Non-invasive evaluation of vascular permeability in formalin-induced orofacial pain model using infrared thermography

Enhanced vascular permeability at the site of injury is a prominent feature in acute inflammatory pain models, commonly assessed through the Evans Blue test. However, this invasive test requires euthanasia, thereby precluding further investigations on the same animal. Due to these limitations, the integration of non-invasive tools such as IRT has been sought. Here, we aimed to evaluate the use of thermography in a common orofacial pain model that employs formalin as a chemical irritant to induce local orofacial inflammation. Male Hannover rats (290-300 g, N = 43) were used. In the first approach, radiometric images were taken before and after formalin administration, assessing temperature changes and extravasated Evans Blue. The second approach included capturing pre- and post-formalin test radiometric images, followed by cytokine measurements in excised vibrissae tissue. Rats were anesthetized for vibrissae tissue collection, allowing correlations between thermographic patterns, nocifensive behavior duration, and cytokine levels in this area. Our findings revealed a positive correlation between local temperature, measured via thermography, and vascular permeability in the contralateral (r2 = 0.3483) and ipsilateral (r2 = 0.4502) side, measured using spectrophotometry. The obtained data supports the notion that thermography-based temperature assessment can effectively evaluate vascular permeability in the orofacial region.

Orofacial anti-hypernociceptive effect of citral in acute and persistent inflammatory models in rats

Orofacial pain has significant psychological and physiological effects. Citral (3,7-dimethyl-2,6-octadienal) is the main component of Cymbopogon citratus (DC) Stapf, an herb with analgesic properties. Although citral has been considered a potent analgesic, its putative effects on orofacial pain are still unknown.

OBJECTIVE

The objective of this study is to test the hypothesis that citral modulates orofacial pain using two experimental models: formalin-induced hyperalgesia in the vibrissae area and during persistent temporomandibular hypernociception using Complete Freund's Adjuvant - CFA test.

METHODS

For the formalin test, citral (100 and 300 mg/kg, oral gavage) or its vehicle (Tween 80, 1 %) were given 1 h before the formalin injection subcutaneously (sc) into the vibrissae area. For the CFA model, we analyzed the prophylactic (100 mg/kg of citral by oral gavage, 1 h before CFA injection) and the chronic therapeutic (citral treatment 1-hour post-CFA injection and daily post-CFA injection) effect of citral or its vehicle in animals treated with CFA for 8 days.

RESULTS

Citral caused a decrease in formalin-induced local inflammation and the time spent performing nociceptive behavior in a dose-dependent fashion. Similarly, prophylactic and therapeutic citral treatment decreased the CFA-induced persistent mechanical hypernociception in the temporomandibular area.

CONCLUSION

Our data strengthen the notion that citral plays a powerful antinociceptive role by decreasing orofacial hypernociception in formalin and CFA models.

Mineralocorticoid receptors contribute to ethanol-induced vascular hypercontractility through reactive oxygen species generation and up-regulation of cyclooxygenase 2

The effects on blood pressure produced byethanol consumption include both vasoconstriction and activation of the renin-angiotensin-aldosterone system (RAAS), although the detailed relationship between these processes is yet to be accomplished. Here, we sought to investigate the contribution of mineralocorticoid receptors (MR) to ethanol-induced hypertension and vascular hypercontractility. We analyzed blood pressure and vascular function of male Wistar Hannover rats treated with ethanol for five weeks. The contribution of the MR pathway to the cardiovascular effects of ethanol was evaluated with potassium canrenoate, a MR antagonist (MRA). Blockade of MR prevented ethanol-induced hypertension and hypercontractility of endothelium-intact and -denuded aortic rings. Ethanol up-regulated cyclooxygenase (COX)2 and augmented vascular levels of both reactive oxygen species (ROS) and thromboxane (TX)B₂, a stable metabolite of TXA₂. These responses were abrogated by MR blockade. Hyperreactivity to phenylephrine induced by ethanol consumption was reversed by tiron [a scavenger of superoxide (O₂∙^-)], SC236 (a selective COX2 inhibitor) or SQ29548 (an antagonist of TP receptors). Treatment with the antioxidant apocynin prevented the vascular hypercontractility, as well as the increases in COX2 expression and TXA₂ production induced by ethanol consumption. Our study has identified novel mechanisms through which ethanol consumption promotes its deleterious effects in the cardiovascular system. We provided evidence for a role of MR in the vascular hypercontractility and hypertension associated with ethanol consumption. The MR pathway triggers vascular hypercontractility through ROS generation, up-regulation of COX2 and overproduction of TXA₂, which will ultimately induce vascular contraction.

Chronic molecular hydrogen inhalation mitigates short and long-term memory loss in polymicrobial sepsis

The central nervous system (CNS) is one of the first physiological systems to be affected in sepsis. During the exacerbated systemic inflammatory response at the early stage of sepsis, circulatory inflammatory mediators are able to reach the CNS leading to neuroinflammation and, consequently, long-term impairment in learning and memory formation is observed. The acute treatment with molecular hydrogen (H₂) exerts important antioxidative, antiapoptotic, and anti-inflammatory effects in sepsis, but little is known about the mechanism itself and the efficacy of chronic H₂ inhalation in sepsis treatment. Thus, we tested two hypotheses. We first hypothesized that chronic H₂ inhalation is also an effective therapy to treat memory impairment induced by sepsis. The second hypothesis is that H₂ treatment decreases sepsis-induced neuroinflammation in the hippocampus and prefrontal cortex, important areas related to short and long-term memory processing. Our results indicate that (1) chronic exposure of hydrogen gas is a simple, safe and promising therapeutic strategy to prevent memory loss in patients with sepsis and (2) acute H₂ inhalation decreases neuroinflammation in memory-related areas and increases total nuclear factor E2-related factor 2 (Nrf2), a transcription factorthat regulates a vast group of antioxidant and inflammatory agents expression in these areas of septic animals.

Photobiomodulation induces hypotensive effect in spontaneously hypertensive rats

To evaluate whether acute photobiomodulation can elicit a hypotensive effect in spontaneously hypertensive rats (SHR). Male SHR were submitted to the implantation of a polyethylene cannula into the femoral artery. After 24 h, baseline measurements of the hemodynamic parameters: systolic, diastolic, and mean arterial pressure, and heart rate were accomplished for 1 h. Afterwards, laser application was simulated, and the hemodynamic parameters were recorded for 1 h. In the same animal, the laser was applied at six different positions of the rat's abdomen, and the hemodynamic parameters were also recorded until the end of the hypotensive effect. The irradiation parameters were red wavelength (660 nm); average optical power of 100 mW; 56 s per point (six points); spot area of 0.0586 cm²; and irradiance of 1.71 W/cm² yielding to a fluency of 96 J/cm² per point. For measuring plasma NO levels, blood was collected before the recording, as well as immediately after the end of the mediated hypotensive effect. Photobiomodulation therapy was able to reduce the systolic arterial pressure in 69% of the SHR submitted to the application, displaying a decrease in systolic, diastolic, and mean arterial pressure. No change in heart rate was observed. Nevertheless, there was an increase in serum nitric oxide levels in the SHR responsive to photobiomodulation. Our results suggest that acute irradiation with a red laser at 660 nm can elicit a hypotensive effect in SHR, probably by a mechanism involving the release of NO, without changing the heart rate.

Molecular hydrogen potentiates hypothermia and prevents hypotension and fever in LPS-induced systemic inflammation

Molecular hydrogen (H₂) exerts anti-oxidative, anti-apoptotic, and anti-inflammatory effects. Here we tested the hypothesis that H₂ modulates cardiovascular, inflammatory, and thermoregulatory changes in systemic inflammation (SI) induced by lipopolysaccharide (LPS) at different doses (0.1 or 1.5 mg/kg, intravenously, to induce mild or severe SI) in male Wistar rats (250-300 g). LPS or saline was injected immediately before the beginning of 360-minute inhalation of H₂ (2% H₂, 21% O₂, balanced with nitrogen) or room air (21% O₂, balanced with nitrogen). Deep body temperature (Tb) was measured by dataloggers pre-implanted in the peritoneal cavity. H₂ caused no change in cardiovascular, inflammatory parameters, and Tb of control rats (treated with saline). During mild SI, H₂ reduced plasma surges of proinflammatory cytokines (TNF-α and IL-6) while caused an increase in plasma IL-10 (anti-inflammatory cytokine) and prevented fever. During severe SI, H₂ potentiated hypothermia, and prevented fever and hypotension, which coincided with reduced plasma nitric oxide (NO) production. Moreover, H₂ caused a reduction in surges of proinflammatory cytokines (plasma TNF-α and IL-1β) and prostaglandin E₂ [(PGE₂), in plasma and hypothalamus], and an increase in plasma IL-10. These data are consistent with the notion that H₂ blunts fever in mild SI, and during severe SI potentiates hypothermia, prevents hypotension reducing plasma NO production, and exerts anti-inflammatory effects strong enough to prevent fever by altering febrigenic signaling and ultimately down-modulating hypothalamic PGE₂ production.

Differential Role of Neurohypophysial Hormones in Hypotension and Nitric Oxide Production During Endotoxaemia

Besides their well-established endocrine roles, vasopressin and oxytocin are also important regulators of immune function, participating in a complex neuroendocrine-immune network. In the present study, we investigated whether and how vasopressin and oxytocin could modulate lipopolysaccharide (LPS)-induced nitric oxide (NO) production in a well-established model of experimental endotoxaemia. Male Wistar rats were previously treated i.v. with vasopressin V1 or oxytocin receptor antagonists and then received either an i.v. LPS injection to induce endotoxaemia or a saline imjection as a control. The animals were divided into two groups: in the first group, blood was collected at 2, 4 and 6 h after LPS injection; in the second group, mean arterial blood pressure (MABP) and heart rate (HR) were recorded over 6 h. Plasma vasopressin and oxytocin values were higher in LPS- compared to saline-injected animals at 2 and 4 h but returned to basal levels at 6 h. NO levels exhibited an opposite pattern, showing a progressive increase over the entire period. The previous administration of a vasopressin V1 receptor antagonist significantly reduced NO plasma concentrations at 2 and 4 h but not at 6 h. By contrast, oxytocin receptor agonist pre-treatment had no effect on the NO plasma concentration. In relation to MABP, previous treatment with vasopressin V1 receptor antagonist reversed the LPS-induced hypotension at 4 h, although this was not the case for oxytocin antagonist-treated animals. None of the antagonists affected HR. Our findings indicate that vasopressin (but not oxytocin) has effects on NO production during endotoxaemia in rats, although they do not lend support to the proposed anti-inflammatory actions of vasopressin during endotoxaemia.

Interleukin-1 receptor antagonist decreases cerebrospinal fluid nitric oxide levels and increases vasopressin secretion in the late phase of sepsis in rats

The aim of this study was to analyze the effect of IL-1ra (an Interleukin-1 receptor antagonist) on sepsis-induced alterations in vasopressin (AVP) and nitric oxide (NO) levels. In addition, IL-1ra effect on the hypothalamic nitric oxide synthase (NOS) activities and survival rate was also analyzed. After Wistar rats were intracerebroventricular injected with IL-1ra (9 pmol) or vehicle (PBS 0.01 M), sepsis was induced by cecal-ligation and puncture (CLP). Blood, CSF, and hypothalamic samples were collected from different groups of rats (n = 8/group) after 4, 6, and 24 h. AVP and NO levels were greatly increased in CLP. Both total NOS and inducible NOS (iNOS) activities were also greatly increased in CLP rats. These changes in AVP, NO, and NOS were not observed in sham-operated control rats. IL-1ra administration did not alter plasma AVP levels after 4 and 6 h as compared to vehicle in CLP animals but after 24 h were significantly (P < 0.01) higher in IL-1ra-treated animals. IL-1ra administration significantly (P < 0.01) decreased NO concentration in CSF but not in plasma. Both total NOS and iNOS activities were also significantly decreased by IL-1ra at 24 h in CLP animals. Moreover, the 24 h survival rate of IL-1ra-treated rats increased by 38 % in comparison to vehicle administered animals. The central administration of IL-1ra increased AVP secretion in the late phase of sepsis which was beneficial for survival. We believe that one of the mechanisms for this effect of IL-1ra is through reduction of NO concentration in CSF and hence lower hypothalamic iNOS activities in the septic rats.

Estradiol and thermoregulation in adult endotoxemic rats exposed to lipopolysaccharide in neonatal life

AIMS

Early life immune challenge has been considered an adaptive defense strategy against potential pathogens when the innate immune system is not completely developed. This study assesses whether neonatal endotoxin challenge alters body temperature response in adult female rats during endotoxemic shock and also, whether ovarian hormones may participate in this response.

METHODS

Rats were intraperitoneally injected with lipopolysacharide (LPS) or saline at post-natal day 14, then as adults they were submitted to endotoxemic shock.

RESULTS

The LPS injection in adult neonatal Saline rats caused an initial hypothermia, followed by a febrile response. However, neonatal LPS showed an increased hypothermic response and an attenuation of fever. The bilateral ovariectomy abolished the difference in body temperature between the neonatal LPS and saline rats. To determine the dependence of ovarian hormones, ovariectomized rats treated with estradiol cypionate (ECP) restored hypothermia and the suppressed febrile response. However, the same results were not obtained when the animals were supplemented with ECP and medroxyprogesterone acetate (MPA). The neonatal LPS rats displayed a significant reduction in TNF-α levels and an increase in IL-10 levels when compared with saline animals. The ECP injection significantly enhanced IL-10 and suppressed TNF-α in neonatal LPS, but it did not change the inflammatory response in the saline rats. The ECP + MPA regiment in the neonatal LPS rats reduced TNF-α, but eliminated IL-10 stimulation in comparison with the saline group.

CONCLUSION

The present investigation shows that neonatal LPS challenge alters the thermoregulatory response during endotoxemic shock in adulthood and the mechanism for this difference could be mediated by sex hormones, especially estradiol.

nNOS is involved in behavioral thermoregulation of newborn rats during hypoxia

The present study was undertaken to investigate the role of nitric oxide (NO) pathway in the behavioral thermoregulation of newborn rats in cold and hypoxia. We predicted that injection of L-NAME (non-selective NO synthase (NOS) inhibitor) and SMTC (neuronal NOS (nNOS) inhibitor) would restore the huddling behavior and eliminate the reduction of Tb caused by hypoxia. Experiments were performed on Wistar rat pups of 7-9 days old. We measured Tb and analyzed the huddling behavior by means of the calculation of the total surface area occupied by 5 pups and the number of single pups grouped in the center of a chamber at 20 degrees C, before and after L-NAME, SMTC or their respective vehicles (D-NAME and saline) s.c. injections. Subsequently, the pups were exposed to hypoxia (10% O(2)) during 30 min, whereas control animals were kept under normoxia. The experiments were monitored by a digital camera. All animals were hypothermic when exposed to 20 degrees C. There was no significant difference in Tb, total area and number of single pups in normoxia after treatments. During hypoxia, the drop in Tb was higher in control groups, and this effect was attenuated by L-NAME and SMTC injections. Hypoxia increased the area occupied by the pups in saline, D-NAME and L-NAME groups, while SMTC attenuated this response. The data indicate that NO pathway is involved in the inhibition of huddling behavior and in the reduction of Tb caused by hypoxia, but plays no role during normoxia. Furthermore, NO seems to arise from the nNOS isoform.

Respiratory and body temperature modulation by adenosine A1 receptors in the anteroventral preoptic region during normoxia and hypoxia

The present study was undertaken to investigate adenosine as a simultaneous mediator of hypoxia-induced hyperventilation and regulated hypothermia in the anteroventral preoptic region (AVPO), the thermointegrative region of the central nervous system (CNS). Accordingly, we predicted that injection of aminophylline and DPCPX, non-selective and A(1) receptor antagonists, respectively, into the AVPO would exacerbate the ventilatory response and lessen the drop in body temperature (T(b)) caused by hypoxia. We measured ventilation (V ) and T(b) of conscious Wistar rats before and after AVPO injection of aminophylline (1 and 10 microg/100 nL) or DPCPX (17.5 and 175 ng/100 nL), or their respective vehicles, followed by 30 min of hypoxia (7% O(2)). Vehicles and the lower doses of both antagonists had no effect on V and T(b) during normoxia or hypoxia. The higher doses of aminophylline and DPCPX increased (P<0.05) the hypoxia-induced hyperventilation, whereas the drop in T(b) elicited by hypoxia was attenuated (P<00.05) by DPCPX only. This higher DPCPX dose also increased T(b) during normoxia. The present data is consistent with the notion that adenosine plays an inhibitory role in respiratory and metabolic regulation, in a way that A(1) receptors stimulation in the AVPO inhibits ventilatory drive during hypoxia and tonically modulates basal T(b).

Evidence for thermoregulation by dopamine D1 and D2 receptors in the anteroventral preoptic region during normoxia and hypoxia

Hypoxia causes a regulated decrease in body temperature (Tb), a response that has been called anapyrexia. Stimulation of dopamine receptors in the central nervous system (CNS) reduces Tb in rats, and dopamine D1 and D2 receptors seem to be involved in this response. Thus, we predicted that injection of SCH 23390 and haloperidol, D1 and partly D2 receptor antagonists, respectively, into the anteroventral preoptic region (AVPO, the thermointegrative region of the CNS) would lessen the hypoxia-induced anapyrexia. We measured Tb of conscious Wistar rats before and after injection of SCH 23390 (50 and 100 ng/100 nl) or haloperidol (50 e 500 ng/100 nl) or their respective vehicles (saline and DMSO 5%) into the AVPO followed by 30 min of hypoxia (7% O2). Vehicles and the lower doses of SCH 23390 and haloperidol had no effect on Tb during normoxia or hypoxia. The higher doses of SCH 23390 and haloperidol attenuated (P<0.05) the drop in Tb elicited by hypoxia. However, this higher haloperidol dose also increased Tb during normoxia. The present data is consistent with the notion that dopamine is an important thermoregulatory neurotransmitter in a way that D2 receptors are mainly involved with maintenance of Tb in euthermia, while D1 receptors are activated to induce hypoxic anapyrexia in the AVPO.

Is there a link between salt-intake and atrial natriuretic peptide system during hypertension induced by nitric oxide blockade?

Long-term nitric oxide (NO) blockade is known to induce a severe and progressive hypertension. The influence of the salt-intake on atrial natriuretic peptide (ANP) system in this hypertension model is unknown. The aim of this study was to evaluate ANP plasma levels, content and mRNA in atria of male Wistar rats chronically treated with oral Nomega-nitro-L-arginine methyl ester (L-NAME) after 4 weeks of high-salt diet. The high-salt diet induced an increase (P < 0.05) in ANP plasma levels in normotensive rats and no significant changes in hypertensive animals. We observed a significant increase in the ANP content in the left and right atria of hypertensive rats (P < 0.001) when compared to normotensive ones. However, no significant changes were observed during high-salt diet in normotensive and hypertensive animals. Northern blot analysis revealed that ANP gene expression is higher in the right and left atria of hypertensive rats when compared to normotensive rats. However, we found no significant changes in ANP mRNA of rats treated with high-salt diet in normotensive and hypertensive rats when compared to low-salt diet. The present observations indicate no interaction between salt-intake and activation of the ANP system during chronic nitric oxide synthase (NOS) inhibition.

Regulation of breathing and body temperature of a burrowing rodent during hypoxic–hypercapnia

Burrowing mammals usually have low respiratory sensitivity to hypoxia and hypercapnia. However, the interaction between ventilation (V), metabolism and body temperature (Tb) during hypoxic-hypercapnia has never been addressed. We tested the hypothesis that Clyomys bishopi, a burrowing rodent of the Brazilian cerrado, shows a small ventilatory response to hypoxic-hypercapnia, accompanied by a marked drop in Tb and metabolism. V, Tb and O(2) consumption (V?O(2)) of C. bishopi were measured during exposure to air, hypoxia (10% and 7% O(2)), hypercapnia (3% and 5% CO(2)) and hypoxic-hypercapnia (10% O(2)+ 3% CO(2)). Hypoxia of 7% but not 10%, caused a significant increase in V, and a significant drop in Tb. Both hypoxic levels decreased V?O(2) and 7% O(2) significantly increased V/V?O(2). Hypercapnia of 5%, but not 3%, elicited a significant increase in V, although no significant change in Tb, V?O(2) or V/V?O(2) was detected. A combination of 10% O(2) and 3% CO(2) had minor effects on V and Tb, while V?O(2) decreased and V/V?O(2) tended to increase. We conclude that C. bishopi has a low sensitivity not only to hypoxia and hypercapnia, but also to hypoxic-hypercapnia, manifested by a biphasic ventilatory response, a drop in metabolism and a tendency to increase V/V?O(2). The effect of hypoxic-hypercapnia was the summation of the hypoxia and hypercapnia effects, with respiratory responses tending to have hypercapnic patterns while metabolic responses, hypoxic patterns.

[Therapeutic use of nitric oxide: implications for nursing]

Nitric oxide (NO) is a gas that transmits signals in the organism. Such signal transmission takes place by means of the gas synthesis and release in different cell types. After it is released, the gas penetrates the membrane of a neighboring cell and regulates its function. Such mechanism represents an entirely new signaling principle in biological systems. The discoverers of NO as a signaling molecule were awarded the Nobel Prize in Medicine and Physiology in 1998. This discovery has revolutionized medicine and originated new treatments for old problems. In this study, we review the role of NO in some pathologies such as sepsis, arterial hypertension and pulmonary hypertension and Nitric Oxide is explained in terms of its current merit for treatment and its impact on nursing care.

Effects of a neuronal nitric oxide synthase inhibitor on lipopolysaccharide-induced fever

It has been demonstrated that nitric oxide (NO) has a thermoregulatory action, but very little is known about the mechanisms involved. In the present study we determined the effect of neuronal nitric oxide synthase (nNOS) inhibition on thermoregulation. We used 7-nitroindazole (7-NI, 1, 10 and 30 mg/kg body weight), a selective nNOS inhibitor, injected intraperitoneally into normothermic Wistar rats (200-250 g) and rats with fever induced by lipopolysaccharide (LPS) (100 microg/kg body weight) administration. It has been demonstrated that the effects of 30 mg/kg of 7-NI given intraperitoneally may inhibit 60% of nNOS activity in rats. In all experiments the colonic temperature of awake unrestrained rats was measured over a period of 5 h at 15-min intervals after intraperitoneal injection of 7-NI. We observed that the injection of 30 mg/kg of 7-NI induced a 1.5 degrees C drop in body temperature, which was statistically significant 1 h after injection (P<0.02). The coinjection of LPS and 7-NI was followed by a significant (P<0.02) hypothermia about 0.5 degrees C below baseline. These findings show that an nNOS isoform is required for thermoregulation and participates in the production of fever in rats.

Elevated levels of natriuretic peptides in lungs of hamsters with genetic cardiomyopathy

Various alterations in the natriuretic peptide system have been observed in heart and plasma in humans and animals with heart failure. However, there is limited information about these hormones in hamster lung especially in those with genetic cardiomyopathy, a model of human congestive heart failure. Therefore, the aim of the present study was to investigate the content of the three natriuretic peptides (atrial natriuretic peptide (ANP); brain natriuretic peptide (BNP); C-type natriuretic peptide (CNP) and their gene expression in lungs of normal and cardiomyopathic hamsters. The presence of mRNA coding for ANP and BNP in lungs and heart was investigated by Northern blot and confirmed by reverse transcription polymerase chain reaction (RT-PCR). The peptide contents and plasma concentrations were determined by specific radioimmunoassays. Plasma ANP increased in hamsters with moderate to severe cardiomyopathy (aged 230 days) from control levels of 71.8+/-15.8 to 243.1+/-44.0 pg/ml (P < 0.01). Plasma BNP also increased from 79.7+/-23.5 to 227.9+/-51.6 pg/ml (P < 0.01). The levels of the three peptides in lungs of 30- and 120-day-old cardiomyopathic (CMO) hamsters were not different from their corresponding age-matched controls. However, lung ANP increased in 230-day-old CMO from 589+/-63 to 1624+/-219 pg/mg protein (P < 0.01). Lung BNP and CNP also increased from 332+/-35 to 531+/-55 pg/mg protein (P < 0.01) and from 118+/-21 to 224+/-29 pg/mg protein (P < 0.01), respectively. Lung ANP mRNA and BNP mRNA were significantly (P < 0.05) higher in 230-day-old hamsters than those detected in age-matched normal controls. Our data demonstrate that the hamster lungs produce ANP, BNP and CNP, and that this production is enhanced in moderate to severe cardiomyopathy. These findings imply that the lung natriuretic peptide system may participate in pulmonary function especially during cardiac dysfunction.

Participation of the nitric oxide pathway in cold-induced hypertension

Several mechanisms are known to participate in cold-induced hypertension, but no information exist on the role of the nitric oxide (NO). In the present study, we assessed the participation of nitric oxide in cold-induced hypertension by means of inhibition of NO synthase. Experiments were performed in rats treated with N omega-nitro-L-arginine (L-NNA) injected i.p. at a dose of 25 mg/kg body weight twice a day for four consecutive days. Control animals received saline injections of the same volume. Two days before the experiment, the femoral artery was cannulated for blood pressure recording. Arterial blood pressure was measured at 25 degrees C for 30 min (control period), followed by a 3.5 hour period at 4 degrees C (cold exposure) and, eventually, a last 3 hour period after removal from cold (back to 25 degrees C). In control animals, at 25 degrees C, mean arterial blood pressure was 112.5+/-3.6 mmHg and heart rate was 380+/-3.5 bpm. L-NNA treatment caused an increase in blood pressure to 155.0+/-3.5 mmHg (P<0.01) and in heart rate to 410+/-6.0 bpm (P<0.05). Exposure to cold caused blood pressure to increase up to 131.5+/-3.6 mmHg (P<0.05) in the control group, whereas no significant change could be measured in treated animals. Recovery from cold exposure led to a decrease in blood pressure in control animals, but not in treated animals. These results indicate that NO plays a role in the development of cold-induced elevation of blood pressure.