Influence of nitric oxide on the hemodynamic response to hemorrhage in conscious rabbits

Role of Nitric Oxide in Cardiovascular Adaptation to Intermittent Hypoxia

Hypoxia is one of the most frequently encountered stresses in health and disease. The duration, frequency, and severity of hypoxic episodes are critical factors determining whether hypoxia is beneficial or harmful. Adaptation to intermittent hypoxia has been demonstrated to confer cardiovascular protection against more severe and sustained hypoxia, and, moreover, to protect against other stresses, including ischemia. Thus, the direct and cross protective effects of adaptation to intermittent hypoxia have been used for treatment and prevention of a variety of diseases and to increase efficiency of exercise training. Evidence is mounting that nitric oxide (NO) plays a central role in these adaptive mechanisms. NO-dependent protective mechanisms activated by intermittent hypoxia include stimulation of NO synthesis as well as restriction of NO overproduction. In addition, alternative, nonenzymic sources of NO and negative feedback of NO synthesis are important factors in optimizing NO concentrations. The adaptive enhancement of NO synthesis and/or availability activates or increases expression of other protective factors, including heat shock proteins, antioxidants and prostaglandins, making the protection more robust and sustained. Understanding the role of NO in mechanisms of adaptation to hypoxia will support development of therapies to prevent and treat hypoxic or ischemic damage to organs and cells and to increase adaptive capabilities of the organism.

Visceral pain decreases tolerance to blood loss in conscious female but not male rabbits

Pain is a component of traumatic blood loss, yet little is known about how pain alters the response to blood loss in conscious animals. We evaluated the effects of colorectal distension on the cardiorespiratory response to blood loss in six male and six female conscious, chronically instrumented New Zealand White rabbits. The goal of these experiments was to test the hypotheses that 1) colorectal distension would increase tolerance to hemorrhage (i.e., increase the blood loss required to decrease mean arterial pressure <or= 40 mmHg); and 2) the increase in tolerance would be similar in male and female rabbits. For hemorrhage, venous blood was withdrawn until mean arterial pressure decreased to <or=40 mmHg. Conscious rabbits underwent three treatments in a balanced design: a control hemorrhage, hemorrhage with a colorectal balloon present but not inflated (sham CRD), and hemorrhage in the presence of colorectal distension (CRD). Colorectal distension reproducibly increased mean arterial pressure, decreased respiratory rate, and did not change heart rate. There was no difference in control blood loss between males (21.8 +/- 0.3 ml/kg) and females (21.6 +/- 0.3 ml/kg). However, although CRD blood loss did not change in males (22.8 +/- 0.3 ml/kg), it was significantly less than control in females (19.1 +/- 0.3 ml/kg; P = 0.004). Thus, in conscious rabbits, colorectal distension alters cardiovascular control during hemorrhage. Furthermore, colorectal distension did not improve tolerance to blood loss in males or females as hypothesized but instead decreased tolerance to blood loss only in females.

Cardiovascular responses to oxygen inhalation after hemorrhage in anesthetized rats: hyperoxic vasoconstriction

Oxygen inhalation is recommended for the initial care of trauma victims. The improved survival seen in early hemorrhage is normally associated with an increase in blood pressure. Although clinical use of oxygen can occur late after hemorrhage, the effects of late administration have not been specifically examined. Anesthetized rats were studied using an isobaric hemorrhage model with target pressures of either 70 or 40 mmHg. At various times after hemorrhage, the feedback control of the blood pressure was stopped and the inspired gas was changed from room air to 100% oxygen. The results show that shortly after hemorrhage to 70 mmHg, oxygen inhalation results in an increase in mean arterial blood pressure of 60 +/- 3 mmHg, which is associated with a large increase in total peripheral resistance from 0.89 +/- 0.05 to 1.25 +/- 0.1 peripheral resistance units. The blood pressure response is essentially unchanged with time, and it is not altered by a 10-min exposure to N(G)-nitro-l-arginine methyl ester. At a target pressure of 40 mmHg, the initial blood pressure response to oxygen is the same, but it gradually decreases as the animal develops a lactic acidosis. We conclude that the therapeutic value of oxygen needs to be separately evaluated for late hemorrhage.

Brisk production of nitric oxide and associated formation ofS-nitrosothiols in early hemorrhage

The results of previous inhibitor studies suggest that there is some increase in nitric oxide (NO) production from constitutive NO synthase in early hemorrhage (H), but the magnitude of NO production early after H has not been previously assessed. It is generally believed that only modest production rates are possible from the constitutively expressed NO synthases. To study this, anesthetized male Sprague-Dawley rats were subjected to 90 min of isobaric (40 mmHg) H. During this period of time, the dynamics of accumulation of NO intermediates in the arterial blood was assessed using electron paramagnetic resonance spectroscopy, chemiluminescence, fluorescence imaging, and mass spectrometry. Electron paramagnetic resonance-detectable NO adducts were also measured with spin traps in blood plasma and red blood cells. H led to an increase in the concentration of hemoglobin-NO from 0.9 ± 0.2 to 4.8 ± 0.7 μM. This accumulation was attenuated by a nonselective inhibitor of NO synthase, NG-nitro-l-argininemethyl ester (l-NAME), but not by NG-nitro-d-argininemethyl ester (d-NAME) or 1400W. Administration of l-NAME (but not 1400W or d-NAME) during H produced a short-term increase in mean arterial pressure (∼90%). In H, the level of N oxides in red blood cells increased sevenfold. S-nitrosylation of plasma proteins was revealed with “biotin switch” techniques. The results provide compelling evidence that there is brisk production of NO in early H. The results indicate that the initial compensatory response to H is more complicated than previously realized, and it involves an orchestrated balance between intense vasoconstrictor and vasodilatory components.

Hemodynamic effects of blood loss during a passive response to a stressor in the conscious rabbit

In the conscious rabbit, exposure to an air jet stressor increases arterial pressure, heart rate, and cardiac output. During hemorrhage, air jet exposure extends the blood loss necessary to produce hypotension. It is possible that this enhanced defense of arterial pressure is a general characteristic of stressors. However, some stressors such as oscillation (OSC), although they increase arterial pressure, do not change heart rate or cardiac output. The cardiovascular changes during OSC resemble those seen during freezing behavior. In the present study, our hypothesis was that, unlike air jet, OSC would not affect defense of arterial blood pressure during blood loss. Male New Zealand White rabbits were chronically prepared with arterial and venous catheters and Doppler flow probes. We removed venous blood until mean arterial pressure decreased to 40 mmHg. We repeated the experiment in each rabbit on separate days in the presence and absence (SHAM) of OSC. Compared with SHAM, OSC increased arterial pressure 14 ± 1 mmHg, central venous pressure 3.3 ± 0.4 mmHg, and hindquarter blood flow 34 ± 4% while decreasing mesenteric conductance 32 ± 3% and not changing heart rate or cardiac output. During normotensive hemorrhage, OSC enhanced hindquarter and renal vasoconstriction. Contrary to our hypothesis, OSC (23.5 ± 0.6 ml/kg) increased the blood loss necessary to produce hypotension compared with SHAM (16.8 ± 0.6 ml/kg). In nine rabbits, OSC prevented hypotension even after a blood loss of 27 ml/kg. Thus a stressful stimulus that resulted in cardiovascular changes similar to those seen during freezing behavior enhanced defense of arterial pressure during hemorrhage.

Defense reaction alters the response to blood loss in the conscious rabbit

The interaction of sensory stressors with the cardiovascular response to blood loss has not been studied. The cardiovascular response to a stressor (i.e., the defense reaction) includes increased skeletal muscle blood flow and perhaps a reduction in arterial baroreflex function. Arterial pressure maintenance during blood loss requires baroreflex-mediated skeletal muscle vasoconstriction. Therefore, we hypothesized that the defense reaction would limit arterial pressure maintenance during blood loss. Male, New Zealand White rabbits were chronically prepared with arterial and venous catheters and Doppler flow probes. We removed venous blood in conscious rabbits until mean arterial pressure decreased to <40 mmHg. We repeated the experiment with (air) and without (sham) simultaneous exposure to an air jet stressor. Air resulted in a defense reaction (e.g., mean arterial pressure = 94 +/- 1 and 67 +/- 1 mmHg for air and sham, respectively). Contrary to our hypothesis, air increased the blood loss necessary to produce hypotension (19.3 +/- 0.2 vs. 16.9 +/- 0.2 ml/kg for sham). Air did not reduce skeletal muscle vasoconstriction during normotensive hemorrhage. However, air did enhance renal vasoconstriction (97 +/- 3 and 59 +/- 3% of baseline for sham and air, respectively) during the normotensive phase. Thus the defense reaction did not limit but rather extended defense of arterial pressure during hemorrhage.