Modulation of the centrally-evoked visceral alerting/defence response by changes in CSF pH at the ventral surface of the medulla oblongata and by systemic hypercapnia

Transient hypercapnic stress causes exaggerated and prolonged elevation of cardiac and renal interstitial norepinephrine levels in conscious hypertensive rats

The responses of sympathetic nerve activity to transient stress can be exaggerated in salt-sensitive (SS), hypertensive subjects. Cardiac and renal interstitial norepinephrine (iNE) levels during and after transient hypercapnia were investigated in conscious SS rats. Dahl SS and salt-resistant (SR) 6-wk-old rats were fed a high-salt diet, and at 12 wk iNE levels in the heart and kidney were determined using microdialysis with probes inserted in the left ventricular (LV) wall and kidney. A telemetry system determined blood pressure and heart rate (HR) in separate animals. After recovery from the operation, data were collected before, during, and after exposure to normoxic 10% CO2 for 25 min under unanesthetized conditions. The plasma NE concentrations at baseline did not differ between the two strains. Both cardiac and renal iNE levels were much higher in SS rats than in SR rats at baseline as well as during hypercapnic stress. After stress, the markedly increased iNE levels of SS rats were prolonged in the LV as well as in the kidney. During hypercapnic stress, HR decreased in both SS and SR rats, while sudden increases in HR immediately after the withdrawal from stress were followed by its slower reduction in SS rats compared with SR rats. In conclusion, transient hypercapnic stress causes exaggerated and prolonged elevation of iNE levels in the heart as well as in kidneys of SS animals.

Interaction of chemoreceptor and baroreceptor reflexes by hypoxia and hypercapnia - a mechanism for promoting hypertension in obstructive sleep apnoea

Asphyxia, which occurs during obstructive sleep apnoeic events, alters the baroreceptor reflex and this may lead to hypertension. We have recently reported that breathing an asphyxic gas resets the baroreceptor-vascular resistance reflex towards higher pressures. The present study was designed to determine whether this effect was caused by the reduced oxygen tension, which affects mainly peripheral chemoreceptors, or by the increased carbon dioxide, which acts mainly on central chemoreceptors. We studied 11 healthy volunteer subjects aged between 20 and 55 years old (6 male). The stimulus to the carotid baroreceptors was changed using graded pressures of -40 to +60 mmHg applied to a neck chamber. Responses of vascular resistance were assessed in the forearm from changes in blood pressure (Finapres) divided by brachial blood flow velocity (Doppler) and cardiac responses from the changes in RR interval and heart rate. Stimulus-response curves were defined during (i) air breathing, (ii) hypoxia (12% O(2) in N(2)), and (iii) hypercapnia (5% CO(2) in 95% O(2)). Responses during air breathing were assessed both prior to and after either hypoxia or hypercapnia. We applied a sigmoid function or third order polynomial to the curves and determined the maximal differential (equivalent to peak sensitivity) and the corresponding carotid sinus pressure (equivalent to 'set point'). Hypoxia resulted in an increase in heart rate but no significant change in mean blood pressure or vascular resistance. However, there was an increase in vascular resistance in the post-stimulus period. Hypoxia had no significant effect on baroreflex sensitivity or 'set point' for the control of RR interval, heart rate or mean arterial pressure. Peak sensitivity of the vascular resistance response to baroreceptor stimulation was significantly reduced from -2.5 +/- 0.4 units to -1.4 +/- 0.1 units (P < 0.05) and this was restored in the post-stimulus period to -2.6 +/- 0.5 units. There was no effect on 'set point'. Hypercapnia, on the other hand, resulted in a decrease in heart rate, which remained reduced in the post-stimulus period and significantly increased mean blood pressure. Baseline vascular resistance was significantly increased and then further increased in the post-control period. Like hypoxia, hypercapnia had no effect on baroreflex control of RR interval, heart rate or mean arterial pressure. There was, also no significant change in the sensitivity of the vascular resistance responses, however, 'set point' was significantly increased from 74.7 +/- 4 to 87.0 +/- 2 mmHg (P < 0.02). This was not completely restored to pre-stimulus control levels in the post-stimulus control period (82.2 +/- 3 mmHg). These results suggest that the hypoxic component of asphyxia reduces baroreceptor-vascular resistance reflex sensitivity, whilst the hypercapnic component is responsible for increasing blood pressure and reflex 'set point'. Hypercapnia appears to have a lasting effect after the removal of the stimulus. Thus the effect of both peripheral and central chemoreceptors on baroreflex function may contribute to promoting hypertension in patients with obstructive sleep apnoea.

Hypercapnia selectively attenuates the somato-sympathetic reflex

The effects of hyperoxic hypercapnia (5, 10 or 15% CO2 in O2) on splanchnic sympathetic nerve activity (sSNA) and sympathetic reflexes such as the somato-sympathetic reflex or baroreflex were studied in urethane anaesthetised, paralysed, artificially ventilated and vagotomized Sprague-Dawley rats. Hypercapnia caused a small increase in mean arterial blood pressure (MAP) in the 10% CO2 group and a fall in heart rate (HR) in all three groups. sSNA increased in all three groups. Phrenic frequency and amplitude increased during hypercapnia, with frequency adapting back towards baseline during the CO2 exposure. The somato-sympathetic reflex was attenuated in the 5% CO2 group and abolished in the 10 and 15% CO2 groups, whereas there was little effect on the sSNA baroreflex. Hypercapnia significantly affects phrenic nerve activity (PNA), sSNA and selectively inhibits the somato-sympathetic reflex with little effect on the sSNA baroreflex.

Antihypertensive monotherapy and cardiovascular responses to an acoustic startle stimulus

To determine contribution of the autonomic nervous system to cardiovascular reactivity to noise, acoustic startle stimulus (110 dB, 1-20 kHz, 0.150 s) was administered to 35 subjects (19 women, 16 men) with mild essential hypertension. Among these patients, 10 were unmedicated and 25 were receiving long-term monotherapy (10 were taking 100 mg atenolol, 5 were taking 10 mg prazosin, and 10 were taking 50 mg losartan daily). Polygraphic recordings were obtained in supine position. Blood pressure (BP) and heart rate (HR) levels were stable until the noise was administered. In the unmedicated group BP and HR were elevated during the first 10 s. BP returned to resting levels after this period. The calculated hemodynamic indexes showed a biphasic change in total peripheral resistance (TPR), with an overall vasoconstriction associated with the BP rise phase, preceding a delayed vasodilation. The lowest HR changes were observed in the beta-blocker group with increases of 6 beats/min and 3 beats/min after the first and second noise stimulations, compared with 10 beats/min and 5 beats/min in the unmedicated group. Prazosin significantly reduced the BP rises to 7 mm Hg and 6 mm Hg for systolic BP and diastolic BP after the first stimulation compared with 22 mm Hg and 17 mm Hg in the untreated group (p < 0.01). The second stimulation after prazosin determined -5 mm Hg and 1 mm Hg changes for systolic BP and diastolic BP respectively, compared to rises of 13 mmHg for systolic BP and 10 mmHg for diastolic BP in the untreated group (p < 0.01). The hemodynamic percentage changes resulting from the first stimulation indicated prazosin markedly reduced the noise-induced rise in TPR (p < 0.05). No effect of beta-blocker was detectable using percentage changes. The rises in BP were amplified in the losartan-treated subjects compared with the other groups. Because of a low resting TPR in this group, the percentage changes in TPR resulting from noise were amplified in the subjects treated with the AT1 receptor antagonist. In conclusion the acoustic startle stimulus appeared as a simple and reliable procedure for inducing transient increases due to a rise in TPR. Cardiovascular responses differed according to the antihypertensive monotherapy, with a limited effect of noise in the prazosin-treated group.

Baroreflex control of heart rate during hypoxia and hypercapnia in chronically hypertensive rabbits

1. It has been proposed that hypertension alters the respiratory and cardiovascular responses to chemoreceptor stimulation. However, in studies of human hypertension or in genetic animal models of hypertension it has been difficult to unequivocally attribute the changes to hypertension per se, rather than to a genetic predisposition towards an altered chemoreflex response independent of hypertension. 2. In the present study a group of seven rabbits were made hypertensive via a continuous 7 week infusion of angiotensin II (AngII; 50 ng/kg per min, i.v.). Animals were studied twice before AngII treatment commenced, twice during infusion and 48 h after stopping infusion. At each of these times the relationship between heart rate (HR) and mean arterial pressure (MAP) was determined under normoxic, acute hypoxic (10% O2 + 3% CO2) and acute hypercapnic (18% O2 +, 6.5% CO2) conditions for 20 min. A group of six animals also served as time controls. 3. Angiotensin II infusion increased arterial pressure from control levels of 80 +/- 2 to 114 +/- 8 mmHg and maintained it at this level throughout the 7 week period. After 1 week of AngII infusion there was a rightward shift in the heart rate-baroreflex curve, indicating that the baroreflex was now operating at an increased level of pressure. These changes were associated with reductions in the gain from -7.6 +/- 1.6 to -3.0 +/- 0.2 b.p.m./ mmHg, HR range and curvature of the baroreflex. These effects were maintained throughout the 7 weeks of hypertension and were reversed within 2 days of ceasing AngII infusion. Acute hypoxia and hypercapnia in normotensive animals caused a reduction in the HR range of 19 +/- 7 and 15 +/- 7 b.p.m., respectively, but caused no change in the gain (sensitivity) of the baroreflex. Despite the marked changes in the baroreflex produced by the hypertension, the effect of hypoxia or hypercapnia on the HR baroreflex was not different in the hypertensive group. 4. It is concluded that chronic experimental AngII-based hypertension does not alter the HR baroreflex response to hypoxia or hypercapnia and suggests that the altered responses seen in other studies is due to a genetic predisposition as opposed to the effect of raised arterial pressure.

Effect of halothane and isoflurane on in situ diameter responses of small mesenteric veins to acute graded hypercapnia

The purpose of the present study was to quantify the inhibitory effect of inhaled halothane and isoflurane on acute hypercapnia-induced responses of capacitance-regulating veins and related cardiovascular variables in response to sequential 40-s periods of 5%, 10%, 15%, and 20% inspired CO2 (FICO2). Measurements were made in normoxic alpha-chloralose-anesthetized rabbits before, during, and after either 0.75 minimum alveolar anesthetic concentration inhaled halothane or isoflurane. The graded hypercapnia caused graded venoconstriction and bradycardia but minimal pressor responses. Hypercapnia-induced venoconstriction was blocked by prior local superfusion of the exposed veins with 3 x 10(-6) M tetrodotoxin. Both the hypercapnia-induced venoconstriction and bradycardia responses were significantly attenuated by halothane or isoflurane and did not fully recover after removal of the anesthetics from the circulation. Both anesthetics produced a significant baseline (i.e., prehypercapnia) hypotension and a tendency toward a resultant tachycardia. The baseline hypotension did not recover completely after elimination of the anesthetic. Neither anesthetic altered baseline vein diameter. These results agree with previous studies demonstrating that hypercapnic acidosis produces mesenteric venoconstriction by elevating excitatory sympathetic efferent neural input via activation of peripheral and central chemoreceptors and that bradycardia results from activation of compensatory baroreflexes. The neural components of these reflexes are possible primary sites for attenuation of these cardiovascular responses by halothane and isoflurane.

Kainic acid on the rat ventral medullary surface depresses hypoxic and hypercapnic ventilatory responses

Kainic acid, topically applied to the ventral surface of the medulla immediately caudal to the trapezoid body in the urethane/chloralose anaesthetised rat, led to a depression of ventilation and a sustained rise in blood pressure; ventilatory responses to hypercapnia (10% carbon dioxide) and hypoxia (11% oxygen) were slightly depressed. Widespread application of kainic acid to an area at and slightly rostral to the rootlets of the hypoglossal nerve produced a stimulation of ventilation and an unsustained rise in blood pressure. Apnea ensued 12-28 min after application. Ventilatory responses to hypercapnia and hypoxia were markedly attenuated; more discrete bilateral application revealed two regions, one immediately rostral and lateral to the hypoglossal rootlets and the other over the point of exit of the hypoglossal nerve rootlets, which specifically contributed to the diminution of the chemosensory responses. These results raise questions about the medullary circuitry which mediates the chemoreflex regulation of breathing.

Analysis of factors that contribute to cardiovascular changes induced in the cat by graded levels of systemic hypoxia

1. In cats anaesthetized with Saffan, which does not block afferent activation of the brain stem defence areas, we have analysed the cardiovascular changes induced by 3 min periods of graded systemic hypoxia (fraction of O2 in inspirate, Fi,O2, 0.15, 0.12, 0.08, 0.06). 2. At light levels of Saffan anaesthesia, hypoxia (particularly Fi, O2 0.08 and 0.06) or selective stimulation of carotid chemoreceptors evoked the pattern of tachycardia, decrease in renal and mesenteric vascular conductance (RVC, MVC), but increase in femoral vascular conductance (FVC) which is characteristic of the alerting-defence response. This supports our view that activation of the defence areas is an integral part of the response to systemic hypoxia. 3. Hypoxia also induced an increase in frequency of augmented breaths which was graded with the level of hypoxia: 0.6 min-1 at Fi, O2 0.21 to 1.1 min-1 at Fi, O2 0.06; in some cats each of these was accompanied by a transient fall in arterial pressure (ABP) and increase in FVC. It is proposed that these responses were all part of a reflex elicited by lung irritant receptors and facilitated by peripheral chemoreceptors. However, their low rate of occurrence and the liability of the vasodilatation suggests they do not make major contributions to the overall response. 4. The above short-lasting responses were superimposed upon gradual changes whose magnitudes were graded with the level of hypoxia: hyperventilation, slight tachycardia, but bradycardia at Fi, O2 0.6, small increases in ABP, FVC and MVC allowing femoral and mesenteric blood flow to increase, but decreases in RVC which maintained renal blood flow constant. 5. Vagotomy had no significant effect on these changes. Further, hyperinflation of the lungs with pressures of 10 mmHg evoked the Breuer-Hering reflex but had no noticeable cardiovascular effect. It is proposed that, in the cat, reflex tachycardia and vasodilatation elicited by lung stretch receptors play no significant part in the response to hypoxia. 6. By contrast, after pneumothorax, with ventilation and thereby arterial PCO2 (Pa, CO2) maintained constant, graded hypoxia produced graded bradycardia, decrease in MVC and RVC and no change in FVC. Taken together, these results suggest that in the spontaneously breathing cat, the response to hypoxia is dominated by the effects of hypocapnia secondary to hyperventilation, which by inhibiting peripheral and central chemoreceptor activity effectively counteracts the primary bradycardia and peripheral vasoconstriction elicited by hypoxic stimulation of peripheral chemoreceptors. 7. These proposals are compared with those drawn for other species.

Influences on the cardiovascular response to graded levels of systemic hypoxia of the accompanying hypocapnia in the rat

1. In spontaneously breathing, anaesthetized rats, a study was made of the effects upon the graded cardiovascular responses to systemic hypoxia (inspiratory fractional O2 concentration, Fi, O2: 0.15, 0.12, 0.08, 0.06) of maintaining arterial CO2 pressure (Pa,CO2) at the air-breathing level by adding CO2 to the inspirate (eucapnic hypoxia), rather than allowing Pa,CO2 to fall (hypocapnia hypoxia). 2. At each Fi,O2, maintenance of eucapnia significantly reduced the increase in respiratory frequency, but significantly accentuated the increase in tidal and minute volume: as a result the fall in Pa,O2 at each Fi,O2 was significantly reduced. 3. Concomitantly, maintenance of eucapnia reduced the increase in heart rate (HR) and fall in arterial pressure (ABP), the effects being significant at Fi,O2 0.08 and/or 0.06. There was also a tendency for the increases in renal and femoral vascular conductances (RVC, FVC) to be reduced; at Fi,O2 0.06 mean increases from control were 2 +/- 10 vs. 16 +/- 7% (eucapnia vs. hypocapnia) for RVC, and 62 +/- 11 vs. 106 +/- 27% for FVC. 4. As maintenance of eucapnia reduced the fall in Pa,O2 at each Fi,O2, the above results were also considered as a function of Pa,O2. Then, maintenance of eucapnia had similar significant effects on the changes in respiration and HR as described above and reduced the mean increase in RVC (16 +/- 11 vs. 23 +/- 10%, at Pa,O2 31 mmHg, which was attained at Fi,O2 0.06 with eucapnia and 0.08 with hypocapnia). However, maintenance of eucapnia had no effect on the falls in ABP and accentuated the mean increase in FVC (74.9 +/- 13 vs. 57 +/- 10% at Pa,O2 31 mmHg). 5. These findings indicate that, in the rat, the hypocapnia that accompanies the hyperventilatory response to systemic hypoxia facilitates the tachycardia and may accentuate the renal vasodilation, but attenuate the hypoxia-induced vasodilatation in skeletal muscle. Possible mechanisms are discussed.