Effects of hypercapnia on metabolism, temperature, and ventilation during heat and fever.

We wished to determine whether 1) acute hypercapnia in cats changes metabolic heat production to affect temperature regulation and 2) heat exposure and fever affect the normal response. The effects of breathing 4% CO2 on O2 consumption (VO2), rectal temperature (Tre), and ventilation (V) were measured in five conscious resting cats. Cats were exposed to a normal (24-27 degrees C) chamber temperature (Ta) and a warm (33-34 degrees C) chamber Ta. Fever was produced by intravenous injection of Escherichia coli endotoxin (0.02 microgram/kg) at a normal Ta. In normothermic cats, hypercapnia decreased VO2 by approximately 40%, despite an increased V (approximately 100%), but Tre decreased only transiently and slightly compared with studies in which air was breathed throughout. During heat studies, average V was elevated but VO2 was markedly lower than at the normal Ta; Tre gradually increased. Hypercapnia combined with heat did not cause additional increases in VO2, nor did it cause a decrease in VO2, as at a normal Ta; however, the rate of rise of Tre during heat was slowed by hypercapnia. During febrigenesis, hypercapnia prevented the transient increase in VO2 observed when air was breathed and delayed the rate of rise in Tre. As Tre was falling in fever, hypercapnia depressed VO2 but did not affect Tre compared with fever studies in which air was breathed. Unlike heat exposure, hypercapnia had a further additive stimulatory effect on the increase in V at the onset of fever, and it increased V during the falling phase of fever when V had returned to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiration in awake cats: sympathectomy and deafferentation of carotid bifurcations.

Respiratory effects of sympathectomy of the carotid bifurcations and, in a subsequent experiment, bilateral carotid sinus nerve section were examined in six awake resting cats. In each intact and denervated state, sequential breaths were analyzed at 10-min intervals up to 80 min. Individual breath frequency (f), tidal volume (VT), and ventilation (V = f X VT) were determined. In individual cats, sympathectomy or deafferentation could cause significant increases or decreases in ventilation or no change. Thus the range of spontaneous variability in breath V as well as minute ventilation (VE), averaged for the group, were not consistently altered in the same direction by either sympathectomy or deafferentation of the carotid bifurcations. Interestingly, in most cats after both sympathectomy (5 of 6) and deafferentation (4 of 6), VT increased and f decreased relative to V. Despite this, after sinus nerve section in two cats arterial PO2 decreased and arterial PCO2 tended to increase relative to VE, suggesting possible effects of deafferentation on ventilation-perfusion balance. Sympathectomy also affected timing such that inspiratory time began to exceed 0.5 of the breath duration at a lower breath f; this effect of sympathectomy was reversed to intact values by subsequent sinus deafferentation. Thus, in eupneic awake cats, sympathetics normally suppress reflex modulation of central timing from carotid chemoreceptors and/or baroreceptors.

Ventilation is stimulated by small reductions in arterial pressure in the awake dog.

Changes in arterial pressure commonly accompany respiratory adaptations. The purpose of this study was to determine, in awake dogs (n = 6), the degree to which small acute decreases in arterial pressure affect ventilation and acid-base balance. Mean arterial pressure (MAP) was reduced by 6 +/- 2, 10 +/- 3, and 16 +/- 2% by intravenous infusion of sodium nitroprusside for sequential 20-min periods. In another experiment, the ventilatory response to hypercapnia was determined during MAP reduction of 16 +/- 3%. Step reductions in MAP were accompanied by increases in minute ventilation (maximum increase 152 +/- 75%) and step reductions in arterial PCO2 (PaCO2; maximum reduction -4.8 +/- 0.8 Torr). Although eupneic PaCO2 threshold was lowered during MAP reduction, ventilatory sensitivity to CO2 remained unchanged. Despite the lowered PaCO2, arterial [H+] remained constant (acid-base balance was maintained) as a result of a concurrent decrease in strong ion difference. Plasma renin activity increased during MAP reduction (93 +/- 39%) and may have contributed to the increase in minute ventilation, inasmuch as angiotensin II can stimulate respiration by a central mechanism. Evidence is provided that nitroprusside is unlikely to be a primary factor in these hypotensive responses. We conclude that relatively modest decreases in MAP have a consistent stimulatory effect on respiratory control. Therefore it is important to take into account effects of small changes in MAP when interpreting mechanisms for respiratory responses in awake animals.

During acute hypercapnia vasopressin inhibits an angiotensin drive to ventilation in conscious dogs.

Intravenous infusion of arginine vasopressin (AVP) depresses the slope of the ventilatory response to CO2 during acute hypercapnia. We therefore tested the hypothesis that AVP V1-receptor blockade would increase the slope of the ventilatory response to CO2. After a 20-min control period, an AVP V1-receptor antagonist (d(CH2)5[Tyr(Me)2]AVP) was injected into six conscious resting dogs. Thirty minutes after AVP V1-receptor blockade, dogs were exposed to sequential 20-min periods of 5 and 6.5% inspired CO2 in air. A second protocol (no AVP V1-receptor blockade) was conducted as a control. As predicted, AVP V1-receptor blockade enhanced ventilation during inhalation of 6.5% CO2 in association with an increased metabolic rate and increased plasma angiotensin II (ANG II). In eupneic dogs, stimulation of respiration by AVP V1-receptor blockade is mediated by ANG II. A third protocol with ANG II-receptor blockade (intravenous infusion of saralasin) combined with AVP V1-receptor blockade indicated that ANG II mediated the increase in metabolism and the augmented ventilation during inhalation of 6.5% CO2. We conclude that during acute hypercapnia of sufficient magnitude, and perhaps duration, AVP inhibits an ANG II-mediated stimulation of metabolism and respiration.

Angiotensin mediates stimulation of ventilation after vasopressin V1 receptor blockade.

We tested the hypothesis that respiration would be stimulated after vasopressin (AVP) V1 receptor blockade because of disinhibition and activation of the renin-angiotensin system. Intravenous infusion of angiotensin II (ANG II) stimulates respiration, presumably centrally, via circumventricular organs. In the present study, the AVP V1 receptor antagonist [1-(beta-mercapto-beta,beta-cyclopentamethylene propionic acid),2-(O-methyl)tyrosine]-Arg8-AVP (PMP; 10 micrograms/kg i.v.) was administered to six awake resting dogs. Measurements were made 30 min prior, and 60 min subsequent, to injection of PMP (protocol 1). In three other protocols, the ANG II blocker saralasin (0.5 microgram.kg-1.min-1 i.v.) was infused starting 20 min before PMP (protocol 2) and 30 min after PMP (protocol 4) and saline was infused (0.2 ml/min) over 90 min as a control (protocol 3). After PMP in protocol 1, alveolar ventilation increased and arterial PCO2 decreased (approximately 3 Torr). ANG II receptor blockade prevented (protocol 2) and reversed (protocol 4) respiratory stimulation by PMP. Despite ventilatory stimulation, plasma renin activity and ANG II were not increased after PMP relative to control (protocol 3). We conclude that AVP acts at V1 receptors to inhibit formation of brain ANG II. Brain ANG II must modulate respiratory control via a circumventricular organ, because systemically administered saralasin, which does not cross the blood-brain barrier, blocked stimulation of respiration.

Ventilation and acid-base balance in awake dogs exposed to heat and CO2.

Some awake quiet dogs pant at cool ambient temperature (Ta) and some do not pant even when acutely exposed to heat. The purpose of the study was to determine whether this puzzling variability in respiratory behavior diminished during prolonged heat. The contributions of thermal and CO2 drives to respiratory adaptations were also examined. Five awake dogs acclimated to 20 degrees C were studied before and 2 and 48 h following exposure to 30-31 degrees C. Rectal temperature did not change; the important thermal stimulus, even at 48 h, appeared to be the increase in peripheral temperature. Variability between nonpanting and panting persisted over 48 h. On the average, ventilation (VE) doubled during heat, largely due to increased dead space ventilation. Nonpanting dogs at cool Ta decreased the threshold of the ventilatory response to CO2. A panting dog at cool Ta changed its slope of the ventilatory response from negative to positive. During hypercapnia in acute heat, ventilatory pattern changed so that frequency increased and tidal volume decreased for a given VE. By 48 h of heat, the ventilatory response to CO2 returned to control in only two dogs, but the ventilatory pattern during hypercapnia returned to control in four dogs. Since thermal stimuli remained unchanged at 48 h, adaptations of respiratory control may have been related to progressive adjustments of strong ions and acid-base balance.

Renin-angiotensin system stimulates respiration during acute hypotension but not during hypercapnia.

We reported that intravenous infusion of angiotensin II (ANG II) stimulated ventilation (VE) in conscious dogs. Other studies in our laboratory have demonstrated that increases in respiration occurred in association with activation of the renin-angiotensin system during acute hypotension and during hypercapnia. Therefore, in conscious dogs (n = 5), we examined the effects of ANG II receptor blockade with intravenous saralasin (0.5 micrograms.kg-1.min-1) on respiratory responses during progressive nitroprusside-induced hypotension and during the ventilatory response to increased inspired fraction of CO2 (VRC). During hypotension (mean arterial pressure decreased approximately 20%) combined with ANG II receptor blockade, VE, heart rate, and arginine vasopressin increases were attenuated compared within unblocked studies. With ANG II receptor blockade during hypotension, alveolar ventilation and arterial PCO2 (PaCO2) were unchanged, which contrasted with a doubling of alveolar ventilation and a decrease of 4.8 +/- 1 Torr in PaCO2 in unblocked studies. During hypercapnia, the slope of the VRC was not affected by ANG II receptor blockade, but with 6.5% inspired CO2 fraction, VE and PaCO2 were lower than in unblocked studies. These results indicated that ANG II contributed to the respiratory response to a modest hypotension but did not affect respiratory sensitivity to CO2.

Respiration during acute hypoxia: angiotensin- and vasopressin-receptor blocks.

In normoxic conscious dogs, increased angiotensin II (ANG II), or activation (disinhibition) of the renin-angiotensin system by vasopressin (AVP) V1-receptor block, increases ventilation and decreases arterial PCO2. Both hormones can be increased during hypoxia and might modulate ventilatory drive. Six conscious dogs were studied before and during hypocapnic, isocapnic, and hypercapnic hypoxia. To study potential hormonal effects during hypocapnic hypoxia, experiment 1 included three protocols in which 12.8% O2 was breathed for 60 min: protocol 1, control studies without block; protocol 2, AVP V1 receptors were blocked at the onset of hypoxia; and protocol 3, ANG II receptors were blocked 20 min before hypoxia. To study potential effects of acid-base changes during acute hypoxia, experiment 2 included two protocols (with and without AVP V1-receptor block). A 40-min period of hypocapnic hypoxia was followed by two successive 20-min periods with hypoxia maintained but inspired CO2 progressively increased. Neither hormonal block affected respiration during the hypoxic conditions. Unlike normoxia in conscious dogs, during acute hypoxia, respiratory control by ANG II is not modulated by AVP and acid-base effects on receptors do not account for this difference.

Circulatory and metabolic responses in awake dogs to infusion of iso-rANP/(rBNP).

We reported that a second rat atrial peptide, iso-atrial natriuretic peptide (iso-rANP(1-45)) and a potential putative homologue, iso-rANP(17-45) (identical with rat brain natriuretic peptide except for one amino acid) elicited circulatory and renal responses in anesthetized rats. In the present studies, low-dose intravenous infusions of iso-rANP(1-45) (6.3-25 pmol kg-1 min-1) and iso-rANP(17-45) (12.5-50 pmol kg-1 min-1) into conscious dogs produced subtle circulatory effects compared to control studies. Relative to oxygen consumption, cardiac output was lower and total peripheral resistance higher with both iso-rANP(1-45) and iso-rANP(17-45). Heart rate tended to be slightly lower relative to control studies during peptide infusions, and the highest infusion doses caused a decrease in mean arterial pressure. Plasma protein increased and plasma osmolality decreased with iso-rANP(1-45); infusion of iso-rANP(17-45) caused a decrease in the respiratory exchange ratio. The mechanism of action of iso-rANP may have been direct, via an active receptor. However, we previously reported for these same experiments that infusion of iso-rANP(1-45) and iso-rANP(17-45) increased plasma ANP and decreased plasma renin activity. Thus, circulatory changes during infusion of iso-rANP were consistent with an indirect mechanism related to increased endogenous ANP.

The antidiuretic effect of pneumadin requires a functional arginine vasopressin system.

Pneumadin is an antidiuretic decapeptide, recently isolated from rat and human lung. Bolus intravenous injection of 5 nmol of pneumadin into water-loaded rats caused a rapid and significant antidiuresis and a reduction in Na+ and Cl- excretion. Pneumadin administration did not alter mean arterial pressure, right atrial pressure, heart rate or haematocrit. Bolus intravenous injection of 20 nmol of pneumadin into non-water-loaded rats caused a significant increase in arginine vasopressin (AVP) within 10 min. Pneumadin administration also increased circulating atrial natriuretic peptide (ANP) but did not alter aldosterone or plasma renin activity levels. Injection of pneumadin into water-loaded Brattleboro rats, which genetically lack circulating AVP, did not change urine flow, confirming that the pneumadin induced antidiuresis is AVP dependent. Radioactive pneumadin was cleared from the circulation with a t1/2 beta of 480.3 s. Radioactive pneumadin, isolated from plasma, eluted at an altered position on reverse phase HPLC, which indicated that the peptide was modified in vivo. This modification was also observed when synthetic pneumadin was incubated in rat plasma in vitro. Purification and sequencing of the modified synthetic peptide indicated that the modification is not a proteolytic cleavage. These results indicate that pneumadin injected into the rat caused an antidiuresis by altering circulating AVP levels.

Plasma clearance and tissue binding of rANP[99-126] and iso-rANP[1-45] in the rat.

Plasma clearance and tissue binding of atrial natriuretic peptide (ANP) and iso-ANP were compared in Inactin-anaesthetized rats. It was found that the plasma half-life of iso-ANP was comparable to ANP. Appearance of trichloroacetic acid-soluble radioactivity of iso-ANP in the plasma was considerably slower than that of ANP, suggesting that the metabolic process of these two peptides may be different. Although the binding distribution of these two peptides was similar, the total binding of iso-ANP to organs other than the kidney was much lower. The kidney, lung, heart and adrenal gland appeared to be major target organs for iso-ANP. Autoradiography showed that iso-ANP bound specifically to the renal glomerulus and proximal part of the proximal tubule. This latter binding site in the kidney was not apparent with ANP, suggesting that iso-ANP may exerts its physiological action at different sites in this organ.

The physicochemistry of [H+] and respiratory control: roles of PCO2, strong ions, and their hormonal regulators.

I describe how the dietary intake of strong ions potentially affects the regulation of ventilation and the PCO2 of body fluids in two ways. First, changing the dietary intake of NaCl can alter the concentration difference between strong cations and strong anions (the [SID] of Stewart) of body fluids. Experimental observations indicate that the [SID] in brain fluids or cerebrospinal fluid ([SID]CSF) could be the stimulus to central chemoreceptors. [SID]CSF consistently predicts ventilatory regulation of PCO2, whereas [H+]CSF does not. PCO2 acts as a stimulus to ventilation independently of [SID]CSF and possibly at higher as well as lower centers of the nervous system. I relate the concept of [SID] regulation of arterial PCO2 to the alphastat hypothesis of protein function, respiratory control, and [H+] homeostasis. Second, altering the dietary intake of NaCl changes the levels of hormones involved in salt and water balance. Angiotensin II acts centrally to stimulate ventilation. Evidence for the roles of both the renal and brain renin-angiotensin systems in respiratory control, and the modulation of respiratory control by vasopressin are reviewed. These peptide systems probably act via circumventricular organs of the brain to affect respiratory control and (or) by changing strong ion concentrations in brain fluids. Questions to be resolved on the role of [SID]CSF and hormones in respiratory adaptations, and experiments required to improve our understanding of the control of ventilation, are addressed in the concluding comments.

Respiratory control during exercise: hormones, osmolality, strong ions, and PaCO2.

For optimal performance of exercising muscle, the charge state of proteins must be maintained; the pH environment of protein histidine imidazole groups must be coordinated with their pK. During exercise, increasing temperature and osmolality as well as changes in strong ions affect the pK of imidazole groups. Production of strong organic anions also decreases the concentration difference between strong cations and anions (strong ion difference, or [SID]), causing a metabolic acidosis in peripheral tissues. Central chemoreceptors regulate PCO2 in relation to the [SID] of brain fluids to maintain a "constant" brain [H+]. In addition, increased osmolality, angiotensin II, and vasopressin during exercise may stimulate circumventricular organs of the brain and interact with chemical control of ventilation. Changes in [SID] of brain fluids during exercise are negligible compared to systemic decreases in [SID]; thus, regulation of PCO2 to maintain brain [H+] homeostasis cannot simultaneously compensate for greater changes in [SID] in peripheral tissues.

Breathing for protein function and [H+] homeostasis.

Based on a physicochemical analysis of H+ homeostasis, we hypothesize that PCO2 and strong ions, and not [H+], act independently on chemosensors in the central nervous system to regulate ventilation. [H+] in body fluids and the pK of histidine imidazole groups of proteins must be regulated in relation to each other to preserve protein conformation and function. Three independent variables regulate [H+] in body fluids: P(CO2), the strong ion difference ([SID]; ([Na+] + [K+]) - ([Cl-] + [lactate-])), and total weak anion. Temperature, osmolality and strong ions affect the pK of proteins. Our data and the literature support the hypothesis that [SID] is the stimulus to central medullary chemoreceptors and ventilation. The resulting change in PCO2 counterbalances change in [SID] and maintains [H+] constant. For example, a diet low in NaCl predisposes to a high [SID] (acts to decrease [H+]); increased [SID] increases the PaCO2 threshold of the ventilatory response to CO2, decreases alveolar ventilation, and increases PCO2 to maintain [H+] 'constant'. Because ventilation is stimulated by changes in PCO2 at constant [SID], PCO2 acts independently of [SID]. As well, change in osmolality and/or angiotensin II level, associated with alterations in water and electrolyte balance, act as stimuli to ventilation and interact with chemical control. Establishing the contributions of these neural, humoral and chemical mechanisms in respiratory adaptations will provide a challenge for future investigation.

Body temperature and ventilatory responses to CO2 during chronic respiratory acidosis.

During acute hypercapnia (5% carbon dioxide) in resting conscious dogs, ventilation (Ve) attained a new level above control within 5 min, but rectal temperature decreased gradually to reach a steady state lower than control after 40-60 min. At 2 days of breathing 5% carbon dioxide, Ve remained elevated, as in acute hypercapnia, but Paco2 increased and the threshold of the ventilatory response shifted to a higher Paco2. By 2 days of hypercapnia, rectal temperature (Tr) had returned to normal, reflecting an alteration of hypothalamic temperature control that might be expected to result in enhanced respiratory drive. Surprisingly, despite blood acid-base compensation between 2 and 14 days of hypercapnia, Ve did not decrease, whereas Paco2 decreased to the level observed during acute hypercapnia, and the threshold of the ventilatory response returned to normal. Therefore, at 14 days of respiratory acidosis, acid-based compensation resulting from increase in bicarbonate was not associated with reduced respiratory drive. This result could not be accounted for on the basis of a temperature mechanism because temperature adaptation occurred earlier.

Cardiorespiratory effects of prolonged angiotensin II block in resting conscious dogs.

Intravenous (iv) infusion of the angiotensin II (ANG II) receptor blocker saralasin in resting conscious dogs during physiological pertubations, such as hypotension and prolonged hypoxia, indicates the presence of an ANG II drive to increase respiration and decrease the arterial partial pressure of CO2 (PaCO2). In contrast, in eupneic resting dogs on a regular chow diet, iv infusion of saralasin for short periods (up to 30 min) provides no evidence of a tonic effect of circulating levels of ANG II on acid-base balance, respiration, metabolism, or circulation. However, ANG II influences physiological processes involving salt, water, and acid-base balances, which are potentially expressed beyond a 30 min time period, and could secondarily affect respiration. Therefore, we tested the hypothesis that blocking ANG II with iv saralasin would affect respiration and circulation over a 4-h period. Contrary to the hypothesis, iv infusion of saralasin in resting conscious eupneic dogs on a regular chow diet over a 4-h period had no effects on plasma strong ions, osmolality, acid-base balance, respiration, metabolism, or circulation when compared with similar control studies in the same animals. Thus, ANG II does not play a tonic modulatory role in respiratory control under "normal" physiological conditions.

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.

Effects of hypercapnia on variability of normal respiratory behavior in awake cats.

Resting quiet awake cats breathing air in a steady state have a range of respiratory behavior, and this encompasses nonpurring and purring (D. B. Jennings and P. C. Szlyk, Can. J. Physiol. Pharmacol. 63: 148-154, 1985). On a given study day, individual cats usually breathed in a limited part of their potential respiratory range. Respiratory pattern, such as average breath frequency (f) and average tidal volume (VT) utilized for a given level of ventilation (V), could be predicted when cats breathed air; as well, inspiratory (TI) and expiratory (TE) times were specific for a given breath f. Inhalation of 2% and 4% CO2 in air caused an average increase in ventilation of 16 and 100%, respectively but breath-to-breath variability of V, f, and VT persisted at each fractional concentration of inspired CO2 (FICO2). The range of different V utilized breath to breath when breathing 2% CO2 overlapped with V during air control studies. Substantial overlap with control V also occurred in three of six cats when breathing 4% CO2. The most consistent effect of progressive hypercapnia was to increase VT and decrease f at a given level of V; increase in V during hypercapnia was accounted for by an increase in mean inspiratory flow (VT/TI). Hypercapnia also caused the fraction of breathing cycle devoted to inspiration (TI/TT) to increase at low f but not at high f.

H+ homeostasis, osmolality, and body temperature during controlled NaCl and H2O intake.

Arterial PCO2, arterial [H+] ([H+]a), electrolytes, and osmolality, as well as rectal temperature (Tre), were monitored in six awake dogs over sequential 12- or 13-day periods in which their NaCl intake was first less than 5 meq/day, then approximately 120 meq/day, and finally less than 5 meq/day. Water intake was maintained constant at 77 ml.kg-1.day-1 throughout. During low-NaCl periods, decreases in body and plasma water, indicated by weight loss did not prevent lower arterial [Na+] ([Na+]a), arterial [Cl-] ([Cl-]a), and osmolality relative to the high-NaCl period. During high dietary NaCl, the arterial strong ion difference [[SID]a = ([Na+]a + [K+]a) - (arterial [lactate-] + [Cl-]a)] was lower. From physicochemistry, this lowered [SID]a results in a higher [H+]a. However, independent of NaCl intake, [H+]a was positively correlated with plasma osmolality; moreover, [H+]a, relative to plasma osmolality, was higher at lower Tre than at higher Tre. We speculate that this spectrum of plasma osmolality and body temperature may contribute to the creation of an appropriate protein pK to match plasma [H+]a. We also found that the difference between plasma [protein] (measured by the biuret test) and [ATOT]a (an estimation of plasma protein as total weak acid from physicochemistry) was related to plasma osmolality, [SID]a, and [Na+]a. These latter relations may reflect the effect of plasma water concentration (osmolality) and strong ions on the pK of plasma proteins.