The predominant role of brain angiotensinogen and angiotensin in environmentally induced hypertension

Rats exposed chronically to a cold environment (5 degrees C/4 degrees F) develop hypertension. This cold-induced hypertension (CIH) is a non-genetic, non-pharmacological, non-surgical model of environmentally induced hypertension in rats. The renin-angiotensin system (RAS) appears to play a role in both initiating and/or maintaining the high blood pressure in CIH. The goal of the present study was to evaluate the role of central and peripheral circulating RAS components, angiotensinogen (AGT), angiotensin-converting enzyme (ACE) and angiotensin (Ang) II, in CIH. Seventy-two Sprague-Dawley adult male rats were used. Thirty-six rats were kept in cold room at 5 degrees C while the other 36 were at 24 degrees C as controls for 5 weeks. Systolic blood pressure (SBP) was recorded by tail cuff. The SBP was increased in rats exposed to cold within 1 week, and this increase was significant for the next 2-5 weeks of the cold exposure (p<0.01). Three subgroups of the cold-treated and control rats (n=12) were sacrificed at 1, 3 and 5 weeks. The brain and liver were removed and plasma was saved. The AGT mRNA significantly increased in the hypothalamus and liver in cold-treated rats from the first week of exposure to cold, and was maintained throughout the time of exposure to cold (n=4, p<0.01). The AGT protein levels in the brain, liver and plasma did not differ significantly between cold-treated and control rats (p>0.05, n=4). The hypothalamic Ang II levels were significantly increased, whereas plasma Ang II levels significantly decreased, in the rats of 5 weeks of cold exposure (n=8, p<0.05). Plasma ACE significantly increased in the rats of 1 week of cold exposure (p<0.05, n=12). The results show differential regulation of RAS components, AGT, ACE and Ang II, between brain and periphery in cold-exposed rats. We conclude that the exposure to low temperature initially increases plasma RAS but with continuous exposure to cold, the brain RAS maintains the hypertension, probably by sustained sympathetic activation, which would provide increased metabolism but also vasoconstriction leading to hypertension.     

Changes of renal function and structure in rats exposed to cold

The kidney appears to play a crucial role in both initiating and maintaining the high blood pressure in cold-induced hypertension (CIH). The aim of the present study was to evaluate the changes of renal function and structure in rats exposed to cold for 2, 4 and 6 weeks. Systolic blood pressure increased significantly after 2 weeks of cold exposure and was maintained throughout the whole experiment. Upregulation of angiotensin type 1 receptor (AT₁R) expression was seen in the vascular zone and distal tubule after 4 and 6 weeks of cold exposure. This was accompanied by an increase in malondialdehyde (MDA) levels and decreases in superoxide dismutase (SOD), nitric oxide synthase (NOS) activities and nitric oxide (NO) content in kidney. Structural changes were also observed in glomeruli, tubules and arteries in cold-treated rats. These results suggest that upregulation of kidney AT₁R plays a critical role in the development of CIH, and its interaction with oxidative stress, NO and NOS may be involved in changes of renal function and structure.     

Role of the sympathetic nervous system in cold-induced hypertension in rats

Hypertension develops in rats exposed chronically to cold [6 +/- 2 degrees C (SE)] and includes both an elevation of mean arterial pressure and cardiac hypertrophy. Previous studies suggest that cold-exposed animals, at least initially, have a large sustained increase in the activity of their sympathetic nervous system, suggesting a failure of the baroreceptor system to provide sufficient negative feedback to the central nervous system. The present study was designed to investigate whether alterations in the activity of the sympathetic nervous system, including the baroreceptor reflex, occur during exposure to cold and whether they contribute to cold-induced hypertension. Twenty male rats were prepared with indwelling catheters in the femoral artery and vein. Ten of the rats were exposed to cold (6 +/- 2 degrees C) chronically, while the remaining 10 were kept at 26 +/- 2 degrees C. Withdrawal of arterial blood samples (less than 5 ml/kg), measurement of direct arterial pressures, and measurement of baroreflex function were carried out at 0800 h at intervals throughout the experiment. Norepinephrine and epinephrine concentrations in plasma were also determined at intervals throughout the experiment. Systolic, diastolic, and mean blood pressures of cold-exposed rats were increased to levels significantly above those of controls. The sensitivity of the baroreflex (delta heart period/delta mean arterial pressure) was decreased in the cold-treated group. The concentration of norepinephrine in plasma increased after 24 h of exposure to cold and remained elevated throughout the experiment, whereas the concentration of epinephrine in plasma increased initially but returned to control levels after 19 days of exposure to cold.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of chronic exposure to cold on isoprenaline-induced cAMP accumulation and relaxation in the rat aorta

Rats chronically exposed to cold (5 degrees C for 5 weeks) develop hypertension. Isoprenaline-induced vascular smooth muscle relaxation is increased in these animals. Our main objective was to compare isoprenaline-induced relaxation of aortae isolated from control and cold-acclimated rats and attempt to relate the differences to changes in receptor parameters (affinity and reserve) and signaling mechanisms. Isoprenaline (10(-9)-10(-5) M)-induced relaxation was enhanced significantly (p < 0.05) in aorta segments from cold-acclimated rats. There was a significant (p < 0.05) increase in the potency of isoprenaline but with no change in affinity. Isoprenaline produced 50% of the maximum response while occupying about 50% and about 15% of the receptors in isolated rat aorta segments from control and cold-treated rats, respectively. Forskolin and db-cAMP also concentration-dependently relaxed aorta segments from control and cold-acclimated rats. There was no difference in potency or maximum response to forskolin (which directly activates adenylyl cyclase) and db-cAMP. cAMP concentrations in the presence of isoprenaline were significantly (p < 0.05) higher in aorta segments from rats chronically exposed to cold when compared with aorta segments from control rats. These findings suggested that altered mechanisms upstream of activation of adenylyl cyclase are involved in the increased beta-adrenoceptor-induced relaxation.     

Effect of a reduction in sodium intake on cold-induced elevation of blood pressure in the rat.

Chronic exposure of rats to cold (5 degrees C) induces hypertension within 3 weeks. The objective of this study was to determine the effect of treatment with graded levels of dietary NaCl on the induction of hypertension during chronic exposure to cold. Four groups of male rats were used. The first, given a commercial sodium-deficient diet containing 0.30% NaCl, served as the warm-adapted control group. The second, third, and fourth groups were given the same diet containing 0.075%, 0.15%, and 0.30% NaCl, respectively. Because cold-exposed rats ingest approximately twice as much food as warm-adapted controls, this represented half, the same, and twice the amount of NaCl ingested by the control group. The latter three groups were placed in cold air (5 degrees C). All cold-treated groups had an elevation of systolic blood pressure that was proportional to the concentration of NaCl in the diet by the seventeenth week of exposure to cold. Cardiac hypertrophy occurred to the same extent in all cold-exposed groups and was thus unaffected by the NaCl content of the diet or by the extent of elevation of blood pressure. Hence, cardiac hypertrophy during chronic exposure to cold is supported by other factors, possibly by the increased concentration of either norepinephrine or triiodothyronine, or both, which occurs characteristically in rats under these conditions. The results of this experiment suggest that the amount of NaCl ingested daily plays a role in the cold-induced elevation of blood pressure observed in rats.     

Reversibility of cold-induced hypertension after removal of rats from cold

Chronic exposure of rats to cold air induces hypertension, including elevation of blood pressure and cardiac hypertrophy. The present study was designed to assess reversibility of these changes after removal from cold. Five groups of six male rats each were exposed to cold (5 +/- 2 degrees C) for 39 days, while six control rats were maintained at 26 +/- 2 degrees C. Systolic blood pressures of the rats in one of the cold-treated groups, as well as the controls, were measured twice weekly throughout the experiment. Blood pressure of the cold-exposed rats (150 +/- 3 mmHg; 1 mmHg = 133.3 Pa) became elevated significantly above that of controls (129 +/- 3 mmHg) within 4 weeks. On day 39 of cold exposure, one group (six rats) of the cold-treated rats was sacrificed while still in the cold. The remaining four groups of cold-treated rats were than removed from cold and kept at 26 +/- 2 degrees C. One group of cold-treated rats was sacrificed weekly thereafter. During the last week, the six control rats were also sacrificed. At death, the heart, kidneys, and adrenal glands were removed and weighed. Mean heart weight of the cold-treated group (346 +/- 7 mg/100 g body weight), sacrificed prior to removal from cold, was significantly (p less than 0.01) greater than that of controls (268 +/- 5 mg/100 g body weight). The increased heart weight of the cold-treated group appeared to result mainly from an increase in left ventricular weight.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypertension during chronic exposure to cold: comparison between Sprague-Dawley and Long-Evans strains.

It is now well established that chronic exposure of rats to cold (5-6 degrees C) induces an elevation of systolic, diastolic, and mean blood pressures and cardiac hypertrophy within 3 weeks. Since rats of the Long-Evans (LE) strain are known to be resistant to the induction of deoxycorticosterone salt induced hypertension, their cardiovascular responses to chronic exposure to cold were compared with those of rats of the Sprague-Dawley (SD) strain. The results of these studies revealed clear differences between the LE and SD strains of rats. Thus, rats of the SD strain had a significant elevation in their blood pressure; a significantly increased urinary output of norepinephrine and epinephrine; a significantly greater dipsogenic responsiveness to acute administration of angiotensin II, and significant increases in weights of the heart, kidneys, adrenals, and brown adipose tissue compared with their warm-adapted controls. All of these changes are characteristic of rats acclimated to cold. In contrast, rats of the LE strain appear to be less responsive to cold in that blood pressure failed to rise as sharply and to attain as high a level. Furthermore, urinary outputs of norepinephrine and epinephrine were significantly lower in cold-treated rats of the LE strain compared with cold-treated rats of the SD strain, but dipsogenic responsiveness to angiotensin II was unchanged. Although increases in the weight of the previously mentioned organs were also observed in cold-treated rats of the LE strain compared with their warm-adapted controls, weights of the heart and interscapular brown adipose tissue of both groups were significantly less than those of counterparts of the SD strain.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vascular responses to sodium arachidonate in experimental hypertension

This study characterizes vascular responsiveness to sodium arachidonate (C 20:4) in four models of hypertension [deoxycorticosterone acetate (DOCA) hypertensive rats, two kidney-one clip (2K-1C) renal hypertensive rats, spontaneously hypertensive rats (SHR), and psychosocial hypertensive mice]. Isolated arterial strips (aorta, mesenteric artery, tail artery) were equilibrated under optimal resting tension in physiological salt solution for measurement of isometric force generation. Dose-response curves to arachidonate (10(-10) to 10(-4) g/ml) in arteries from DOCA and 2K-1C hypertensive rats were shifted to the left compared to those in arteries from control rats. In arteries from SHR and psychosocial hypertensive mice, the dose-response relationships were unchanged compared to normotensive values. Arteries from DOCA hypertensive and 2K-1C hypertensive rats developed greater maximal contractile responses to arachidonate than controls; maximal responses in arteries from SHR and psychosocial hypertensive mice were unchanged compared to normotensive values. Contractions to arachidonate were inhibited by indomethacin (0.5 and 5 micrograms/ml) and by aspirin (5 and 50 micrograms/ml). The fatty acid, oleate (C 18:1), had no effect on the contractile state of the arteries, whereas prostaglandin F2 alpha caused contraction. These results indicate altered responsiveness to exogenous arachidonate in arteries from DOCA and 2K-1C hypertensive rats, but not in arteries from SHR and psychosocial hypertensive mice.     

Norepinephrine release in brown adipose tissue remains robust in cold-exposed senescent Fischer 344 rats

Near the end of life, old F344 rats undergo a transition, marked by spontaneous and rapidly declining function. Food intake and body weight decrease, and these rats, which we call senescent, develop severe hypothermia in the cold due in part to blunted brown fat [brown adipose tissue (BAT)] thermogenesis. We tested the hypothesis that this attenuation may involve diminished sympathetic signaling by measuring cold-induced BAT norepinephrine release in freely moving rats using linear microdialysis probes surgically implanted into interscapular BAT 24 and 48 h previously. In response to 2 h at 15 degrees C, senescent rats increased BAT norepinephrine release 6- to 10-fold but did not maintain homeothermy. This increase was comparable to that of old presenescent (weight stable) rats that did maintain homeothermy during even greater cold exposure (2 h at 15 degrees C followed by 1.5 h at 8 degrees C). Tail temperatures, an index of vasoconstrictor responsiveness to cold, exhibited similar cooling curves in presenescent and senescent rats. Thus cold-induced sympathetic signaling to BAT and tail vasoconstrictor responsiveness remain robust in senescent rats and cannot explain their cold-induced hypothermia.     

Elevation of the vascular smooth muscle sensitivity to effects of constrictors after denervation and under decreased blood pressure

Changes in contractile activity of saphenous artery in normotensive rats and in rats with regional hypotension have been investigated. The abdominal aorta was partially occluded in Wistar rats distally to the renal arteries. Four weeks later, a 5-7-mm segment of the femoral nerve in one hindlimb was resected to denervate the saphenous artery. After two weeks, the isometric contraction of innervated and denervated saphenous artery segments was studied. In normotensive rats, the denervation augmented vessel sensitivity to noradrenaline, phenylephrine, serotonin, and KCl (in the presence of phentolamine). Chronic hypotension also augmented vessel sensitivity to constrictor agonists, whereas denervation did not result in further increase of sensitivity. In glyoxilic acid-stained preparations obtained from hypotensive rats, a reduced intensity of fluorescence of adrenergic fibers was observed. It was assumed that the higher sensitivity of vascular smooth muscle in hypotensive rats is due to functional disturbances of sympathetic innervation.     

Interaction between cold and altitude exposure on pulmonary circulation of cattle

Hereford calves were exposed in a temperature-controlled hypobaric chamber to environmental temperatures of -2 to 1 degree C (cold) at altitudes of 1,524 m (resident altitude) and 3,048 m 1) to characterize the effects of cold exposure on the pulmonary circulation; 2) to examine the role of cold-induced hypoventilation on the pulmonary circulation; and 3) to examine the interaction between cold and hypoxia on the pulmonary circulation. Cold exposure produced a significant increase in pulmonary arterial pressure (Ppa), pulmonary arterial wedge pressure (Ppaw), and pulmonary vascular resistance (PVR) at both 1,524 and 3,048 m without affecting cardiac output. Concomitantly, cold exposure caused reductions in minute ventilation, respiratory rate, end-tidal O2 tension (PETO2), and arterial O2 tension (PaO2). Tidal volume, end-tidal CO2 tension, and arterial CO2 tension increased. Neither arterial pH nor O2 consumption changed during cold exposure. These results indicated that both pulmonary arterial and venous vasoconstriction were responsible for the pulmonary hypertension associated with cold exposure. Acute exposure to 3,048 m during cold exposure produced increases in Ppa and PVR that were similar to those elicited by cold exposure at 1,524. It was concluded that altitude exposure neither attenuated nor potentiated the effect of cold exposure on the pulmonary circulation; rather, altitude and cold exposure interacted additively. O2 administered during cold exposure to restore PETO2 and PaO2 to control values partially restored Ppa and PVR to control values. This suggested that a portion of the pulmonary hypertension associated with cold exposure was due to hypoxic pulmonary vasoconstriction elicited by the cold-induced alveolar hypoventilation.     

Impact of Corticotropin-Releasing Hormone on Extracellular Norepinephrine in Prefrontal Cortex After Chronic Cold Stress

Abstract: We have previously demonstrated that exposing rats to cold (5°C) for 3–4 weeks potentiates the increase in extracellular norepinephrine (NE) in the medial prefrontal cortex produced by acute tail shock. In the present study, we used microdialysis to determine the duration of cold exposure required to produce this sensitization and explored the mechanism of the phenomenon. Tail shock elicited a twofold greater increase in extracellular NE in the medial prefrontal cortex of rats exposed to cold for 2 weeks than in naive control rats or in rats exposed to cold for 1 week and tested either immediately or after a 2-week delay. Local infusion of 10 µ M d -amphetamine or 30 m M K⁺ increased extracellular NE in the medial prefrontal cortex (∼350 and 190%, respectively) comparably in control rats and rats exposed to cold for 3 weeks. In contrast, intraventricular administration of 3.0 µg of corticotropin-releasing hormone increased extracellular NE in the medial prefrontal cortex by 65% in rats exposed to cold for 2 weeks, but only 35% in control rats. These results indicate that an enhanced responsiveness of noradrenergic neurons to acute tail shock (1) requires ∼2 weeks of cold exposure to develop and (2) may be mediated by a change at the level of the noradrenergic cell bodies rather than the nerve terminals.     

Cyclosporine augments reactivity of isolated blood vessels

Administration of cyclosporine (CS) as an immunosuppressive agent in clinical transplantation is associated with multiple side effects including nephrotoxicity and hypertension. These two effects could be related in that the renal changes may be secondary to alterations in organ blood flow. The present studies investigate the ability of CS to augment contractile responsiveness in blood vessels from normotensive rats. Isometric force generation was measured in isolated tail arteries and portal veins. CS (8.3×10⁻⁶M) potentiated tail artery contractile responses to sympathetic nerve stimulation, exogenous norepinephrine, and increases in extracellular potassium concentration. Portal veins undergo spontaneous contractions which are related to the firing of calcium-driven action potentials in the smooth muscle cells. CS significantly increased the frequency of these spontaneous contractile events. These results suggest that components of CS toxicity may involve a direct action on vascular smooth muscle and/or on vascular adrenergic neurotransmission.     

Vasoconstrictor responsiveness of hind body vascular beds is diminished in tail-suspended rats

It is well known that the mechanisms of occurrence of orthostatic intolerance induced by exposure to microgravity deal with multiple factors including alterations of arteries. In the previous works, the diminished contractile responsiveness of abdominal aorta and hind body medium-sized conduit arteries, mesenteric artery and femoral artery, were observed in tail-suspended rats, and the data showed that the femoral artery have subjected to the greatest changes. These results suggested that the vasoreactivity of resistance vessels might be affected by the real or simulated microgravity. Since the arterioles are the main site of peripheral resistance and of its regulation. Therefore, changes in responsiveness of arteriolar network, especially in the lower/hind body region, would be of primary importance in the genesis of postflight orthostatic intolerance. The aim of the present work was to examine whether simulated weightlessness may lead to an impairment in vasoconstrictor responsiveness in hind body vascular beds.     

Stress- and cold-induced adrenocortical responses in repetitively immobilized or cold-acclimated rats

To evaluate the role of adrenocortical hormones in stress- or cold-induced nonshivering thermogenesis, plasma corticosterone (CS) and deoxycorticosterone (DOCS) were measured with the aid of HPLC under various conditions. Repetitive immobilization stress (3 h/day, for 1 or 4 weeks) elevated the resting level (24 h after the last immobilization) of CS, but not DOCS. Acute stress (immobilization for 30 min) or cold exposure (-5 degrees C for 15 min) caused marked increases of CS and DOCS in both nonstressed naive controls and repetitively stressed rats. Four weeks, but not 1 week, of repetitive immobilization stress potentiated the responsiveness of CS to both acute stress and cold, and that of DOCS to acute stress, but not to cold. Cold acclimation (5 degrees C, 4 weeks) significantly elevated both corticosteroids but did not affect the resting levels (18 h after being transferred to 25 degrees C) or the responsiveness of both CS and DOCS to either acute stress or cold. These results suggest that repetitive immobilization stress, but not cold acclimation, could enhance nonshivering thermogenesis, at least in part, through an improvement in the responsiveness of adrenocortical hormone secretion to acute stress or cold.     

Factors affecting cold-induced hypertension in rats

A 3- to 4-week exposure of rats to a cold environment (5 +/- 2 degrees C) induces hypertension, including elevation of systolic, diastolic, and mean blood pressures and cardiac (left ventricular) hypertrophy. The studies described here were designed to investigate some factors affecting both the magnitude and the time course for development of cold-induced hypertension. The objective of the first study was to determine whether there was an ambient temperature at which the cold-induced elevation of blood pressure did not occur. The objective of the second experiment was to determine whether body weight at the time of exposure to cold affected the magnitude and time course for development of hypertension. To assess the first objective, male rats were housed in a chamber whose temperature was maintained at 5 +/- 2 degrees C while others were housed in an identical chamber at 9 +/- 2 degrees C. After 7 days of exposure to cold, the rats exposed to the colder temperature had a significant elevation of blood pressure (140 +/- 2 mm Hg) compared with the group maintained at 9 degrees C (122 +/- 3 mm Hg). The rats exposed to 9 degrees C had no significant elevation of systolic blood pressure at either 27 or 40 days after initiation of exposure to cold. At the latter time, the temperature in the second chamber was reduced to 5 +/- 2 degrees C. By the 25th day of exposure to this ambient temperature, the rats had a significant increase in systolic blood pressure above their levels at 9 degrees C. Thus, there appears to be a threshold ambient temperature for elevation of blood pressure during exposure to cold. That temperature appears to lie somewhere between 5 and 9 degrees C. The second objective was assessed by placing rats varying in weight from approximately 250 to 430 g in air at 5 degrees C. There was a highly significant direct relationship (r = 0.96) between body weight at the time of introduction to cold and the number of days required to increase systolic blood pressure by 10 mm Hg above pre-cold exposure level. The third objective was to make an initial assessment of potential differences among strains of rats with respect to development of cold-induced hypertension. To this end, rats of the Fischer 344 strain were used. Systolic blood pressures of these rats also increased during chronic exposure to cold.(ABSTRACT TRUNCATED AT 400 WORDS)     

Time-course changes of myogenic tone of mesenteric small artery in spontaneously hypertensive rat

To investigate the time-course changes of myogenic tone in mesenteric small artery (MSA) of spontaneously hypertensive rat (SHR), thirty-two 7-week aged SHR rats were randomly divided into four groups (8, 16, 24, 32 weeks of age), and 32 sex- and age-matched Wistar-Kyoto (WKY) rats were assigned to control groups (CON). On the day of the study, segments of MSA were isolated and then cannulated to the two pipettes. Vascular diameters in response to the increased intraluminal pressure (from 0 mmHg to 150 mmHg, by 25 mmHg steps) of isolated MSA under no-flow conditions were recorded by a Pressure Myograph System both in physiologic salt solution (PSS) (active diameter, Da) and calcium-free PSS (passive diameter, Dp). The myogenic tone was calculated by (Dp - Da)/Dp × 100%. The tail artery pressure and vascular myogenic tone in SHR rats were significantly higher than those of the CON rats. Before 24 weeks, the vascular myogenic tone of MSA in SHR group increased monotonically, but at the end of 32 weeks, the vascular myogenic tone decreased in comparison with that in 24-week group, but was significantly higher than that in CON group. The tail artery pressure in SHR group slowly increased monotonically with increasing weeks of age, and the tail arterial pressure in 32-week group remained significantly higher than that in 24-week group. Vascular myogenic tone may participate in the whole process of hypertension. Early in the development of hypertension, because of the compensatory role of vascular tone, the vascular function has been partially compensated, thus guaranteeing adequate blood supply to organs. Late in the development of hypertension, because of the decompensation of myogenic tone, the vascular function is damaged, leading to the occurrence of severe vascular disease.     

Chloride channel activity of vascular smooth muscle in the spontaneous hypertensive rats

To characterize the activity of the Ca2+-activated Cl- channels in vascular smooth muscle (VSM) of the spontaneous hypertensive rats (SHR), the isolated mesenteric vascular beds and tail artery strips were preparated from SHR and Wistar rats aged 7-8 weeks. The changes in contractile response to norepinphrine (NE) were taken as an index of vascular mortion. Results showed that the contractile responses of mesenteric arteries and tail arteries to NE in SHR were significantly greater than that in Wistar rats. The inhibition magnitude of the contractile response by Ca2+-activated Cl- channel blocker, niflumic acid in SHR was significantly less than that in Wistar rats. Decreasing the extracellular Cl- concentration increased the contractile response to NE significantly, but the amplitude of enhanced contractile response in SHR was greater than that in Wistar rats. It can be concluded that NE-induced contraction was enhanced in SHR, which is partly due to an increase in Cl- efflux through the Ca2+-activated Cl- channels. The chloride channel activity may be increased in association with the elevation of blood pressure.     

Impaired pulmonary artery contractile responses in a rat model of microgravity: role of nitric oxide.

Vascular contractile hyporesponsiveness is an important mechanism underlying orthostatic intolerance after microgravity. Baroreceptor reflexes can modulate both pulmonary resistance and capacitance function and thus cardiac output. We hypothesized, therefore, that pulmonary vasoreactivity is impaired in the hindlimb-unweighted (HLU) rat model of microgravity. Pulmonary artery (PA) contractile responses to phenylephrine (PE) and U-46619 (U4) were significantly decreased in the PAs from HLU vs. control (C) animals. N(G)-nitro-L-arginine methyl ester (10(-5) M) enhanced the contractile responses in the PA rings from both C and HLU animals and completely abolished the differential responses to PE and U4 in HLU vs. C animals. Vasorelaxant responses to ACh were significantly enhanced in PA rings from HLU rats compared with C. Moreover, vasorelaxant responses to sodium nitroprusside were also significantly enhanced. Endothelial nitric oxide synthase (eNOS) and soluble guanlyl cyclase expression were significantly enhanced in PA and lung tissue from HLU rats. In marked contrast, the expression of inducible nitric oxide synthase was unchanged in lung tissue. These data support the hypothesis that vascular contractile responsiveness is attenuated in PAs from HLU rats and that this hyporesponsiveness is due at least in part to increased nitric oxide synthase activity resulting from enhanced eNOS expression. These findings may have important implications for blood volume distribution and attenuated stroke volume responses to orthostatic stress after microgravity exposure.