luminal ANG II reduced BK (10(-7) M)-stimulated plateau [Ca(2+)](i) from 180 +/- 40 to 134 +/- 22 nM without causing vasoconstriction. Abluminal ANG II added to the bath after luminal application further reduced [Ca(2+)](i) to 113 +/- 9 nM and constricted the vessels. After thapsigargin (TG) pretreatment, ANG II (10(-8) M) caused [Ca(2+)](i) to fall from 352 +/- 149 to 105 +/- 37 nM. This effect occurred at a threshold ANG II concentration of 10(-10) M and was maximal at 10(-8) M. ANG II inhibited both the rate of Ca(2+) entry into [Ca(2+)](i)-depleted endothelia and the rate of Mn(2+) entry into [Ca(2+)](i)-replete endothelia. In contrast, ANG II raised [Ca(2+)](i) in the medullary thick ascending limb and outer medullary collecting duct, increasing [Ca(2+)](i) from baselines of 99 +/- 33 and 53 +/- 11 to peaks of 200 +/- 47 and 65 +/- 11 nM, respectively. We conclude that OMDVR endothelia are unlikely to be the source of ANG II-stimulated NO production in the medulla but that interbundle nephrons might release Ca(2+)-dependent vasodilators to modulate vasomotor tone in vascular bundles.     

Strong activation of cyclooxygenase I and II catalytic activity by dietary bioflavonoids

Cyclooxygenases (COXs) catalyze the conversion of arachidonic acid to prostaglandins (PGs), thromboxanes, and hydroxyeicosatetraenoic acids. In the present study, we investigated several dietary bioflavonoids for their ability to modulate the catalytic activity of COX I and II in vitro and also in cultured cells. We found that some of them are the most powerful direct stimulators of the catalytic activity of COX I and II known to date, increasing the formation of prostaglandin products in vitro by up to 11-fold over the controls. This stimulatory effect of bioflavonoids is enzyme specific because none of them stimulates the catalytic activity of a number of lipooxygenases tested. Compared with phenol, a prototypical COX stimulator commonly used in vitro, the naturally occurring bioflavonoids are up to 29 times more efficacious in stimulating the COX activity. Additional studies using intact cells in culture showed that some of the dietary compounds that were active in the biochemical assays also activated the formation of PGE(2) (a representative PG) when they were present at 0.01 to 1 muM concentrations. The stimulatory effect of dietary compounds on COX-mediated PG formation is far more potent in intact cells than in the in vitro assays. Mechanistically, bioflavonoids mainly acted to slow down the suicidal inactivation of the COX enzymes, but they did not appear to reactivate the inactivated enzymes. The finding of this study suggests that some of the bioflavonoids likely will serve as the naturally occurring cofactors for the COX enzymes in humans.     

Mineralocorticoid treatment attenuates activation of oxytocinergic and vasopressinergic neurons by icv ANG II

Central oxytocin (OT) neurons limit intracerebroventricular (icv) ANG II-induced NaCl intake. Because mineralocorticoids synergistically increase ANG II-induced NaCl intake, we hypothesized that mineralocorticoids may attenuate ANG II-induced activation of inhibitory OT neurons. To test this hypothesis, we determined the effect of deoxycorticosterone (DOCA; 2 mg/day) on icv ANG II-induced c-Fos immunoreactivity in OT and vasopressin (VP) neurons in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus and also on pituitary OT and VP secretion in male rats. DOCA significantly decreased the percentage of c-Fos-positive (%c-Fos+) OT neurons in the SON and PVN, both in the magnocellular and parvocellular subdivisions, and the %c-Fos+ VP neurons in the SON after a 5-ng icv injection of ANG II. DOCA also significantly reduced the %c-Fos+ OT neurons in the SON after 10 ng ANG II and tended to attenuate 10 ng ANG II-induced OT secretion. However, the %c-Fos+ OT neurons in DOCA-treated rats was greater after 10 ng ANG II, and DOCA did not affect the %c-Fos+ OT neurons in the PVN nor VP secretion or c-Fos immunoreactivity in either the SON or PVN after 10 ng ANG II. DOCA also did not significantly alter the effect of intraperitoneal (ip) cholecystokinin (62 microg) on %c-Fos+ OT neurons or of ip NaCl (2 ml of 2 M NaCl) on the %c-Fos+ OT and VP neurons. These findings indicate that DOCA attenuates the responsiveness of OT and VP neurons to ANG II without completely suppressing the activity of these neurons and, therefore, support the hypothesis that attenuation of OT neuronal activity is one mechanism by which mineralocorticoids enhance NaCl intake.     

Central mechanisms of acute ANG II modulation of arterial baroreflex control of renal sympathetic nerve activity

Short-term intravenous infusion of angiotensin II (ANG II) into conscious rabbits reduces the range of renal sympathetic nerve activity (RSNA) by attenuating reflex disinhibition of RSNA. This action of ANG II to attenuate the arterial baroreflex range is exaggerated when ANG II is directed into the vertebral circulation, which suggests a mechanism involving the central nervous system. Because an intact area postrema (AP) is required for ANG II to attenuate arterial baroreflex-mediated bradycardia and is also required for maintenance of ANG II-dependent hypertension, we hypothesized that attenuation of maximum RSNA during infusion of ANG II involves the AP. In conscious AP-lesioned (APX) and AP-intact rabbits, we compared the effect of a 5-min intravenous infusion of ANG II (10 and 20 ng x kg(-1) x min(-1)) on the relationship between mean arterial blood pressure (MAP) and RSNA. Intravenous infusion of ANG II into AP-intact rabbits resulted in a dose-related attenuation of maximum RSNA observed at low MAP. In contrast, ANG II had no effect on maximum RSNA in APX rabbits. To further localize the central site of ANG II action, its effect on the arterial baroreflex was assessed after a midcollicular decerebration. Decerebration did not alter arterial baroreflex control of RSNA compared with the control state, but as in APX, ANG II did not attenuate the maximum RSNA observed at low MAP. The results of this study indicate that central actions of peripheral ANG II to attenuate reflex disinhibition of RSNA not only involve the AP, but may also involve a neural interaction rostral to the level of decerebration.     

L-arginine-induced glomerular hyperfiltration response: the roles of insulin and ANG II

Infusion of L-arginine produces an increase in glomerular filtration via kidney vasodilation, correlating with increased kidney excretion of nitric oxide (NO) metabolites, but the specific underlying mechanisms are unknown. We utilized clearance and micropuncture techniques to examine the whole kidney glomerular filtration rate (GFR) and single nephron GFR (SNGFR) responses to 1) L-arginine (ARG), 2) ARG+octreotide (OCT) to block insulin release, 3) ARG+OCT+insulin (INS) infusion to duplicate ARG-induced insulin levels, and 4) losartan (LOS), an angiotensin AT-1 receptor blocker, +ARG+OCT. ARG infusion increased GFR, while increasing insulin levels. OCT coinfusion prevented this increase in GFR, but with insulin infusion to duplicate ARG induced rise in insulin, the GFR response was restored. Identical insulin levels in the absence of ARG had no effect on GFR. In contrast to ARG infusion alone, coinfusion of OCT with ARG reduced proximal tubular fractional and absolute reabsorption potentially activating tubuloglomerular feedback. Losartan infusion, in addition to ARG and OCT (LOS+ARG+OCT), restored the increase in both SNGFR and proximal tubular reabsorption, without increasing insulin levels. In conclusion, 1) hyperfiltration responses to ARG require the concurrent, modest, permissive increase in insulin; 2) inhibition of insulin release after ARG reduces proximal reabsorption and prevents the hyperfiltration response; and 3) inhibition of ANG II activity restores the hyperfiltration response, maintains parallel increases in proximal reabsorption, and overrides the arginine/octreotide actions.     

Biological activity of the novel cyclic angiotensin II analogue [Sar1,Lys3,Glu5]ANG II

Summary This study was undertaken to investigate the biological activity of the cyclic amide-linked analogue of angiotensin II (ANG II), [Sar¹,Lys³,Glu⁵]ANG II, in both ex vivo and in vivo experiments. This constrained analogue was designed on the basis of a recently suggested conformational model for ANG II-induced receptor activation, which is characterized by a Tyr-Ile-His backbone bend and the clustering of the three aromatic rings (Tyr, His, Phe). After [Sar¹,Lys³,Glu⁵]ANG II was found to have contractile activity (15% of ANG II in the rat uterus assay), it was administered in anesthetized rabbits where it produced an immediate and dose-dependent increase in blood pressure, which peaked within minutes, was sustained as long as the drug was given, and was gradually returned to baseline after discontinuation of the drip. The blood pressure response to the cyclic analogue was of less magnitude compared to that elicited by an isovolemic and equimolar solution of ANG II. These data confirm the importance of a properly oriented ring cluster, allowing the charge-relay conformation proposed for ANG II.     

Differential response of cardiac fibroblasts from young adult and senescent rats to ANG II

The intracardiac ANG II-forming pathway is activated in the senescent myocardium, raising the possibility of enhanced ANG II effects on cardiac fibroblasts. This study established an in vitro model of cultured cardiac fibroblasts from aged rats to examine if the response of these cells to ANG II is modified in the aged heart. Levels of mRNA encoding renin, angiotensinogen, and the AT(1) receptor subtype in cardiac fibroblasts from young adult and senescent rats were quantified by RT-PCR, net collagen production by a hydroxyproline-based assay, and transforming growth factor (TGF)-beta levels using a commercial kit. In cardiac fibroblasts from young adult rats, ANG II significantly enhanced AT(1) mRNA levels, net collagen production, and TGF-beta production. In fibroblasts from the aged myocardium, ANG II downregulated AT(1) mRNA expression, had a less pronounced effect on net collagen production, and had no effect on TGF-beta production. Such age-related modification of the response of cardiac fibroblasts to ANG II may counteract the effects of augmented intracardiac ANG II production in the senescent heart, limiting fibrogenesis.     

Role of plasma ANG II in the excretion of acute sodium load in a bird with salt glands (Anas platyrhynchos)

This study was designed to further examine the role of plasma ANG II in the excretion of sodium in the Pekin duck, a bird with salt glands. Renal and extrarenal (salt gland) excretion of an intravenously administered isotonic saline load was monitored over a 4-h period in a group of eight birds under two conditions: the control condition, in which isotonic saline infusion decreased endogenous plasma ANG II from 102.6 to 16.5 pg/ml, and the experimental condition, in which ANG II suppression was prevented by intravenous infusion of a 3.5 ng. kg(-1). min(-1) dose of synthetic ANG II. ANG II infusion significantly decreased the total sodium excretion (by 15%), primarily via an inhibition of salt gland output. The results suggest that ANG II suppression facilitates the excretion of an administered sodium load in birds with salt glands.     

Desensitization to ANG II in guinea pig ileum depends on membrane repolarization: role of maxi-K+ channel

Desensitization of ANG II tonic contractile response of theguinea pig ileum is related to membrane repolarization determined byCa²⁺-activatedK⁺(maxi-K⁺) channelopening. ANG II-stimulated depolarized myocytes presented sustainedactivation of maxi-K⁺ channels,characterized by reduction from 415 to 12 ms of the closed timeconstant. ANG II desensitization was prevented by 100 nM iberiotoxin,being reversible within 30 min. Depolarization by KCl, higher than 4 mM, impaired desensitization, suggesting that the membrane potentialmust attain a threshold to counteract the repolarization induced bymaxi-K⁺ channel opening. Once thisvalue is attained, there is no time dependency because thedesensitization process was shut off by addition of KCl along the timecourse of the tonic response. In contrast, the sustained ACh toniccomponent was not altered by these maneuvers. We conclude thatdesensitization of the ANG II tonic component is foremost due to theopening of maxi-K⁺ channels,leading to membrane repolarization, thus closing the voltage-dependentCa²⁺ channels responsible for theCa²⁺ influx that sustains thetonic component in this muscle.

     

Renal nerve inhibition by central NaCl and ANG II is abolished by lesions of the lamina terminalis

The lamina terminalis is situated in the anterior wall of the third ventricle and plays a major role in fluid and electrolyte homeostasis and cardiovascular regulation. The present study examined whether the effects of intracerebroventricular infusion of hypertonic saline and ANG II on renal sympathetic nerve activity (RSNA) were mediated by the lamina terminalis. In control, conscious sheep (n = 5), intracerebroventricular infusions of 0.6 M NaCl (1 ml/h for 20 min) and ANG II (10 nmol/h for 30 min) increased mean arterial pressure (MAP) by 6 +/- 1 (P < 0.001) and 14 +/- 3 mmHg (P < 0.001) and inhibited RSNA by 80 +/- 6 (P < 0.001) and 89 +/- 7% (P < 0.001), respectively. Both treatments reduced plasma renin concentration (PRC). Intracerebroventricular infusion of artificial cerebrospinal fluid (1 ml/h for 30 min) had no effect. In conscious sheep with lesions of the lamina terminalis (n = 6), all of the responses to intracerebroventricular hypertonic saline and ANG II were abolished. In conclusion, the effects of intracerebroventricular hypertonic saline and ANG II on RSNA, PRC, and MAP depend on the integrity of the lamina terminalis, indicating that this site plays an essential role in coordinating the homeostatic responses to changes in brain Na(+) concentration.