Chronic hypoxia (CH) attenuates systemic vasoconstriction to a variety of agonists in conscious rats. Recent evidence suggests that similarly diminished responses to vasoconstrictors in aortic rings from CH rats may be due to increased endothelial heme oxygenase (HO) activity and enhanced production of the vasodilator carbon monoxide (CO). Thus we hypothesized that a hypoxia-induced increase in HO activity is responsible for decreased vasoconstrictor responsiveness observed in conscious CH rats. CH (4 wk at 0.5 atm) and control rats were renal denervated and instrumented for the measurement of renal blood flow (RBF) and blood pressure. First, renal vasoconstrictor responses to graded intravenous infusion of phenylephrine (PE) were assessed in conscious rats. CH rats demonstrated significantly diminished renal vasoconstrictor responses to PE compared with control responses that persisted even with acute restoration of normoxia. In additional experiments, CH rats exhibited increased renal vascular resistance and decreased RBF in response to the HO inhibitor zinc protoporphyrin IX (11 micromol/kg iv), whereas renal hemodynamics were unaffected by the inhibitor in control animals. Furthermore, we demonstrated greater HO enzyme activity in renal tissue from CH rats compared with controls. These data suggest that enhanced HO activity contributes a tonic vasodilatory influence in the renal vasculature of CH rats that may be responsible for the diminished sensitivity to vasoconstrictor agonists observed under these conditions.

Water deprivation is associated with increased excitatory amino acid (EAA) drive of the rostral ventrolateral medulla (RVLM), but the mechanism is unknown. This study tested the hypotheses that the increased EAA activity is mediated by decreased blood volume and/or increased osmolality. This was first tested in urethane-anesthetized rats by determining whether bilateral microinjection of kynurenate (KYN, 2.7 nmol) into the RVLM decreases arterial pressure less in water-deprived rats after normalization of blood volume by intravenous infusion of isotonic saline or after normalization of plasma osmolality by intravenous infusion of 5% dextrose in water (5DW). Water-deprived rats exhibited decreased plasma volume and elevated plasma osmolality, hematocrit, and plasma sodium, chloride, and protein levels (all P < 0.05). KYN microinjection decreased arterial pressure by 24 +/- 2 mmHg (P < 0.05; n = 17). The depressor response was not altered following isotonic saline infusion but, while still present (P < 0.05), was reduced (P < 0.05) to -13 +/- 2 mmHg soon after 5DW infusion. These data suggest that the high osmolality, but not low blood volume, contributes to the KYN depressor response. To further investigate the action of increased osmolality on EAA input to RVLM, water-replete rats were also studied after hypertonic saline infusion. Whereas KYN microinjection did not decrease pressure immediately following the infusion, a depressor response gradually developed over the next 3 h. Lumbar sympathetic nerve activity also gradually increased to up to 167 +/- 19% of control (P < 0.05) 3 h after hypertonic saline infusion. In conclusion, acute and chronic increases in osmolality appear to increase EAA drive of the RVLM.

Chronic hypoxia (CH) is associated with both blunted agonist-induced and myogenic vascular reactivity, possibly due to an enhanced production of heme oxygenase (HO)-derived carbon monoxide (CO). However, the cellular location of the HO responsible for these effects has not been clearly established. Therefore, we examined the response to administration of the substrate for HO, heme-l-lysinate (HLL), in endothelium-intact and endothelium-denuded small mesenteric arteries from CH male Sprague-Dawley rats. Mesenteric arteries were isolated and mounted on glass cannulas, pressurized to 60 mmHg, and superfused with physiological saline solution. All experiments were performed in the presence of 100 microM N(omega)-nitro-l-arginine. The vasodilator response to HLL or exogenous CO was examined. HLL experiments were performed in the presence and absence of the HO inhibitor zinc protoporphyrin IX (ZnPPIX). HLL administration resulted in a dose-dependent vasodilator response that was abolished in the presence of ZnPPIX or by endothelial removal. Exogenous CO produced a vasodilator response that was independent of an intact endothelium. Cellular localization of HO was verified through immunohistochemistry in sections of the gut and aorta from CH and control animals. Staining for HO-1, HO-2, and endothelial nitric oxide synthase was confined to the endothelium. Thus we conclude that CO is a product of HO located within the endothelium.

Brooks, Virginia L., Yue Qi, and Theresa L. O'Donaughy. Increased osmolality of conscious water-deprived rats supports arterial pressure and sympathetic activity via a brain action. Am J Physiol Regul Integr Comp Physiol 288: R1248 -R1255, 2005. First published January 20, 2005 doi:10.1152/ajpregu.00638.2004.-To test the hypothesis that high osmolality acts in the brain to chronically support mean arterial pressure (MAP) and lumbar sympathetic nerve activity (LSNA), the osmolality of blood perfusing the brain was reduced in conscious water-deprived and water-replete rats by infusion of hypotonic fluid via bilateral nonoccluding intracarotid catheters. In waterdeprived rats, the intracarotid hypotonic infusion, estimated to lower osmolality by ϳ2%, decreased MAP by 9 Ϯ 1 mmHg and LSNA to 86 Ϯ 7% of control; heart increased by 25 Ϯ 8 beats per minute (bpm) (all P Ͻ 0.05). MAP, LSNA, and heart rate did not change when the hypotonic fluid was infused intravenously. The intracarotid hypotonic fluid infusion was also ineffective in water-replete rats. Prior treatment with a V1 vasopressin antagonist did not alter the subsequent hypotensive and tachycardic effects of intracarotid hypotonic fluid infusion in water-deprived rats. In summary, acute decreases in osmolality of the carotid blood of water-deprived, but not waterreplete, rats decreases MAP and LSNA and increases heart rate. These data support the hypothesis that the elevated osmolality induced by water deprivation acts via a region perfused by the carotid arteries, presumably the brain, to tonically increase MAP and LSNA and suppress heart rate. lumbar sympathetic nerve activity; heart rate; vasopressin; intracarotid infusion ALTHOUGH IT IS WELL ESTABLISHED that body fluid osmolality is a sensitive, critical regulator of vasopressin secretion and thirst, more recently, evidence increasingly supports a similar role in the modulation of blood pressure and activity of the sympathetic nervous system [for review, see (37)]. Acute increases in osmolality increase arterial pressure and activate the sympathetic nervous system in a regionally specific manner (1,12,40). Studies in water-deprived animals, in which plasma osmolality is more chronically increased, suggest that this rapid sympathoexcitatory effect can be sustained. First, there is evidence that the basal tone of sympathetic nerves activated by acute increases in osmolality is elevated; adrenal nerve activity and lumbar sympathetic nerve activity (LSNA) appear to be increased (6, 29), but basal renal nerve activity may not be altered (30). One index of overall sympathetic tone, plasma catecholamines, consistently exhibits a 50% elevation; however, this elevation has not always reached statistical significance (2, 10, 18, 38). Nevertheless, the depressor response to ganglionic blockade is usually enhanced (13, 34), consistent with the concept that water deprivation is associated with a teleologically appropriate activation of the sympathetic nervous system. Second, increased osmolality per se contributes to...

Rats exposed to chronic hypoxia (CH; 4 wk at 0.5 atm) exhibit attenuated renal vasoconstrictor reactivity to phenylephrine (PE). Preliminary studies from our laboratory suggest that this response is mediated by hypoxic induction of heme oxygenase (HO) and subsequent release of the endogenous vasodilator carbon monoxide. Because vascular HO mRNA is increased within hours of hypoxic exposure, we hypothesized that the onset of reduced reactivity may occur fairly rapidly and correlate with HO expression. Therefore, we examined the onset of attenuated vasoconstriction on CH exposure as well as the duration of hyporeactivity on return to a normoxic environment. Renal vascular resistance (RVR) responses to graded intravenous infusion of PE were measured in conscious rats under control conditions and after 24 h, 48 h, and 4 wk of CH exposure. Vasoreactivity responses were also determined in 4-wk CH rats 1, 5, 24, and 96 h after return to normoxia. We found that RVR responses to PE were significantly blunted after 48 h and 4 wk but not after 24 h of hypoxic exposure. Inhibition of HO with zinc protoporphyrin IX increased RVR and decreased renal blood flow in 48-h CH rats but not controls. Although reactivity to PE was gradually restored after 4 wk of CH, responsiveness was still slightly blunted at 96 h after return to normoxia. Western blot analysis demonstrated a correlation between HO-1 protein levels and attenuated vasoconstrictor response in CH and posthypoxic rats. These data suggest that the onset and offset of physiologically relevant vascular HO expression occur within 2--3 days.