Donald B. Jennings

Breath timing, volume and drive to breathe in conscious rats: comparative aspects.

In conscious animals, respiratory frequency (f) and tidal volume (VT) vary breath to breath. Examining the average value of variables associated with specific bins of another variable, such as breath f, provides a unique tool to examine respiratory behaviour. In conscious Sprague-Dawley rats respiratory breath timing, tidal volume (VT) and drive (VT/TI) were characterized using a plethysmograph. In the majority of rats at low breath f, expiratory time (TE) exceeded inspiratory time (TI) and these times became equal as f exceeded 150 breaths/min; there was no evidence for TI greater than TE at higher f, as observed in cats and dogs. When VT is normalized per kg, rat breath VT and VT/TI, binned by breath f, are continuous with those for the cat and non-panting dog at the lowest breath f. Relative to breath f, breath VT and VT/TI in rats are greater than in normothermic panting dogs (20 degrees C), but only slightly greater than those variables in panting dogs in the heat (30 degrees C). Lower values of breath VT/TI, binned by breath f or V, in cats and dogs are compensated for by a greater TI relative to the duration of a given breath. This comparative analysis suggests continuities of respiratory pattern generation among species.

Angiotensin II stimulates respiration in awake dogs and antagonizes baroreceptor inhibition

The effects of intravenous infusions of physiologic doses of angiotensin II (AII) on expired ventilation (VE) and acid-base balance were determined in awake dogs. A control infusion of saline was followed by AII infusion, initially with mean arterial pressure (MAP) raised 15%, and then with MAP at control levels by concurrent infusion of sodium nitroprusside (SNP). To control for SNP, the protocol was repeated using arginine vasopressin (AVP). Ventilatory responses to CO2 (VRC) were measured at the end of these protocols and separately with MAP elevated during infusion of AII. With AVP, increased MAP inhibited VE, heart rate (HR) and metabolism. However, with MAP elevated during AII infusion, stimulation by AII opposed baroreceptor reflexes and these variables, as well as plasma AVP, did not change. When MAP was lowered to control during AII infusion all variables increased. With AII, PaCO2 followed VE changes, decreasing 3 Torr with MAP at control levels; however, [H+] remained constant due to a decrease in arterial strong ion difference. The stimulatory effects of AII were not due to SNP; SNP did not stimulate VE during AVP infusion. The slope of the VRC was unaltered by AII infusion or MAP; however, AVP reduced the VRC slope. Physiological increases in AII stimulate VE and other systems at normal MAP and maintain several regulatory systems at control levels during baroreceptor inhibition.

Isolation and characterization of iso-rANP, a new natriuretic peptide from rat atria

Using a specific radioimmunoassay we have isolated and sequenced a new 45-amino acid peptide from rat atria which exhibits similar physiological and pharmacological properties to rat atrial natriuretic peptide (rANP). We have termed the new peptide iso-rANP, because of its functional and structural similarities to rANP. Amino acid sequence differences show that iso-rANP is genetically distinct from rANP. Iso-rANP has a single disulfide bond between residues 23-39 and this portion of the peptide shows substantial homology to rANP and to porcine brain natriuretic peptide (BNP). Little homology is evident at the N- and C-termini of iso-rANP and ANP. Iso-rANP is equipotent with rANP in eliciting diuresis, natriuresis and hypotension in the bioassay rat.

Respiratory effects of pressor and depressor agents in conscious rats

We hypothesized that the respiratory baroreflex in conscious rats is either more transient, or has a higher pressure threshold than in other species. To characterize the effect of arterial pressure changes on respiration in conscious rats, ventilation (V) was measured by the plethysmographic technique during injections, or infusions, of pressor and depressor agents. Bolus injections of angiotensin II (Ang II) or arginine vasopressin (AVP), transiently increased mean arterial pressure (MAP; mean +/- SE) 43+/-6 and 28+/-5 mm Hg (1 mm Hg = 133.3 Pa), respectively, and immediately reduced tidal volume (Vt) and, in the case of AVP, V. In contrast, by 10 min of a sustained elevation of MAP (40+/-3 mm Hg) with infusion of Ang II, Vt, f, and V were not different from control levels. Bolus injection of sodium nitroprusside (SNP) to lower MAP (-28+/-3 mm Hg) immediately increased breathing frequency (f) and V, whereas sustained infusion of SNP to lower MAP (-21+/-3 mm Hg) did not change for V at 10 and 20 min. In conscious rats, both injection and infusion of the pressor agent PE (+40 to 50 mm Hg) stimulated f and V; this contrasted with anesthetized rats where PE inhibited f and V, as reported by others. In conscious rats, respiratory responses associated with baroreflexes adapt rapidly and, in the case of PE, can be overridden by some other mechanism.

Ventilatory effects of angiotensin and vasopressin in conscious rats

Angiotensin II (ANG II) stimulates ventilation (V), when ventilatory baroreceptor reflexes are taken into account, and arginine vasopressin (AVP) causes baroreflex inhibition of V in conscious and anesthetized dogs. To study mechanisms of hormonal modulation of V, a conscious rat model was investigated. V and metabolism were measured during steady-state intravenous infusions of ANG II and AVP in Sprague-Dawley rats (mean arterial pressure (MAP) increased 20 mmHg (1 mmHg = 133.3 Pa)). These data were compared with observations during equal pressor infusions of phenylephrine (PE), an agent classically used to study baroreceptor reflexes. V, respiratory frequency (f), and tidal volume (Vt) were maintained during the increased MAP associated with ANG II infusions, a response identical with that reported in conscious dogs. However, unlike dogs, AVP infusion did not depress V and metabolism in rats. PE in conscious rats caused an unexpected increase in Vt and V in association with increased metabolism. None of the pressor agents affected breath timing when the latter was binned by breath f. Since there was no obvious baroreflex inhibition of V with AVP and PE, potential stimulatory effects of ANG II on respiration could not be discerned. As well, the ventilatory baroreceptor pressure threshold may be higher or adaptation of the reflex may be faster in conscious rats than in dogs.

The effects of O2 and CO2 and of ambient temperature on ventilatory patterns of dogs

Interrelationships between minute ventilation and respiratory rate were examined in conscious dogs at rest and during exercise. Studies were carried out at both cool (19–23 °C) and warm (27–32 °C) ambient temperatures while the dogs breathed room air or different mixtures of O2 and CO2. The relation between respiratory rate and minute ventilation in a cool room was sigmoidal with a mean respiratory rate of 15 breaths per minute when the mean ventilation was 2.9 L per minute and with a mean respiratory rate of 276 breaths per minute when the mean ventilation was 27.5 L per minute. This fundamental sigmoidal relation in the cool room was shifted to the right slightly by breathing 10% O2 or by light exercise and more markedly by breathing 5 % CO2. In a warm room the patterns established in the cool room were shifted to the left. The respiratory response of the animals to a change in the inspired oxygen or carbon dioxide mixture was shown to depend on the initial frequency-ventilation relation.

Acute changes in osmolality and renin and respiratory control of arterial PCO2 and [H+].

To investigate whether osmoreceptor mechanisms or the renin-angiotensin system might be involved in respiratory regulation of H+ homeostasis, plasma osmolality was acutely lowered by approximately 10 mOsm in 7 awake mongrel dogs by a gastric water load (20 ml.kg-1 distilled, deionized water). Plasma renin activity (PRA) was measured as an indicator of angiotensin II levels. During these studies PaCO2 and [H+]a reflected the spontaneous level of ventilation (VE); higher levels of VE were correlated with lower PaCO2 and [H+]a, indicating a nonchemical drive to breathe. Stimulation of ventilation to lower PaCO2 following the water load was positively correlated with increase in PRA and decrease in plasma osmolality, but not with change in osmolality alone. An increased VE, a decreased ventilatory response curve (VRC) threshold for PaCO2, and a lower PaCO2 occurred with increase in PRA. Conversely, a lower or acutely decreased PRA, due to administration of arginine vasopressin, was correlated with a lower VE, an increase in the VRC threshold for PaCO2, and a higher PaCO2. Ventilatory control of PaCO2 during acute lowering of osmolality may be related to a central inter-action between osmolality and the renin-angiotensin system.

Angiotensin II modulates respiratory and acid-base responses to prolonged hypoxia in conscious dogs

We tested the hypothesis that angiotensin II (ANG II) contributes to ventilatory and acid-base adaptations during 3-4 h of hypoxia (partial pressure of O2 in arterial blood ≈ 43 Torr) in the conscious dog. Three protocols were carried out over 3-4 h in five dogs: 1) air control, 2) 12% O2 breathing, and 3) 12% O2 breathing with ANG II receptors blocked by infusion of saralasin (0.5 μg ⋅ kg-1 ⋅ min-1). After 2 h of hypoxia, expired ventilation and alveolar ventilation progressively increased, and the partial pressure of CO2 in arterial blood and the difference between the arterial concentrations of strong cations and strong anions ([SID]) decreased. When the hypoxic chemoreceptor drive to breathe was abolished transiently for 30 s with 100% O2, the resultant central apneic time decreased between 0.5 and 2.5 h of hypoxia. All these adaptive responses to hypoxia were abolished by ANG II receptor block. Because plasma ANG II levels were lower during hypoxia and hypoxic release of arginine vasopressin from the pituitary into the plasma was prevented by ANG II receptor block, the brain renin-angiotensin system was likely involved. It is possible that ANG II mediates ventilatory and acid-base adaptive responses to prolonged hypoxia via alterations in ion transport to decrease [SID] in brain extracellular fluid rather than acting by a direct neural mechanism.

PCO2 modulation of ventilation and HCO−3 buffer during chronic metabolic acidosis

Ventilation and acid-base balance were studied in 6 conscious dogs during chronic eucapnic and hypocapnic metabolic acidosis. The dogs had tracheostomas for respiratory studies, exteriorized carotid arteries for obtaining arterial blood and cannulae for sampling cisternal cerebrospinal fluid (CSF). Measurements were obtained on a control diet, and then, during metabolic acidosis induced by adding HCl (7 mmol/kg per day). Initially during metabolic acidosis, PaCO2 was maintained normal by having the dogs breathe 3% CO2 (eucapnia); then the dogs breathed air (hypocapnia). Chronically, arterial and CSF [HCO-3] were related to PCO2. No respiratory compensation occurred during chronic hypocapnic metabolic acidosis since [HCO-3] decreased more than PCO2; consequently, the acidosis worsened. At any [H+], ventilation was related to PCO2. Thus, during hypocapnic metabolic acidosis, ventilation was not increased relative to increase in arterial and CSF [H+]. Modulation of ventilation by PCO2 during severe acidosis may be crucial because stimulation of ventilation by [H+] of arterial blood or CSF would have progressively reduced PCO2 and [HCO-3], resulting in a worsening of the metabolic acidosis.

Ventilatory and metabolic effects of hypercapnia in conscious rats: AVP V1 receptor block.

In conscious dogs, arginine vasopressin (AVP) inhibits an angiotensin II drive to ventilation during air breathing and during acute hypercapnia. To determine whether AVP inhibits respiration in rats, as in dogs, respiration and metabolism were measured in six male Sprague-Dawley rats using a plethysmograph. Rats breathed air, followed by 5% and 6.5% CO2 with or without AVP V1 receptor block. In unblocked experiments, minute ventilation (V) increased to a comparable level during inhalation of both CO2 gas mixtures, resulting in a flattening of the ventilatory response to increased Paco2. However, oxygen consumption decreased during 6.5% CO2, compared with 5% CO2, so that the ventilatory equivalent for O2 increased in a more linear manner with respect to Paco2. The main effect of AVP V1 receptor block was to increase mean arterial blood pressure; there was no significant effect of AVP V1 receptor block on respiratory responses. AVP does not inhibit respiration in conscious rats as it does in conscious dogs.