Unanesthetized Goats Research Articles

Hypoxia does not increase CSF or plasma beta-endorphin activity.

The ability of moderate (30-50 Torr arterial PO2) and severe (less than 30 Torr arterial PO2) hypoxia to generate endogenous opioids that modulate ventilation was studied in unanesthetized goats. Ventilation and its components, arterial blood gas tensions and pH, and plasma and cerebrospinal fluid (CSF) beta-endorphin activity were measured before and after 4 h of sustained moderate or severe hypoxia. Ventilation, as expected, increased with hypoxia. There were no significant changes in either plasma or CSF beta-endorphin activity after sustained hypoxia. To rule out elaboration of endogenous opioids other than beta-endorphin after hypoxia, naloxone or saline was administered to five of the seven goats exposed to 4 h of severe hypoxia, and their ventilatory responses were compared for 30 additional min of hypoxic breathing. No significant differences in ventilation occurred in the two treatment groups during this time period. We conclude that, unlike increases in airway resistance, moderate and severe hypoxia do not cause the elaboration of endogenous opioids that modify respiratory output in unanesthetized adult goats. The apparent ability of hypoxia to cause elaboration of endogenous opioids in the neonate may represent a maturational phenomenon.

Cerebral effects of extended hyperventilation in unanesthetized goats.

Thirty-six adult, male unanesthetized goats were hyperventilated to a PaCO2 level of 16-18 mm Hg for 6 hours. Arterial and sagittal sinus blood and cerebrospinal fluid were analyzed for pH, blood gases, bicarbonate, lactate, and pyruvate before hyperventilation, during hyperventilation, and after the termination of hyperventilation. Total cerebral blood flow, regional brain blood flows, and cerebral metabolic rate for oxygen were calculated from the distribution of radioactive microspheres. Intracranial pressure was measured in either the right or left cerebral ventricle. With the initiation of hyperventilation, cerebral blood flow and cerebral metabolic rate for oxygen fell significantly (64 +/- 5 ml/100 g/min to 41 +/- 3; 4.6 +/- 0.3 ml O2/100 g/min to 3.6 +/- 0.2), but both returned to prehyperventilation values within 6 hours of hyperventilation. With termination of hyperventilation, cerebral blood flow and cerebral metabolic rate for oxygen increased significantly above control levels (64 +/- 5 vs. 105 +/- 9; 4.6 +/- 0.3 vs. 5.4 +/- 0.4). Intracranial pressure was unaffected by hyperventilation or its termination. Arterial and sagittal sinus blood and cerebrospinal fluid pH increased with hyperventilation but returned to control values by 6 hours. However, pH was still significantly elevated at 6 hours. Lactate and pyruvate followed a similar pattern except in the cerebrospinal fluid, where both increased throughout the course of hyperventilation. There were no significant differences in the lactate:pyruvate ratio. On termination of hyperventilation, pH of the arterial and sagittal sinus blood and cerebrospinal fluid fell below control levels. Bicarbonate values decreased in all fluid compartments and were still below control values 2 hours after the cessation of hyperventilation.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of endogenous opioids in the ventilatory response to acute flow-resistive loads.

The ability of acute, short-term, inspiratory flow-resistive loading to generate endogenous opioids was studied in 6 unanesthetized goats. Endogenous opioid generation was assessed by measurement of immunoreactive beta-endorphin levels in the cisternal cerebrospinal fluid (CSF) after high (80 cm H2O/L/s) and moderate (50 cm H2O/L/s) resistive loading. The results show that CSF levels of beta-endorphin were significantly increased by both the high and moderate resistive loads (40 +/- 4 SEM pg/ml and 33.7 +/- 3.4 pg/ml, respectively) when compared with the same animals during unloaded control conditions (19.5 +/- 3.8 pg/ml). Both levels of loading also caused a significant progressive decline in tidal volume (to 82 +/- 8 and 89 +/- 8% of baseline tidal volume with the high and moderate loads, respectively). Naloxone administration (0.1 mg/kg) resulted in a transient but significant increase in tidal volume from the sixth through the twentieth minute (to 37 +/- 5 and 34 +/- 5% peak tidal volume increase with high and moderate loads, respectively). In addition, there was a significant correlation between the percent decline in tidal volume and mean inspiratory flow rate after loading and the level of beta-endorphin in the cisternal CSF. We conclude that relatively short-term, high-level, inspiratory flow-resistive loading results in elaboration of endogenous opioids within the central nervous system and that these opioids play a role in the progressive decline in tidal volume and mean inspiratory flow rate exhibited during these conditions.

GABA receptors mediate cerebral vasodilation in the unanesthetized goat

The effects of γ-aminobutyric acid (GABA) and muscimol upon cerebral blood flow were evaluated in the unanesthetized goat. Cerebral blood flow was continuously measured by means of an electromagnetic flow probe chronically implanted on the internal maxillary artery after occlusion and thrombosis of the distal extracerebral vessels. Administration of GABA (1–100 μg) directly into the cerebral circulation produced dose-dependent increases in cerebral blood flow, without accompanying systemic effects. Muscimol mimicked the effects of GABA at doses 10 times lower. Administration of picrotoxin (1–3 mg) into the internal maxillary artery did not significantly change cerebral blood flow, but inhibited in a dose-dependent manner the vasodilation induced by GABA. Selective blockade of β-adrenergic or muscarinic cholinergic receptors by propranolol or atropine, respectively, did not modify the cerebrovascular response to the GABAergic agonists. These results indicate that GABA increases total cerebral blood flow, acting on specific receptor sites in the cerebral blood vessels. The absence of influence of picrotoxin on resting cerebral blood flow suggests that the GABAergic receptors are not tonically activated under physiological conditions.

Catecholamine infusion in the unanesthetized goat lymph fistula model

This study has demonstrated in an awake goat the differential effects of infusing low dose epinephrine and norepinephrine. Epinephrine increases pulmonary fluid exchange, possibly as a result of β stimulation to the heart; whereas norepinephrine results in little change in pulmonary fluid balance, primarily affecting vascular vasoreactivity. This study underscores the importance of testing drug responses in awake animal models.

Selective inhibition of thromboxane synthesis during experimental endotoxemia in the goat: effects on pulmonary haemodynamics and lung lymph flow.

1 The effects of a selective thromboxane synthetase inhibitor, dazoxiben (UK 37248), on haemodynamics, eicosanoid levels, and lung vascular permeability were assessed in three unanesthetized goats with chronic lung lymph fistulae following infusions of E. coli endotoxin (1 microgram/kg). Each animal received an infusion of endotoxin in both a control and treatment trial. In the control trial endotoxin alone was infused. In the treatment trial endotoxin administration was preceded by a bolus intravenous infusion of dazoxiben 25 mg/kg followed by a maintenance infusion of 10 mg/kg/h. 2 In animals receiving endotoxin alone the peak elevation in pulmonary artery and wedge pressures, the initial increase in lymph flow, and the fall in cardiac output correlated with the peak elevation in plasma thromboxane B2 (TXB2) concentration at 30 min after endotoxin. Although TXB2 levels had returned to baseline by 3 h, lung lymph flow remained elevated for at least 6 h after endotoxin. The lymph/plasma protein ratio (L/P) did not fall. 3 In dazoxiben-treated animals the rise in plasma concentrations of TXB2 after endotoxin was prevented and, in concert, the increase in pulmonary artery and wedge pressures, the fall in cardiac output, and the initial increase in lymph flow were greatly attenuated. The increase in lymph flow at 2-6 h after endotoxin, however, was not prevented in dazoxiben-treated animals, and again L/P did not change. 4 Plasma 6-keto-prostaglandin F1 alpha concentrations were not significantly altered by dazoxiben treatment except for an earlier peak. 5 We conclude that selective total inhibition of thromboxane synthesis can be achieved by dazoxiben. The inhibition ameliorates many of the adverse haemodynamic consequences of experimental endotoxaemia and reduces, but does not prevent pulmonary oedema formation. This seems to be due to an increase in lung microvascular permeability which is not mediated by TXA2.

Cerebrovascular responses to CO2 after inhibition of sympathetic activity.

The effects of inhalation of 10% CO2 in air on cerebral blood flow (CBF) and cerebrovascular resistance (CVR) were examined in 13 unanesthetized goats before and after impairment of the sympathetic activity present in cerebral vessels. Under control conditions, inhalation of CO2 increased CBF from 112 +/- 10.2 (SE) to 191 +/- 17.2 ml X min-1 X 100 g-1 and decreased CVR from 0.91 +/- 0.02 to 0.55 +/- 0.03 Torr X ml-1 X min X 100 g. After treatment with reserpine CO2 increased CBF to levels similar to those before treatment, but values for CVR were significantly lower. Inhalation of CO2 1-2 days after removal of both superior cervical sympathetic ganglia produced a larger increase in CBF and a greater drop in CVR, whereas at 6-20 days after ganglionectomy the effects of CO2 were analogous to those measured before the operation. The results suggest that the existence of an intact tonic adrenergic activity is a factor in the level of CVR achieved in hypercapnic acidosis.

Brain hypoxia and control of breathing: role of the vagi.

Vagally mediated reflexes play an important role in the generation of respiratory responses to various stimuli. This study examined the role of vagally mediated mechanisms in the generation of the respiratory responses to progressive brain hypoxia secondary to carboxyhemoglobinemia (HbCO 0-55%) in six unanesthetized goats. Ventilation, respiratory cycle timing, and the lung inflation reflex were measured before and during CO inhalation in intact and bilaterally vagotomized animals. Our results indicate that vagal reflexes contribute a small magnitude of the hyperpnea caused by carboxyhemoglobinemia. Furthermore, in contrast to that reported for CO2 inhalation, the tachypneic nature of the ventilatory response to CO is not a vagally mediated phenomenon. CO inhalation had a biphasic influence on the strength of the lung inflation reflex measured as the ratio of inspiratory time during occlusion (TIoccl) to inspiratory time of the preceding spontaneous breath (TIspont). At HbCO levels of 35%, TIoccl/TIspont was enhanced, whereas at HbCO levels of 55% of ratio fell to unity, indicating abolition of the reflex. After vagotomy, this ratio was unity at all levels of carboxyhemoglobinemia.

Reduction of cerebrovascular reactivity during hypercapnia.

The effects on cerebral blood flow of alpha- or beta-adrenergic receptor stimulation of cerebral vessels were examined in 13 unanesthetized goats before and during hypercapnia produced by inhalation of 10% CO2 in air. This procedure increased the PCO2 from 34 to 52 and was accompanied by a fall in pH from 7.39 to 7.26. Electrical stimulation of the cervical sympathetic nerve and injections of norepinephrine and tyramine into the internal maxillary artery produced reductions in cerebral blood flow that were abolished or reduced in hypercapnia. The increase in cerebral blood flow in response to beta-adrenergic stimulation with isoproterenol was also reduced. Hypercapnia caused a similar depression of the constrictor and dilatory effects of the nonadrenergic drugs vasopressin and diazoxide. The results show a decreased response of cerebral vessels to adrenergic and nonadrenergic stimuli in hypercapnia. The findings do not suggest any difference between the refractoriness of cerebral vessels in hypercapnia and that described in other vascular beds.

Role of carotid chemoreflex in respiratory acclimatization to hypoxemia in goat and sheep

The role of the carotid body chemoreflex in the ventilatory acclimatization to chronic hypoxia was studied in the unanesthetized goat and sheep. The time-course of changes in ventilation, P CO 2 , pH and P CO 2 of arterial blood and cisternal fluid (CF) were measured before and following exposure to a simulated altitude of 3660–5000 m, with and without intact carotid sinus nerves. At sea level, after section of carotid sinus nerves most animals hypoventilated chronically, and developed mild arterial hypoxemia and hypercapnia. Upon exposure to acute hypoxia, all of the intact animals hyperventilated and CF pH increased from 7.310 to 7.380 whereas after chemodenervation, the increase in ventilation was small and delayed, and CF pH decreased from 7.285 to 7.143. During exposure of the intact animals to chronic hypoxia, hyperventilation accompanied by decreases in arterial and CF P CO 2 reached its peak in two days; these changes partially subsided during the next few days. Partial compensation of respiratory alkalosis occurred during the first day. In contrast, several chemodenervated animals died during chronic hypoxia; the survivors showed either a small decrease or an increase in Pa CO 2 . Thus, an intact peripheral chemoreflex drive during hypoxia in necessary for ventilatory acclimatization which raises the arterial and presumably tissue P CO 2 in spite of alkalosis. The new proposal is that a central tissue metabolic acidosis resulting from a direct effect of acute hypoxia is partly compensated as hypoxia is prolonged. This central compensation decreases ventilatory drive and hence opposes the ventilatory acclimatization during chronic hypoxia initiated by the peripheral chemoreflexes.

In vivo and In vitro studies on the cerebrovascular dilatation induced by diazoxide in normotensive and renal hypertensive goats.

We studied the in vivo and in vitro effects of diazoxide on the cerebral circulation of 8 normotensive (mean arterial pressure = 100 mm Hg) and 5 renal hypertensive (mean arterial pressure = 146 mm Hg) goats. Injections of diazoxide (0.3-27 mg) into the internal maxillary artery of unanesthetized goats produced dose-dependent increases in cerebral blood flow (electromagnetic flowmeter), this effect being significantly higher in hypertensive goats. Intravenous injection of 5 mg/kg of diazoxide into normotensive goats increased cerebral blood flow 40 ml/min/100 g and mean arterial pressure dropped 22 mm Hg whereas in hypertensive goats cerebral blood flow was unchanged and mean arterial pressure decreased 50 mm Hg. The increase in heart rate due to intravenous diazoxide was similar in normotensive and hypertensive goats (35 beats/min). Cumulative applications of diazoxide (10(-5) to 10(-3)M) on isolated middle cerebral arteries produced dilatory responses both under resting conditions and after previous tonic contraction by serotonin. This relaxation was significantly greater in arterial segments from hypertensive goats. The results indicate that diazoxide exerts powerful dilatatory effects on cerebral vessels, both in vivo and in vitro, and that these effects are particularly evident in hypertensive animals.

Brain hypoxia and control of breathing: neuromechanical control.

The effects of graded brain hypoxia on respiratory cycle timing, the lung inflation reflex, and respiratory compensation for an inspiratory flow-resistive load were studied in unanesthetized goats. Two models, inhalation and CO and acute reduction of brain blood flow (BBF) were used to produce comparable levels of brain hypoxia. The lung inflation reflex was assessed as the ratio of inspiratory time of an occluded breath to that of the preceding spontaneous breath (TIoccl/TIspont). Compensation for flow-resistive loading was assessed as the effect of the load upon the airway occlusion pressure response to rebreathing CO2 (delta P 0.1/delta PCO2). Major findings were 1) severe brain hypoxia (HbCO of 60% or BBF of 42%) caused tachypnea due to a 50% or more reduction of expiratory time but only a 20% or less reduction of inspiratory time; 2) moderate carboxyhemoglobinemia (HbCO of 25-30%) enhanced TIoccl/TIspont from 1.5 +/- 0.1 at control to 2.1 +/- 0.1, while severe brain hypoxia (HbCO of 60% and BBF of 42%) reduced the ratio to 1.0 +/- 0.2; and 3) compensation for a flow-resistive load, manifested by increases of delta P 0.1/delta PCO2 of 75-300% in the control state, was abolished at HbCO of 45-50% and BBF of 60%. The data suggest that in unanesthetized animals brain hypoxia elicits tachypnea largely by an effect on the expiratory phase of the bulbopontine timing mechanism. The observed enhancement of the lung inflation reflex and abolition of flow-resistive load compensation are best explained by hypoxic depression of higher than brain stem neural function.

Analysis of cerebrovascular action of diazoxide in conscious goats.

The effects of diazoxide on cerebral blood flow were evaluated in unanesthetized goats under control conditions and after selective blockade of adrenergic or cholinergic receptors in cerebral vessels. Injections of diazoxide (1-27 mg) into the internal maxillary artery produced dose-dependent increases in cerebral blood flow, an increase of 90% occurring with the highest dose. Administration of phentolamine, propranolol, or atropine into the internal maxillary artery did not modify the cerebrovascular response to diazoxide. In reserpine-treated animals the cerebral effects of diazoxide were also unchanged. Intravenous injections of diazoxide (150-400 mg) produced sustained hypotension and tachycardia whereas cerebral blood flow was maintained within normal values or increased slightly. The normal cerebral vasoconstriction obtained with injections of norepinephrine directly into the internal maxillary artery was unaffected during the diazoxide induced-hypotension. These findings show that diazoxide exerts a powerful vasodilatory effect on cerebral vessels through mechanisms other than blockade of alpha-adrenergic receptors or inhibition of adrenergic activity. The results also indicate that activation of beta-adrenergic or atropine-sensitive vascular receptors in the cerebral response to diazoxide is negligible.

Effects of graded reduction of brain blood flow on chemical control of breathing.

We measured ventilatory responses to CO2 (delta VI/delta PCO2) and transient hypoxia (delta VI/delta SaO2) during reductions of brain blood flow (BBF) to 70% and 50% of control in unanesthetized goats. Increase in inspiratory volume per change in CO2 tension (delta VI/delta PCO2) was measured during rebreathing with sampling of both arterial and cerebral venous blood; increase in inspiratory volume per fall in arterial oxygen saturation (delta VI/delta SaO2) was assessed by the transient N2 inhalation method. Delta VI/delta SaO2 did not significantly change at 70% BBF, but was depressed at 50% BBF. Delta VI/delta PCO2 increased (0.94 +/- 0.18 to 1.29 +/- 0.24 l . min-1 . Torr-1) at 70% BBF if arterial CO2 tension were used to represent the CO2 stimulus but was unchanged if venous CO2 tension were used. At 50% BBF, delta VI/delta PCO2 was depressed (0.38 +/- 0.13 l . min-1 . Torr-1) for both representations of the CO2 stimulus. Brain ischemia increased blood pressure and heart rate but blunted the increase in BBF caused by hypercapnia. We conclude that 1) moderate brain ischemia (70% BBF) does not affect chemosensitivity to hypoxia and CO2, 2) delta VI/delta PCO2 may not be accurately determined from PaCO2 during brain ischemia because cerebrovascular reactivity to CO2 is depressed, and 3) severe brain ischemia (50% BBF) blunts delta VI/delta SaO2 and delta VI/delta PCO2, probably as a consequence of hypoxic depression of the respiratory neurons.

Composition of cerebral fluids in goats adapted to high altitude.

We explored the ionic composition of cerebral interstitial fluid (cISF) in six unanesthetized goats at sea level (SL) and again after 5 days at a simulated high altitude (HA) of 4,300 m. By measuring net transependymal fluxes of HCO3-, Cl-, and lactate during ventriculocisternal perfusions with lactate-free artificial cerebrospinal fluid (CSF) with various [HCO3-] and [Cl-], we determined [HCO3-] and [Cl-] in the inflowing perfusate that produced zero flux, which are estimates of the concentrations of these ions in cISF. Ventilatory acclimatization to HA was established in the goats with alkaline shift in cisternal CSF pH. At SL zero flux of HCO3- and of Cl- occurred when [HCO3-] and [Cl-] in the perfusate were equal to those in CSF. At HA Cl- flux again was zero when [Cl-] in perfusate and in the goat's own CSF were equal; however, for HCO3-, zero flux occurred at HA when [HCO3-] in perfusate was significantly lower than in CSF. Mean transependymal washout of lactate was 16 times larger at HA than at SL. We conclude that at SL [HCO3-] and [Cl-] in CSF were the same as in cISF. In goats adapted to HA [Cl-] in cISF and in CSF were again equal, whereas [HCO3-] in cISF was lower and [lactate] presumably higher than in CSF. The fluid surrounding the central chemoreceptors appears to be more acidic in goats acclimatized to HA than at SL despite the alkalosis in cisternal CSF. This may contribute to ventilatory acclimatization to HA.

Effects of graded reduction of brain blood flow on ventilation in unanesthetized goats.

The ventilatory effects of graded reductions in brain bloow flow (BBF) were studied in unanesthetized goats. At a BBF of 85% of control (PVO2 = 29.2 Torr, PVCO2 = 47.3 Torr) there were no clear ventilatory effects. At BBF of 70% of control (PVO2 = 25.2, PVCO2 = 50.5) and 50% of control (PVCO2 = 22.3, PVCO2 = 53.0) there was hyperpnea, due primarily to an increase of tidal volume. Further reduction of BBF (avg of 42% of control) first produced intense tachypnea and then (30--40% of control) caused apnea that was reversible. At 50% BBF there was a reduction of brain O2 consumption, (4.67--4.00 ml/min) and an increase in systemic O2 consumption. beta-Adrenergic blockade prevented the increase in systemic O2 consumption and reduced the hyperpnea by two-thirds at 50% BBF; the residual hyperpnea was associated with hypocapnia in contrast to the hyperpnea prior to beta-adrenergic blockade, which was virtually isocapnic. The data suggest that hyperpnea due to brain ischemia is a result of both brain acidosis and systemic hypermetabolism. The similarity of the pattern of responses to that previously reported for progressive carboxyhemoglobinemia suggests that brain hypoxia is a determinant of the ventilatory responses to brain ischemia.

Cerebrovascular Response to CO2 Inhalation in Unanesthetized Goats

The effects on cerebral blood flow of inhalation of 9% CO/sub 2/ in air were examined in unanesthetized goats. This procedure increased mean cerebral blood flow from 118 to 185 ml/min per 100 g and was accompanied by a rise in Pco/sub 2/ from 33 to 50 mmHg and a fall in pH from 7.41 to 7.26. Administration of phentolamine, propranolol, or atropine into the internal maxillary artery did not modify the normal cerebrovascular response to CO/sub 2/ inhalation. It is concluded that blockade of vascular adrenergic of cholinergic atropine-sensitive receptors in the goat has no effect on the cerebral vasomotor response to hypercapnia.

The effects of inhalation of carbon monoxide on some aspects of neuromechanical ventilatory control.

We have been involved in a broad study of the effects of brain hypoxia on the control o f breathing. The two major experimental models which we have used are inhalation of carbon monoxide and restriction of brain blood flow in intact, unanesthetized goats. Our findings to date indicate that brain hypoxia may have both inhibitory and facilitatory influences on ventilation and ventilatory responsiveness to chemical stimuli. Although these phenomena seem complex, and undoubtedly represent the interplay of several mechanisms, they are highly reproducible and may be demonstrated in each of the two different models of brain hypoxia which we have studied. In each model the occurrence of any given manifestation of brain hypoxia may be related to a specific range of 02 tension in cerebral venous blood1–4.

Correlation between ventilatory and cerebrovascular responses to inhalation of CO.

To study the determinants of carbon monoxide (CO) induced hyperpnea simultaneous measurements were made of carboxyhemoglobin level in arterial blood (HbCO), ventilation (VE), cerebral blood flow (CBF), O2 delivery to the brain (CBF X O2 content of arterial blood), O2 consumption of the brain (CMRO2), and O2 tension in cerebral venous blood (PVO2) during inhalation of 1% CO in 40% O2 by six unanesthetized goats. HbCO increased to 65% in 10 min; VE remained constant until a HbCO level of approximately 50% was reached and then increased abruptly; CBF increased progressively; O2 delivery to the brain and CMRO2 decreased somewhat with CO inhalation; these decreases reached statistical significance at a HbCO level of 30-40% whereupon the rate of decline with respect to HbCO level increased substantially; and PVO2 decreased progressively from an average of from 31 to 14.6 Torr and averaged 19.2 Torr when hyperpnea was manifest. When considered in the light of previous studies which indicate that CO-induced hyperpnea is not caused by stimulation of the carotid bodies, these data suggest that this phenomenon is related to brain hypoxia. Calculations of brain tissue O2 tension with the Krogh equation support this contention.

Cerebral blood flow during hemorrhagic hypotension in the unanesthetized goat.

Changes in cerebral blood flow (CBF), arterial blood pressure (AP), and cardiac output (CO) were studied during stepwise blood losses in 12 unanesthetized goats. Bleeding was followed by a drop in CBF, AP and CO in a nearly parallel fashion. Control values (means +/- SE) for CBF, AP, and CO were 117 +/- 5.6 ml/min per 100 g, 105 +/- 3.1 mmHg, and 2,825 +/- 124 ml/min, respectively; and after bleeding, 66 +/- 3.9 ml/min per 100 g, 57 mmHg, and 1, 383 ml/min. Arterial blood samples obtained before bleeding, during the hypotensive state, and after reinfusion did not show any significant differences in pH, PCO2, and PO2 values. In phentolamine-treated animals, bleeding produced a drop in AP and CO similar to that observed in the nontreated animals; however, the decrease in CBF was less marked. These findings show that CBF follows pari passu the fall in AP, and they indicate that the alpha-adrenergic receptors of the cerebral vessels are involved in the cerebral blood flow changes in hemorrhage.