P(O(2))-P(CO(2)) stimulus interaction in [Ca(2+)](i) and CSN activity in the adult rat carotid body

Since glomus cell intracellular calcium ([Ca(2+)](i)) plays a key role in generating carotid sinus nerve (CSN) discharge, we hypothesized that glomus cell [Ca(2+)](i) would correspond to CSN discharge rates during P(O(2))-P(CO(2)) stimulus interaction in adult rat carotid body (CB). Accordingly, we measured steady state P(O(2))-P(CO(2)) interaction in CSN discharge rates during hypocapnia (P(CO(2))=8-10 Torr), normocapnia (P(CO(2))=33-35 Torr) and hypercapnia (P(CO(2))=68-70 Torr) in normoxia (P(O(2)) approximately 130 Torr) and hypoxia (P(O(2)) approximately 36 Torr). The results showed P(O(2))-P(CO(2)) stimulus interaction in CSN responses. [Ca(2+)](i) levels were measured in isolated type I cells (2-3 cells/field), using Ca(2+) sensitive fluoroprobe indo-1AM. The [Ca(2+)](i) responses increased with increasing P(CO(2)) in normoxia. In hypoxia, [Ca(2+)](i) did not increase during hypocapnia but increased during normocapnia, showing P(O(2))-P(CO(2)) interaction. However, CSN response during hypoxia was far greater than that for [Ca(2+)](i) response, particularly during hypocapnic hypoxia. Thus, the [Ca(2+)](i) interaction cannot account for the whole CSN interaction. The origin of this CSN P(O(2)-)P(CO(2)) interaction must have occurred in part beyond cellular [Ca(2+)](i) interaction. Interactions at both sites (glomus cell membrane and sinus nerve endings) are reminiscent of reversible O(2)-heme protein reaction with a Bohr effect.

High-pass filtering of carotid-vagal influences on expiration in rat: role of N-methyl-D-aspartate receptors

Repetitive electrical stimulation of the carotid sinus nerve or vagus nerve in rats elicited abrupt reflex shortening or prolongation, respectively, of the inter-burst interval of phrenic nerve activity followed by exponential decay from the initial response. Removal of the stimuli resulted in transient post-stimulus rebound excitation or inhibition that mirrored the corresponding stimulus-evoked responses. The biphasic responses to these complementary inputs approximate the on- and off-transients of full-wave differentiators or high-pass filters. Blockade of N-methyl-D-aspartate (NMDA) receptors abolished the post-stimulus rebounds and transformed both signal pathways into integrators or low-pass filters, thus switching off part or all of the high-pass filters. We suggest that such NMDA receptor-dependent high-pass filtering effects may serve to increase the dynamic range and response speed of sensory neurotransmission to the brain, thereby enhancing closed-loop stability of sensorimotor reflex.

Cellular mechanisms involved in carotid body inhibition produced by atrial natriuretic peptide

Atrial natriuretic peptide (ANP) and its analog, atriopeptin III (APIII), inhibit carotid body chemoreceptor nerve activity evoked by hypoxia. In the present study, we have examined the hypothesis that the inhibitory effects of ANP and APIII are mediated by cyclic GMP and protein kinase G (PKG) via the phosphorylation and/or dephosphorylation of K(+) and Ca(2+) channel proteins that are involved in regulating the response of carotid body chemosensory type I cells to low-O(2) stimuli. In freshly dissociated rabbit type I cells, we examined the effects of a PKG inhibitor, KT-5823, and an inhibitor of protein phosphatase 2A (PP2A), okadaic acid (OA), on K(+) and Ca(2+) currents. We also investigated the effects of these specific inhibitors on intracellular Ca(2+) concentration and carotid sinus nerve (CSN) activity under normoxic and hypoxic conditions. Voltage-dependent K(+) currents were depressed by hypoxia, and this effect was significantly reduced by 100 nM APIII. The effect of APIII on this current was reversed in the presence of either 1 microM KT-5823 or 100 nM OA. Likewise, these drugs retarded the depression of voltage-gated Ca(2+) currents induced by APIII. Furthermore, APIII depressed hypoxia-evoked elevations of intracellular Ca(2+), an effect that was also reversed by OA and KT-5823. Finally, CSN activity evoked by hypoxia was decreased in the presence of 100 nM APIII, and was partially restored when APIII was presented along with 100 nM OA. These results suggest that ANP initiates a cascade of events involving PKG and PP2A, which culminates in the dephosphorylation of K(+) and Ca(2+) channel proteins in the chemosensory type I cells.

Adenosine triphosphate-induced peripheral nerve discharges generated from the cat petrosal ganglion in vitro

Since nucleotides have been postulated as transmitters between glomus cells and chemosensory nerve endings in the carotid body, we studied the effects of their application to the petrosal ganglion, where the perikarya of carotid (sinus) nerve are located. Cat petrosal ganglia were superfused in vitro, while electrical activities of their peripheral processes (carotid nerve and glossopharyngeal branch) were recorded simultaneously. Adenosine triphosphate (ATP) evoked dose-dependent bursts of impulses in carotid nerve, while those in glossopharyngeal branch were less intense and consistent. Adenosine monophosphate was less effective than ATP. ATP-induced carotid nerve responses presented no temporal desensitization and persisted after applying P(2Y) receptor blocker Reactive Blue 2 to the ganglion. The results indicate that ATP has an excitatory effect on the perikarya of the population of petrosal ganglion neurons projecting peripherally through the carotid nerve.

Activation of brain neurons following central hypervolaemia and hypovolaemia: contribution of baroreceptor and non-baroreceptor inputs

In the present study we have used the detection of Fos, the protein product of c-fos, to determine the distribution of neurons in the medulla and hypothalamus that are activated by changes in central blood volume. Experiments were conducted in both barointact and barodenervated conscious rabbits, to determine the contribution of arterial baroreceptors to the pattern of Fos expression evoked by changes in central blood volume, induced either by intravenous infusion of an isotonic modified gelatin solution, or by partial occlusion of the vena cava. These procedures resulted in a significant increase and decrease, respectively, in right atrial pressure over a 60 min period. In control experiments, barointact and barodenervated rabbits were subjected to the identical procedures except that no changes in central blood volume were induced. In comparison with the control observations, central hypervolaemia produced a significant increase in the number of Fos-immunoreactive neurons in the nucleus tractus solitarius, area postrema, the caudal, intermediate and rostral parts of the ventrolateral medulla, supraoptic nucleus, paraventricular nucleus, arcuate nucleus, suprachiasmatic nucleus and median preoptic nucleus. The overall pattern of Fos expression induced by central hypervolaemia did not differ significantly between barointact and barodenervated animals. Similarly, the overall pattern of Fos expression induced by central hypovolaemia did not differ significantly between barointact and barodenervated animals, but did differ significantly from that produced by hypervolaemia. In particular, central hypovolaemia produced a significant increase in Fos expression in the same regions as above, but also in the subfornical organ and organum vasculosum lamina terminalis. In addition, compared with central hypervolaemia, hypovolaemia produced a significantly greater degree of Fos expression in the rostral ventrolateral medulla and supraoptic nucleus. Furthermore, double-labelling for tyrosine hydroxylase immunoreactivity demonstrated that neurons in the ventrolateral medulla that expressed Fos following hypovolaemia were predominantly catecholamine cells, whereas following hypervolaemia they were predominantly non-catecholamine cells. Finally, double-labelling for vasopressin immunoreactivity demonstrated that the number of Fos/vasopressin immunoreactive cells in the supraoptic nucleus was approximately 10 times greater following hypovolaemia compared with hypervolaemia, but there were very few such double-labelled neurons in the paraventricular nucleus in response to either stimulus. The results demonstrate that central hypervolaemia and hypovolaemia each induces reproducible and specific patterns of Fos expression in the medulla and hypothalamus. The degree and pattern of Fos expression was unaffected by arterial baroreceptor denervation, indicating that it is primarily a consequence of inputs from cardiac receptors, together with an increase in the level of circulating hormones such as atrial natriuretic peptide, angiotensin II or vasopressin. Furthermore, the pattern of Fos expression produced by central hypervolaemia and hypovolaemia is distinctly different from that evoked by hypertension and hypotension, respectively [Li and Dampney (1994) Neuroscience 61, 613-634], particularly in hypothalamic regions. These findings therefore indicate that the central pathways activated by changes in blood volume are, at least in part, separate from those activated by changes in arterial pressure.

Ventilatory and central neurochemical reorganisation of O2 chemoreflex after carotid sinus nerve transection in rat

1. The first step of this study was to determine the early time course and pattern of hypoxic ventilatory response (HVR) recovery following irreversible bilateral carotid sinus nerve transection (CSNT). The second step was to find out if HVR recovery was associated with changes in the neurochemical activity of the medullary catecholaminergic cell groups involved in the O2 chemoreflex pathway. 2. The breathing response to acute hypoxia (10% O2) was measured in awake rats 2, 6, 10, 45 and 90 days after CSNT. In a control group of sham-operated rats, the ventilatory response to hypoxia was principally due to increased respiratory frequency. There was a large reduction in HVR in the CSNT compared to the sham-operated rats (-65%, 2 days after surgery). Within the weeks following denervation, the CSNT rats progressively recovered a HVR level similar to the sham-operated rats (-37% at 6 days, -27% at 10 days, and no difference at 45 or 90 days). After recovery, the CSNT rats exhibited a higher tidal volume (+38%) than the sham-operated rats in response to hypoxia, but not a complete recovery of respiratory frequency. 3. Fifteen days after CSNT, in vivo tyrosine hydroxylase (TH) activity had decreased in caudal A2C2 (-35%) and A6 cells (-35%). After 90 days, the CSNT rats displayed higher TH activity than the sham-operated animals in caudal A1C1 (+51%), caudal A2C2 (+129%), A5 (+216%) and A6 cells (+79%). 4. It is concluded that HVR following CSNT is associated with a profound functional reorganisation of the central O2 chemoreflex pathway, including changes in ventilatory pattern and medullary catecholaminergic activity.

[Radiofrequency electrostimulation of carotid sinus nerves for the treatment of bronchial asthma]

Electrostimulators of synocarotid nerves (SCN) were implanted in 30 patients aged from 18 to 62 years for the treatment of bronchial asthma. The procedures of SCN stimulation (from 1 to 10 daily) were carried out with parameters of 30-100 Hz, 0.1-0.8 ms, 0.5-2.0 V. The external respiration function was examined by pneumotachometry and spirography, and cardiovascular system by the method of rheography. The attacks of apnea were effectively eliminated and prevented by electrostimulation (ES) of SCN. The frequency of the attacks 2 and 5 years after the procedures decreased 2.8 fold, and the duration of the attacks--3.5 fold. Daily consumption of antiasthmatic drugs decreased by 2.7 times. Some complications were detected: rejection of the electrostimulator (1 patient), paresis of n. hypoglottis (1), suppuration of the wound (3), abnormal mobility of the electrostimulator (1). ES of SCN can be applied in patients with complicated forms of bronchial asthma and severe drug addiction.

Stimulus-specific signaling pathways in rabbit carotid body chemoreceptors

The carotid body is an arterial chemosensory organ which responds to multiple natural and pharmacological stimuli, including hypoxia and nicotine. Numerous studies have investigated the initial molecular events which activate chemosensory type I cells in the carotid body, but less attention has been focused on later steps in the transduction cascade, which mediate neurotransmitter release from type I cells and excitation of chemoreceptor afferent fibers in the carotid sinus nerve. In the present study, we examined the effects of a highly specific inhibitor of calcium/calmodulin-dependent kinase II, KN-62, and a calmodulin inhibitor, trifluoperazine, on carotid sinus nerve activity and catecholamine release evoked from rabbit carotid bodies superfused in vitro. KN-62 did not alter sinus nerve activity and catecholamine release evoked by hypoxia, but this agent significantly reduced nerve activity and neurotransmitter release evoked by 100 microM nicotine. Trifluoperazine (10 microM), likewise inhibited activity evoked by nicotine, as well as hypoxia. Basal levels of nerve activity and catecholamine release (established in superfusate equilibrated with 100% O2) were unaffected by all drug treatments. Separate biochemical experiments showed that Ca2+/calmodulin-dependent incorporation of 32P into carotid body particulate proteins is significantly reduced following incubation of intact carotid bodies in nicotine, but not following exposure to hypoxia. Our observations suggest that excitation of the carotid body by diverse stimuli may involve the activation of distinct, stimulus-specific transduction pathways. Furthermore, these data correlate with our previous findings which showed that hypoxia, on the one hand, and nicotine on the other, evoke the preferential release of either dopamine or norepinephrine, respectively, from carotid bodies incubated in vitro.

[Physiology and methods for studying the baroreceptor reflex]

The baroreflex is of major importance for the moment-to-moment maintenance of arterial pressure particularly during orthostatic stress. Blood pressure increase stimulates the receptors e.g. in the carotid sinuses and the aortic arch, and rapidly increases the receptor discharge rate. Blood pressure decrease induces arrest of impulse transmission to the nucleus of the solitary tract. The impulses are modulated by the nucleus ambiguous, the rostral ventrolateral medulla, the dorsal nucleus of the vagus nerve, parabrachial and paraventricular nuclei and other central structures. Blood pressure increase induces an increase of cardiovagal activity resulting in cardiodeceleration and a decrease of sympathetic peripheral vasoconstrictor outflow. The receptor firing rates show adaptation and resetting to longer lasting blood pressure changes, hysteresis, i.e. firing rates that are higher with rapid blood pressure increase than during the return to baseline pressure. The receptors interact with respiration, chemoreceptor stimulation, central stimuli, exercise and sleep, etc. Baroreceptor function and interaction e.g. with chemoreceptors is compromised in diseases such as diabetic autonomic neuropathy. Guillain-Barré syndrome, arterial hypertension, heart failure and probably in most stroke patients. Fatal complications may result from baroreceptor malfunction. Subtle analysis of the baroreflex is therefore crucial for a refined pathophysiological understanding of these diseases. Pharmacological testing and "neck chamber" negative pressure stimulation of the receptors are as useful as the non-invasive computerized analysis of the interaction of spontaneous blood pressure and heart rate fluctuations.

Effect of intracarotid administration of adenosine on the activity of area postrema neurons in barodenervated rats

To observe the effect of intracarotid administration of adenosine on the electrical activity of area postrema (AP) neurons, 76 spontaneous active units were recorded from 45 sino-aortic denervated Sprague-Dawley rats using extracellular recording technique. The results obtained are as follows. (1) Following intracarotid administration of adenosine (Ado, 25 micrograms/kg), the discharge rate of 29 out of 42 units decreased markedly from 6.26 +/- 0.75 to 4.74 +/- 0.76 spikes/s (P < 0.01), whereas that of 6 units increased from 4.13 +/- 0.77 to 4.72 +/- 0.83 spikes/s (P < 0.05), and the other 7 showed no response. Blood pressure (BP) and heart rate (HR) were unaltered throughout the experiment. (2) 8-phenyltheophylline (8-PT, 15 micrograms/kg), a nonselective adenosine receptor antagonist, completely blocked the inhibitory effect of Ado in 10 units. (3) Selective A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 50 micrograms/kg), blocked the effect of Ado in 12 units to a remarkable extent. (4) Glibenclamide (500 micrograms/kg), a blocker of ATP-sensitive potassium channel, abolished the effect of Ado in 12 units. The above results indicate that Ado can inhibit spontaneous electrical activity of AP neurons, which is mediated by adenosine A1-receptor with the involvement of ATP-sensitive potassium channels.

Responses to hypoxia of petrosal ganglia in vitro

NaCN is a classical stimulus used to elicit discharges from carotid body chemoreceptors. The effect is assumed to be mediated by glomus (type I) cells, which release an excitatory transmitter for the excitation of carotid nerve endings. Since the sensory perikarya of the glossopharyngeal nerve (from which the carotid nerve branches) are located in the petrosal ganglion, we tested whether application of this drug to the petrosal ganglion superfused in vitro elicits antidromic discharges in the carotid nerve. NaCN did indeed cause an intense and prolonged burst of nerve impulses in the carotid nerve, while provoking a less intense and much briefer burst of discharges in the glossopharyngeal branch. Carotid nerve responses to NaCN were reduced and shortened by prior or following application of dopamine to the ganglion. Sodium azide applied to the petrosal ganglion evoked a less intense and much briefer burst of impulses in the carotid nerve. Ganglionar application of 2,4-dinitrophenol did not induce discharges in the carotid nerve. Switching the superfusion of the ganglion from a normoxic to a hypoxic solution did not evoke discharges in the carotid nerve. Therefore, the perikarya of carotid nerve neurons are sensitive to NaCN, but are not excited by reducing the pO(2) of the superfusing solution.

Is the aortic depressor nerve involved in arterial chemoreflexes in rats?

Recent anatomical and physiological studies showed that chemoreceptor afferent fibers are present in the rat aortic depressor nerve (ADN), which has been considered to contain exclusively baroreceptor afferent fibers. However, it remains to be proven whether the chemoreceptor afferents of the ADN are practically involved in chemoreflexes. The present study was performed in chloralose/urethane-anesthetized rats of either Sprague-Dawley (SD) or Wistar strain to examine whether the ADN carries sufficient information regarding arterial hypoxia and hypercapnia, and whether the ADN indeed participates in chemoreflexes, the circulatory and respiratory components. It was found in either strain that afferent discharges of the ADN were not affected at all by hypoxia or hypercapnia, whereas those of the carotid sinus nerve (CSN) markedly increased due to these stimuli. Hypoxia produced hypertension, transient bradycardia followed by tachycardia, and respiratory facilitation, which characterize the chemoreflexes. Any of these responses was not affected at all by the ADN section, but all were abolished by the CSN section. Intraaortic injection of cyanide also induced transient bradycardia and respiratory facilitation, but any of them was not affected by the ADN section while all were abolished by the CSN section. Furthermore, electrical stimulation of the ADN produced solely baroreflex responses, i.e. hypotension and respiratory suppression, whereas that of the CSN provoked chemoreflex responses, i.e. early, transient hypertension and respiratory facilitation. In conclusion, the rat ADN does not contain a functionally significant number of chemoreceptor afferent fibers, if at all, and does not appreciably contribute to generation of chemoreflexes.

Inhibition of baroreflex vagal bradycardia by selective stimulation of arterial chemoreceptors in rats

We reported recently that hypoxia inhibits baroreflex vagal bradycardia (BVB) in rats and that this inhibition persists following chemoreceptor denervation. However, since it is possible that hypoxia also affects the central processing of chemoreceptive input, the existence of chemoreceptor-mediated inhibition of BVB cannot be ruled out. Therefore, we have studied whether selective chemoreceptor activation affects BVB in normoxic conditions. In pentobarbital-urethane-anaesthetized, succinylcholine-immobilized, artificially ventilated rats, BVB was provoked by electrical stimulation of the aortic depressor nerve. Arterial chemoreceptors were selectively activated by intracarotid injection of a minute amount of sodium cyanide. Cyanide injection consistently increased blood pressure while changing heart rate variably. BVB was inhibited in a dose-dependent manner. This inhibition, as well as changes in blood pressure and heart rate, was abolished following transection of the carotid sinus nerve (CSN) ipsilateral to the injection. Spinal cord transection at the C2 level did not affect the inhibition. On the other hand, intracarotid cyanide had no effect on bradycardia elicited by electrical stimulation of a peripheral cut end of the cervical vagus nerve. We conclude that chemoreceptor activation definitely inhibits BVB and that this inhibition is mediated by the CSN, and predominantly occurs in the central nervous system. The possibility is suggested that severe hypoxia suppresses not only BVB but also the chemoreceptor-mediated inhibition of BVB, both via the direct, central action.

Short-term potentiation of carotid chemoreflex: an NMDAR-dependent neural integrator

Repetitive stimulation of the carotid sinus nerve (CSN) elicits a short-term potentiation (STP) of the reflex response in respiratory motor output in mammals. The input-output transformation approximates a leaky integrator with a time constant of several seconds. Here, we showed that STP induced by CSN stimulation in rats was manifested in the reflex response in the amplitude of rhythmic phrenic nerve activity as well as its duration. Moreover, pharmacological blockade of NMDA receptors (NMDAR) resulted in marked increases in the time constants of the equivalent neural integrator in both the STP induction phase (by 10- to 20-fold) and recovery phase (by 1- to 5-fold). Thus, NMDAR serves as a molecular switch that facilitates the integrative processing of CSN inputs by STP.

Chronic hypoxia enhances the phrenic nerve response to arterial chemoreceptor stimulation in anesthetized rats

Chronic exposure to hypoxia results in a time-dependent increase in ventilation called ventilatory acclimatization to hypoxia. Increased O(2) sensitivity of arterial chemoreceptors contributes to ventilatory acclimatization to hypoxia, but other mechanisms have also been hypothesized. We designed this experiment to determine whether central nervous system processing of peripheral chemoreceptor input is affected by chronic hypoxic exposure. The carotid sinus nerve was stimulated supramaximally at different frequencies (0.5-20 Hz, 0.2-ms duration) during recording of phrenic nerve activity in two groups of anesthetized, ventilated, vagotomized rats. In the chronically hypoxic group (7 days at 80 Torr inspired PO(2)), phrenic burst frequency (f(R), bursts/min) was significantly higher than in the normoxic control group with carotid sinus nerve stimulation frequencies >5 Hz. In the chronically hypoxic group, peak amplitude of integrated phrenic nerve activity ( integral Phr, percent baseline) or change in integral Phr was significantly greater at stimulation frequencies between 5 and 17 Hz, and minute phrenic activity ( integral Phr x f(R)) was significantly greater at stimulation frequencies >5 Hz. These experiments show that chronic hypoxia facilitates the translation of arterial chemoreceptor afferent input to ventilatory efferent output through a mechanism in the central nervous system.

Postoperative hypertension effect of carotid sinus denervation

Postoperative arterial blood pressures were monitored in 43 patients who had undergone bilateral neck dissection during a 6-week period at Ankara Numune Hospital's IInd Otorhinolaryngology Department. During the first operations, all cases received carotid sinus denervation, whereas no denervation was done for the opposite side dissections held 6 weeks later. Study and control groups were composed of the same patients to achieve an objective outcome for the risk of postoperative hypertension. Hypertension was observed in 10 (23%) of 43 patients after the first operations and 12 (28%) of 43 patients after the opposite side dissections, for which no carotid denervation was done. The difference between the rates was insignificant statistically.

Dopamine modulates carotid nerve responses induced by acetylcholine on the cat petrosal ganglion in vitro

We have recently reported that application of acetylcholine (ACh) or nicotine to the petrosal ganglion-the sensory ganglion of the glossopharyngeal nerve-elicits a burst of discharges in the carotid nerve branch, innervating the carotid body and sinus, but not in the glossopharyngeal branch, innervating the tongue and pharynx. Thus, the perikarya of sensory neurons for the carotid bifurcation exhibit selective cholinosensitivity. Since dopamine (DA) modulates carotid nerve chemosensory activity, we searched for the presence of DA sensitivity at the perikarya of these neurons in the cat petrosal ganglion superfused in vitro. Applications of DA in doses of up to 5 mg to the ganglion did not modify the rate of spontaneous discharges in the carotid nerve. However, if DA was applied 30 s before ACh injections, ACh-evoked reactions were modified: low doses of DA enhanced the subsequent responses to ACh, while high doses of DA depressed the responses to ACh. This depressant effect of DA on ACh responses was partially antagonized by adding spiroperone to the superfusate. Our results show that the response to ACh of petrosal ganglion neurons projecting through the carotid nerve is modulated by DA acting on D(2) receptors located in the somata of these neurons. Thus, dopaminergic modulation of cholinosensitivity could be shared also by the membranes of peripheral endings and perikarya of primary sensory neurons involved in arterial chemoreception.

Activation of brain neurons by circulating angiotensin II: direct effects and baroreceptor-mediated secondary effects

Circulating angiotensin II acts on neurons in circumventricular organs, leading to activation of central pathways involved in blood pressure regulation and body fluid homeostasis. Apart from this primary effect, an increase in the level of circulating angiotensin II may also activate brain neurons as a secondary consequence of the associated increase in blood pressure, which will stimulate arterial baroreceptors and thus activate central neurons that are part of the central baroreceptor reflex pathway. The aim of this study was to identify the population of neurons that are activated as a consequence of the direct actions of circulating angiotensin II on the brain, independent of secondary baroreceptor-mediated effects. For this purpose, we have mapped the distribution of neurons in the brainstem and forebrain that are immunoreactive for Fos (a marker of neuronal activation) following intravenous infusion of angiotensin II in conscious rabbits with chronically denervated carotid sinus and aortic baroreceptors. The distribution was compared with that evoked by the same procedure in two separate groups of barointact rabbits, in which angiotensin II was infused either at a rate similar to that in the barodenervated group, or at a rate approximately five times greater. In barodenervated rabbits, angiotensin II infusion evoked a significant increase in Fos expression, compared to control animals infused with the vehicle solution alone, in several forebrain nuclei (organum vasculosum of the lamina terminalis, subfornical organ, median preoptic nucleus, supraoptic nucleus, paraventricular nucleus, bed nucleus of the stria terminalis and suprachiasmatic nucleus), but little or no increase in Fos expression in any lower brainstem region. In barointact rabbits infused with angiotensin II at a similar rate to that in barodenervated rabbits, a similar degree of Fos expression was evoked in all of the above forebrain regions, but in addition a significantly greater degree of Fos expression was evoked in several medullary regions (nucleus tractus solitarius, area postrema, and ventrolateral medulla), even though the angiotensin II-evoked increase in mean arterial pressure (17 +/- 3 mmHg) was less than that evoked in the barodenervated rabbits (26 +/- 2 mmHg). In barointact rabbits infused with angiotensin II at the higher rate, the increase in mean arterial pressure was 29 +/- 3 mmHg. In these animals, the pattern of Fos expression was similar to that evoked in barointact rabbits infused at the lower rate, but the degree of Fos expression in all medullary regions and in some forebrain regions was significantly greater. The results of the present study, together with those of previous studies from our laboratory in which we determined the effects of phenylephrine-induced hypertension on brain Fos expression [Li and Dampney (1994) Neuroscience 61, 613-634; Potts et al. (1997) Neuroscience 77, 503-520], indicate that in conscious rabbits circulating angiotensin II activates primarily circumventricular neurons within the organum vasculosum of the lamina terminalis and subfornical organ, but not the area postrema, and this in turn leads to activation of neurons in other forebrain regions, including the median preoptic, supraoptic, paraventricular and suprachiasmatic nucleus as well as the bed nucleus of the stria terminalis. In contrast, the activation of neurons in medullary regions evoked by an increase in the level of circulating angiotensin II is primarily a secondary effect resulting from stimulation of arterial baroreceptors.

Failure of acute hyperinsulinemia to alter blood pressure is not due to baroreceptor feedback

It is well documented that acute insulin administration stimulates the sympathetic nervous system in both humans and animals. Despite marked sympathetic activation during acute hyperinsulinemia, blood pressure is generally not increased because it is overridden by the vasodilator action of insulin. The maintenance of blood pressure in the face of sympathetic activation is unknown. A possible mechanism includes feedback regulation by the baroreceptor reflex arc. In normotensive states, hyperinsulinemic-induced sympathetic activation may tend to elevate blood pressure, but this change is rapidly sensed by the baroreceptors in the carotid arteries (and aortic arch), and a counterbalancing increase in vasodilation could return blood pressure to normal. Thus, it can be speculated that, in the event of diminished baroreceptor sensitivity and suppressed vasodilator actions of insulin, common abnormalities in hypertension, acute insulin infusion would be expected to increase blood pressure. We undertook the present study to determine whether the baroreceptor reflex arc modulated the blood pressure response to acute hyperinsulinemia. To this end, six normotensive dogs underwent saline or insulin infusions before and after deactivation of the carotid and aortic baroreceptors. Baroreceptor dysfunction was documented after denervation in all cases by an abnormal response to phenylephrine injections. Before denervation, insulin infusions caused a slight but nonsignificant rise in mean arterial pressure (MAP; 110 +/- 5 to 120 +/- 5 mm Hg; P = 0.13). Baroreceptor denervation caused a marked variability in blood pressure. However, basal mean arterial pressure was not significantly altered. Neither saline nor insulin infusions (105 +/- 10 v 105 +/- 8 mm Hg, basal v steady state) caused a significant change in MAP in denervated dogs. Likewise, insulin and saline did not change heart rates significantly in intact or denervated animals. Furthermore, glucose metabolism was similar in both groups of animals. This study demonstrates that the baroreceptor reflex arc does not mediate the blood pressure response to acute hyperinsulinemia.

New simple methods for isolating baroreceptor regions of carotid sinus and aortic depressor nerves in rats

We developed new methods for isolating in situ baroreceptor regions of carotid sinus and aortic depressor nerves in halothane-anesthetized rats. After ligation of the root of the external carotid artery, the internal carotid and pterygopalatine arteries were embolized with two ball bearings of 0.8 mm in diameter. Bilateral carotid sinus pressures were changed between 60 and 180 mmHg in 20-mmHg steps lasting 1 min each. The sigmoidal steady-state relationship between aortic and carotid sinus pressures in 11 rats indicated the maximum gain of the carotid sinus baroreflex to be -2. 99 +/- 0.75 at 120 +/- 5 mmHg. An in situ isolation of the baroreceptor area of the right aortic depressor nerve could be made by ligation of the innominate, common carotid, and subclavian arteries in 9 rats. Pressure imposed on the subclavian baroreceptor was altered between 40 and 180 mmHg in 20-mmHg steps lasting 1 min each. The sigmoidal steady-state relationship between the aortic depressor nerve activity and imposed pressure showed that the baroreceptor gain peaked at 118 +/- 4 mmHg. We established an easy approach to the rat baroreflex and baroreceptor research.

The carotid body of the spontaneous insulin-dependent diabetic rat

The carotid bodies from adult spontaneous insulin-dependent diabetic rats (strain BB/S) were perfusion-fixed at normal arterial blood pressure with 3% phosphate-buffered glutaraldehyde and compared with the organs from control rats (strain BB/Sc) prepared in the same way. Serial 5-micron sections were cut, stained, and using an interactive image analysis system, were analysed to determine the volumes of the carotid body and its vascular and extravascular compartments. There was no evidence of systemic arterial disease in the carotid stem arteries in either group of animals, and the microvasculature of the organs appeared normal by light microscopy. The volume of the carotid body was unchanged 3 months after the onset of diabetes but was increased at 6 months. The total vascular volume of the organ was unchanged, but the volume of the small vessels (5-12 microns) was increased. In the control group the small vessels comprised 5% of the total volume of the carotid body, or about 44% of the vascular compartment. The percentage of small vessels increased at 3 months in the diabetic group, but had returned to normal at 6 months. The extravascular volume followed the same pattern as the total carotid body volume and so did not change appreciably when expressed as a percentage of the total volume of the organ. The increase in size of the carotid body in diabetic rats is due, therefore, to an augmented extravascular volume. In one diabetic specimen the carotid sinus nerve showed signs of diabetic neuropathy, axonal swelling and intramyelinic oedema. The clinical implications of these results are discussed.

Hypoxia inhibits baroreflex vagal bradycardia via a central action in anaesthetized rats

It is known that arterial baroreflexes are suppressed in stressful conditions. The present study was designed to determine whether and how hypoxia affects arterial baroreflexes, especially the heart rate component, baroreflex vagal bradycardia. In chloralose-urethane-anaesthetized rats, baroreflex vagal bradycardia was evoked by electrical stimulation of the aortic depressor nerve, and the effect of 15 s inhalation of hypoxic gas (4% O2) was studied. Inhalation of hypoxic gas was found to inhibit baroreflex vagal bradycardia. The inhibition persisted after bilateral transection of the carotid sinus nerve. Cervical vagus nerves were cut bilaterally and their peripheral cut ends were stimulated to provoke vagal bradycardia of peripheral origin so as to determine whether hypoxia could inhibit vagal bradycardia by acting on a peripheral site. In contrast to baroreflex vagal bradycardia, the vagus-induced bradycardia was not affected by hypoxic gas inhalation. It is concluded that baroreflex vagal bradycardia is inhibited by hypoxia and the inhibition is largely mediated by its direct central action.

A molecular component of the arterial baroreceptor mechanotransducer

Baroreceptor nerve endings detect acute fluctuations in arterial pressure. We tested the hypothesis that members of the DEG/ENaC family of cation channels, which are responsible for touch sensation in Caenorhabditis elegans, may be components of the baroreceptor mechanosensor. We found the gamma subunit of ENaC localized to the site of mechanotransduction in baroreceptor nerve terminals innervating the aortic arch and carotid sinus. A functional role for DEG/ENaC members was suggested by blockade of baroreceptor nerve activity and baroreflex control of blood pressure by an amiloride analog that inhibits DEG/ENaC channels. These data suggest that ENaC subunits may be components of the baroreceptor mechanotransducer and pave the way to a better definition of mechanisms responsible for blood pressure regulation and hypertension.

Effects of carotid sinus nerve transection on changes in neuropeptide Y and indolamines induced by long-term hypoxia in rats

Long-term hypoxia induces changes in neuropeptide-Y-like immunoreactivity (NPY-LI) and/or in the content of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) at the central level. To determine whether these alterations depend on the integrity of carotid body (CB) chemoreceptors, intact rats or those whose carotid sinus nerve was transected (CSNT) were exposed to hypoxia (10% O2) or to normoxia for 14 days. Thereafter, NPY-LI, 5-HT and 5-HIAA levels in discrete brain regions were determined. The increase in NPY-LI in the ventrolateral medulla oblongata (VLM) of intact hypoxic rats was mostly abolished after CSNT and therefore is mainly mediated by CB chemoreceptors. In contrast, other hypoxia-induced changes were similar or even enhanced in CSNT as compared to intact rats and therefore do not depend on the integrity of CB chemoreceptors. This was the case for the increase of NPY-LI in the striatum and the caudal dorsomedian medulla oblongata (DMM), as well as for all the changes in 5-HT and 5-HIAA in the DMM, the VLM, the raphe nuclei, the striatum and the frontal cortex. We propose that long-term hypoxia alters brain NPY-LI and indolamine content through the stimulation of CB chemoreceptors or ancillary chemoreceptors, as well as through local biochemical or morphological mechanisms.

Endogenous prostanoids modulate the ACTH and AVP responses to hypotension in late-gestation fetal sheep

We have previously reported that prostaglandin E2 and thromboxane A2 stimulate endocrine and cardiovascular responses similar to the responses to arterial hypotension. The present experiments were designed to test the hypothesis that prostanoids are involved in the generation of responses to hypotension induced by vena cava occlusion. Fetal sheep were either intact or subjected to a prior carotid sinus denervation and bilateral vagosympathetic nerve section. Indomethacin or vehicle was injected intravenously 90 min before the start of arterial hypotension. In intact fetuses treated with phosphate buffer, ACTH increased significantly from 83 +/- 39 to 3,611 +/- 774 pg/ml, arginine vasopressin (AVP) increased from 3. 9 +/- 0.5 to 1,079 +/- 549 pg/ml, and cortisol increased from 4.7 +/- 0.8 to 9.5 +/- 1.7 ng/ml. Indomethacin treatment significantly reduced the magnitudes of the hormonal responses. Baroreceptor and chemoreceptor denervation attenuated the ACTH and AVP responses, but these responses were not further inhibited by indomethacin. We conclude that endogenous prostanoids partially mediate the reflex hormonal and hemodynamic responses to arterial hypotension in late-gestation fetal sheep.

Time structure, temporal correlation and coherence of chemosensory impulses propagated through both carotid nerves in cats

In spontaneously breathing, pentobarbitone anesthetized cats, we recorded simultaneously the impulses in the chemosensory fibers of both carotid (sinus) nerves, to analyze the correlations between the frequencies of chemosensory discharges (f chi) and their activation (¿df chi/dt¿a) and deactivation (¿df chi/dt¿d) rates. We studied the chemosensory responses to brief exposures to hypoxia (100% N2; 5-s and 10-s) and hyperoxia (100% O2; 30-s), and intravenous injections of excitatory (NaCN 0.2-100 micrograms/kg) and inhibitory (dopamine hydrochloride 0.02-20 micrograms/kg) chemoreceptor agents. Hypoxia increased f chi, with a high temporal correlation between frequency levels in both nerves. Prolonging hypoxic stimulation increased ¿df chi/dt¿d, with preservation of ¿df chi/dt¿a. Hyperoxic exposure produced highly correlated decreases in f chi in both nerves, but reduced correlation in df chi/dt. Increasing doses of NaCN produced analogous increments in f chi, df chi/dt and their correlations, the ¿df chi/dt¿a/¿df chi/dt¿d ratio remaining constant along all the experimental range, except in one animal in which the ratio increased in both nerves alike. Dopamine reduced f chi bilaterally, with chemosensory silencing being reached with doses of about 0.2-0.5 microgram/kg, the correlations between f chi's of both nerves remaining constant within the range analyzed. Maximal ¿df chi/dt¿d was not affected along the range of dopamine doses, except in one animal in which it increased in both nerves. It is concluded that both carotid nerves convey similar quantitative information to the brain stem. Thus, the carotid nerves constitute either cooperative inputs or redundant afferences contributing to a high safety factor.

Excitation of baroreceptors depresses A- and C-components of the somato-cardiac sympathetic reflex in anesthetized rats

The effect of baroreceptor activation on somato-cardiac sympathetic reflex discharges was examined in urethane-anesthetized, vagotomized, and artificially ventilated rats. Single shock stimulation of myelinated (A) and unmyelinated (C) fibers in the tibial nerve of the left hindlimb elicited two separate excitatory reflex discharge components in a branch of the cardiac sympathetic nerve. They are termed the A- and C-components of the somato-cardiac sympathetic reflex discharges. When aortic nerves (AN) and carotid sinus nerves (CSN) were intact, a sudden increase in mean arterial blood pressure to about 150 mmHg induced by I.V. injection of phenylephrine (50 micro/kg) depressed the A- and C-components by up to 47 +/- 5.4 and 37 +/- 7.7% of the control values, respectively. However, bilateral sino-aortic denervation completely abolished the pressure-induced depression of both components. We conclude that baroreceptor afferent signals from the AN and CSN inhibit both A- and C-components of the excitatory somato-cardiac sympathetic reflex discharges. This and other previous evidence mentioned in the text indicate that inhibitory cardiac sympathetic reflexes originating from arterial baroreceptors and excitatory ones originating from somatic afferents interact, probably at the brainstem.

Effects of long-term vasodilator therapy in patients with carotid sinus hypersensitivity

BACKGROUND

In patients affected by carotid sinus hypersensitivity, long-term vasodilator therapy might increase the risk of syncopal episodes by reducing systolic blood pressure and venous return to the heart.

METHODS AND RESULTS

Thirty-two patients (mean age 73 +/- 9 years; 20 men) who met all the following criteria were included: (1) one or more episodes of syncope occurring during long-term (>6 months) treatment with angiotensin-converting enzyme inhibitors, long-acting nitrates, calcium antagonists, or a combination of these; (2) a positive response to carotid sinus massage, defined as the reproduction of spontaneous syncope in the presence of ventricular asystole > or =3 seconds or a fall in systolic blood pressure > or =50 mm Hg; (3) negative workup for other causes of syncope. The patients were randomly assigned to continue or to discontinue use of vasodilators; carotid sinus massage was repeated 2 weeks after randomization. By the end of the study period, the baseline values of systolic blood pressure were significantly different between the 2 groups of patients both in supine (P=.01) and upright (P=.03) positions. Syncope had been induced by carotid sinus massage in 81% of patients in the "on-vasodilator" group and in 62% of patients in the "off-vasodilator" group (P=.21). The cardioinhibitory reflex was of similar magnitude in the 2 groups, being found in 50% of the patients in each group, with a maximum ventricular pause of 7.1 +/- 2.7 and 6.7 +/- 1.8 seconds, respectively. The percentage decrease of blood pressure did not differ between the 2 groups, even if, in absolute values, the baseline difference of blood pressure roughly persisted for the duration of the test. In consequence of that, the rise of blood pressure to similar values was delayed approximately 30 seconds in the "on-vasodilator" group and took more than 2 minutes to return to baseline values.

CONCLUSIONS

In patients affected by carotid sinus hypersensitivity, chronic vasodilator therapy does not have a direct effect on carotid sinus reflexivity, although the delayed recovery of pretest blood pressure values could indirectly potentiate the severity of the clinical manifestations of the syndrome. The persistence of hypotension for a longer time after the end of the massage suggests that vasodilators cause an impairment of compensatory mechanisms.

Catechol activation in rat rostral ventrolateral medulla after systemic isocapnic metabolic acidosis

The catechol signal recorded using in vivo voltammetry within the rat rostral ventrolateral medulla (RVLM) can be interpreted as a catechol-specific index of the integrated activity of RVLM adrenergic barosensitive bulbospinal and nonbulbospinal neurons. To test the hypothesis that systemic acidosis leads to the activation of RVLM adrenergic neurons, the RVLM catechol signal was observed in rats after mild systemic acidosis (pH 7.20-7.25 for 30 min) induced by 1 M HCl under halothane anesthesia, controlled mechanical ventilation, and continuous infusion of Ringer lactate. Particular attention was paid to ensure that changes in mean arterial pressure (MAP) were <15 mmHg during HCl challenge. Saline administration was not associated with any significant change in all considered variables (n = 5). Mild isocapnic systemic acidosis was associated with an increase in catechol signal (n = 5), irrespective of carotid sinus nerve section (n = 5). In keeping with the aim of the study, there were minor (<15 mmHg) but significant changes in MAP among saline, intact, and deafferented groups. Changes in heart rate were not significant. In conclusion, a catechol activation is observed in the RVLM when arterial pressure is maintained during isocapnic systemic metabolic acidosis. This catechol activation appears primarily centrally mediated. Therefore, adrenergic RVLM neurons may relay inputs from the central respiratory generator to the sympathetic system and/or act as chemosensors for H+ next to the surface of the ventrolateral medulla.

Influence of cervical sympathetic nerves on ventilation and upper airway resistance in the rat

The cervical sympathetic trunks innervate the carotid bodies, carotid baroreceptors, thyroid gland and the upper airway mucosa, structures which can influence breathing and upper airway resistance. However, their role in the control of ventilation and upper airway patency is poorly understood. A constant airflow was applied to the upper airway through a high-cervical tracheostomy in anaesthetized rats breathing spontaneously through a low-cervical tracheostomy. The peripheral ends of the cut cervical sympathetic trunks were stimulated electrically and airflow resistance and ventilation were measured. The effects of cervical sympathetic trunk section on ventilation were also measured in conscious rats. In conscious rats, cutting the sympathetic trunks caused a decrease in ventilation during normoxia but only slightly affected ventilatory responses to hypoxia and hypercapnia. In anaesthetized rats, sympathetic trunk stimulation caused an inhibition of breathing which was sometimes followed by excitation. These responses were unaffected by alpha- or beta-adrenoceptor blockade but were abolished by cutting the carotid sinus nerves. Sympathetic stimulation also caused a fall in upper airway resistance which was reduced by bypassing the nose, unaffected by propranolol or carotid sinus nerve section and abolished by phentolamine. It was concluded that the cervical sympathetic nerves exert important influences on ventilation and upper airway resistance.

Strain-associated differences in hypoxic chemosensitivity of the carotid body in rats

Studies in humans indicate genetic effects on the ventilatory response to hypoxia, but the site of these effects is unknown. The present study explores the question of whether there are genetically directed effects on the intrinsic hypoxic chemosensitivity of the carotid body. The approach was to study these responses in two inbred rat strains [spontaneously hypertensive rats (SHR) and Fischer 344 (F-344)] and to measure in vivo carotid chemosensitivity as the change in carotid sinus nerve (CSN) activity during progressive, isocapnic hypoxia and the isolated, in vitro responses of excised superfused carotid bodies, loaded with the fluorimetric indicator fura 2, measured as the cytosolic calcium response to moderate hypoxia (PO2 = 55 mmHg). CSN responses in F-344 rats (n = 12) were uniformly low, with a shape parameter A of 13.8 +/- 6.59 (SE), whereas responses in SHR (n = 15) were sevenfold higher (108 +/- 24.1; P < 0.002) and showed greater variation. In vitro, intracellular calcium responses of superfused carotid bodies estimated from the fluorimetric ratio (340/380 nm) showed a greater peak increase during hypoxia in carotid bodies from SHR (140 +/- 4.7%) than from F-344 rats (114 6.0%; P < 0.01). Our results indicate strain-related differences in hypoxic chemosensitivity that are intrinsic to the carotid body and that could mediate genetic effects on ventilatory responsiveness to hypoxia.

Time domains of the hypoxic ventilatory response

The ventilatory response to hypoxia depends on the pattern and intensity of hypoxic exposure and involves several physiological mechanisms. These mechanisms differ in their effect (facilitation or depression) on different components of ventilation (tidal volume and frequency) and in their time course (seconds to years). Some mechanisms last long enough to affect future ventilatory responses to hypoxia, indicating 'memory' or functional plasticity in the ventilatory control system. A standard terminology is proposed to describe the different time domains of the hypoxic ventilatory response (HVR) and to promote integration of results from different experimental preparations and laboratories. In general, the neurophysiological and neurochemical basis for short time domains of the HVR (seconds and minutes) are understood better than longer time domains (days to years), primarily because short time domains are studied in the laboratory more easily. Understanding the mechanisms for different time domains of the HVR has important implications for both basic and clinical science.

Inhibition of dopamine release with simultaneous chemosensory excitation by hypercapnia with and without [Ca2+]0 in the cat carotid body

The hypothesis that dopamine (DA) overflow corresponds to carotid sinus nerve (CSN) discharge during hypercapnia and is dependent on [Ca2+]0 was tested. We simultaneously measured the time course of DA overflow and CSN discharge of the cat carotid body, perfused/superfused in vitro at 37 degrees C at decreasing [Ca2+]0, during transition from normocapnia (PCO2 approximately 30-35 Torr) to hypercapnia (PCO2 approximately 60-65 Torr). In the presence of normal [Ca2+]0, hypercapnia instantaneously increased nerve discharge to peak levels followed by a decrease to steady states which were above the basal rate of activity. CSN discharge rate did not differ at decreasing [Ca2+]0 between 2.2 and 1.0 mM, and it began to decline at 0.1 mM [Ca2+]0, culminating to zero level in most cases, at zero [Ca2+]0. DA overflow increased slightly during hypercapnic peak CSN activity. Thereafter it declined to steady state levels below those of normocapnic conditions. Decreases in steady state DA levels were significantly less at 0 mM [Ca2+]0 compared to the higher calcium concentrations (0.1, 1.0 and 2.2 mM). Overall, steady state CSN activity and DA overflow were inversely related. Thus, DA release cannot have excitatory implications for carotid chemoreceptors during hypercapnia in the cat.

Hereditarily elongated carotid sinus nerve in a rat colony

A specific colony of Wistar rats was found in which the common carotid artery bifurcates at an unusually caudal position, thereby the carotid sinus nerve that originates from the bifurcation is elongated. The present study was done to determine whether this elongated nerve carries baro- and chemosensations in the same manner as the carotid sinus nerve of conventional rats or of other species. In chloralose-urethane anesthetized rats of this specific colony, the afferent discharges were recorded from the elongated carotid sinus nerve in response to a phenylephrine-induced rise in blood pressure and a fall in oxygen tension, as well as an increase in carbon dioxide tension in the respiratory gas. Reflex effects of electrical stimulation of the nerve were also examined. In nerve recording, the afferent discharges of the elongated carotid sinus nerve were increased by any of the perturbations, hypertensive, hypoxic or hypercapnic. Electrical stimulation of the elongated carotid sinus nerve caused an initial rise and a subsequent fall in blood pressure, bradycardia, and an increase of respiratory volume and rate. These results confirmed that the elongated carotid sinus nerve of rats in this colony contains both baroreceptor and chemoreceptor afferent fibers. It seems that this colony of rats proffers a beneficial material which will facilitate the studies to analyze the features and reflex functions of carotid body chemoreceptors and/or carotid sinus baroreceptors.

Fetal endocrine responses to prolonged reduced uterine blood flow are altered following bilateral sectioning of the carotid sinus and vagus nerves

The present study examines the effect of carotid sinus/vagosympathetic denervation on fetal endocrine responses to prolonged reduced uterine blood flow (RUBF). Fetal sheep had vascular catheters inserted following bilateral sectioning of the carotid sinus and vagus nerves (denervated, n = 7) or sham denervation (intact, n = 7). Uterine blood flow was mechanically restricted at 126.1 +/- 0.7 days (mean +/- S.E.M.) for 24 h, decreasing arterial oxygen saturation by 47.3 +/- 2.6% (P < 0.01). Fetal plasma samples were obtained at -1, 3, 6, 12 and 24 h for subsequent analyses of arginine vasopressin (AVP), angiotensin II and catecholamines. The AVP response to prolonged RUBF was markedly attenuated in denervated fetuses (15.6 +/- 3.6 to 34.9 +/- 6.0 pg/ml) when compared with intact (10.0 +/- 1.4 to 127.3 +/- 28.4 pg/ml). In contrast, intact fetuses demonstrated no change in plasma angiotensin II concentrations with RUBF whereas denervated fetuses demonstrated a marked increase from 47.5 +/- 18.9 to 128.7 +/- 34.2 pg/ml. The norepinephrine and epinephrine responses to prolonged RUBF were attenuated in denervated fetuses (950.1 +/- 308.9 and 155.8 +/- 58.5 to 1268.3 +/- 474.6 and 290.6 +/- 160.2 pg/ml respectively) when compared with intact (1558.3 +/- 384.4 and 547.3 +/- 304.7 pg/ml to 3289.2 +/- 1219.8 and 896.8 +/- 467.8 pg/ml respectively). These results support a role for the peripheral chemoreceptors in mediating fetal endocrine responses to prolonged RUBF, which may in part lead to the altered cardiovascular responses observed in denervated fetuses under these conditions.

Angiotensin II and cardiovascular chemoreflex responses to acute hypoxia in late gestation fetal sheep

1. In six intact and nine carotid sinus denervated (CSD) fetal sheep (125-128 days gestation) we measured heart rate (FHR), mean systemic arterial blood pressure (MAP), femoral and carotid blood flows (FBF and CBF), and femoral and carotid vascular resistances (FVR and CVR). Three experiments were conducted on successive days: normoxia followed by acute isocapnic hypoxia (Pa,O2 to ca 12 mmHg) with infusion of vehicle (HV experiment), the same protocol but with infusion of the angiotensin converting enzyme (ACE) inhibitor, captopril (HC experiment), and normoxia alone with captopril infusion (NC experiment). Plasma angiotensin II concentration ([AII]) was measured in these fetuses, and in a separate group of fetuses (n = 5) that were infused with the nitric oxide (NO) synthesis inhibitor N G-nitro-L-arginine methyl ester (L-NAME) or saline vehicle. 2. During normoxia, cardiovascular parameters and plasma [AII] were unaltered by captopril infusion, apart from a fall in MAP (NC experiment only, P < 0.05) and FHR (HC experiment only, P < 0.05) in intact and CSD fetuses, respectively. No differences were observed between intact and CSD groups. 3. At the onset of hypoxia the rapid initial fall in FHR and rise in FVR was attenuated in CSD fetuses. In all fetuses FHR returned towards prehypoxic levels as hypoxia continued. In contrast, during hypoxia with vehicle infusion (HV experiment) plasma [AII] rose to a similar level in intact and CSD fetuses. 4. In both intact and CSD fetuses, the rise in [AII] during hypoxia was blocked by captopril or L-NAME infusion. In CSD, but not intact, fetuses infused with captopril the rise in MAP was absent, and the fall in FBF and rise in FVR did not reach significance during hypoxia. 5. Thus, during normoxia CSD alone, or combined with ACE inhibition, does not consistently alter basal cardiovascular control in the late gestation fetus. The rise in [AII] during hypoxia is not mediated by carotid reflexes but may involve NO-dependent mechanisms. In intact fetuses, AII does not appear to be pivotal in cardiovascular control during hypoxia. It is only when carotid reflex mechanisms are removed that a role for AII in the regulation of MAP and peripheral blood flow during hypoxia becomes apparent. These findings lend weight to the idea of multiple mechanisms of fetal cardiovascular control during hypoxia.

Suppression of glomus cell K+ conductance by 4-aminopyridine is not related to [Ca2+]i, dopamine release and chemosensory discharge from carotid body

The hypothesis that suppression of O2-sensitive K+ current is the initial event in hypoxic chemotransduction in the carotid body glomus cells was tested by using 4-aminopyridine (4-AP), a known suppressant of K+ current, on intracellular [Ca2+]i, dopamine secretion and chemosensory discharge in cat carotid body (CB). In vitro experiments were performed with superfused-perfused cat CBs, measuring chemosensory discharge, monitoring dopamine release by microsensors without and with 4-AP (0.2, 1.0 and 2.0 mM in CO2-HCO3- buffer) and recording [Ca2+]i by ratio fluorometry in isolated cat and rat glomus cells. 4-AP decreased the chemosensory activities in normoxia but remained the same in hypoxia and in flow interruption. It decreased the tissue dopamine release in normoxia, and showed an additional inhibition with hypoxia. Also, 4-AP did not evoke any rise in [Ca2+]i in glomus cells either during normoxia and hypoxia, although hypoxia stimulated it. Thus, the lack of stimulatory effect on chemosensory discharge, inhibition of dopamine release and unaltered [Ca2+]i by 4-AP are not consistent with the implied meaning of the suppressant effect on K+ current of glomus cells.

Inactivation of baroafferents leads to loss of barosensitivity without changes in nerve morphology

Baroreceptors are stretch-sensitive mechanoreceptors, which are silenced by preventing distension of the receptor zone. Does chronic inactivation of these peripheral afferents alter their function or morphology? Compound action potentials and morphometry of carotid sinus nerves of 10 rabbits were investigated. The baroafferents were inactivated by embedding the pressure-released carotid sinus into silicon gel. The success of this procedure was validated by the absence of spike activity of the sinus nerve during normal and elevated systemic blood pressure. The contralateral vessels of the same animals were sham-operated and also embedded into silicon, but without prevention of wall movements. After 5, 7, 14 or 28 days the nerves were functionally reinvestigated before and after release of the sinus wall. Afterwards, the morphology of the nerve cross-sections was analysed by morphometry of electron micrographs. Baroafferents did not regain spike activity during immobilisation of the sinus wall. After release of the carotid sinus wall only nerves inactivated for five days regained their pulse synchronous baroreceptor discharge. Following seven days of inactivation, baroreceptor discharge could be elicited by maximal pressure elevation in only one of three nerves. At any time later, the baroreceptor response to arterial pressure changes was lost completely. The activity of the control nerves was preserved after 28 days. No obvious differences in fibre size and myelin thickness were observed between inactivated and control nerves. Inactivation of baroafferents for more than one week leads to a loss of pressure-dependent spike activity. Since morphology did not differ between inactivated and control nerves, it is suggested that changes of baroreceptor endings are responsible for this loss of function.

Phase resetting of the respiratory oscillator by carotid sinus nerve stimulation in cats

1. Stimulation of the carotid sinus nerve causes an increase in inspiratory (I) and expiratory (E) neural activities. If central respiratory oscillation is generated by an attractor-cycle process, an increase in its activity can be caused by a centrifugal perturbation of state. We evaluated this hypothesis by comparing the respiratory oscillator's phase responses to carotid sinus nerve stimulations in cats to the phase responses of an attractor-cycle oscillator, the Bonhoeffer-van der Pol (BvP) equations, subjected to centrifugal perturbations. 2. We recorded phrenic activity in seven anaesthetized, vagotomized, glomectomized, paralysed and servo-ventilated cats. Carotid sinus nerve (CSN) stimulation with 0.5-0.8 s electrical pulse trains increased the immediate cycle period and delayed the onset of breaths after stimulation in a highly predictable manner, with the exception that strong stimuli (25 Hz, 0.25-0.90 V) caused unpredictable responses when given at the I-E or the E-I transitions. The resetting plots exhibited focal gaps corresponding to these unpredictable responses, and the size of the gaps increased with increases in the strength of CSN stimulation. Type 0 resetting was not achieved despite the large perturbations in rhythm induced by CSN stimulation. 3. Centrifugal perturbations of the BvP oscillator resulted in phase responses which were similar to those found in the animal experiments. The BvP cycle had two critical phases at which phase resetting was highly irregular and neighbouring state trajectories were highly divergent. The resetting plots had focal gaps that increased in size with increases in the strength of perturbation. The gaps did not represent true discontinuity because at higher computational resolution the resetting plots appeared to be steep but smooth portions of topological Type 1 resetting curves. 4. These studies support the concept that brief carotid sinus nerve stimulations cause a transient outward displacement of the central respiratory state away from its attractor cycle, in contrast to the unidirectional displacements that accompany midbrain reticular or superior laryngeal nerve stimulations. The findings define particular geometrical relationships between oscillatory state trajectories of the rhythm generator and perturbed state trajectories induced by inputs to the oscillator. These relationships provide a framework for developing and testing the validity of neural models of the respiratory oscillator.

[Study of the central and peripheral mechanisms in enhancement of arterial baroreflex in cats during noradrenaline infusion]

An increased suppression of electrical activity of the cardiac inferior nerve by the baroreflex during norepinephrine infusion can be eliminated by a preliminary infusion of monoamine. The data obtained corroborated our previous findings that an increase in arterial pressure due to norepinephrine infusion was essential for catecholamine increasing the baroreflex inhibition of electrical activity in sympathetic nerves.

[Control of the cardiac rhythm in the cat by stimulation of the carotid sinus nerve with pulse bursts]

The cat heart rate responded with a systole to a burst stimulation of the sinocarotid nerve. A change of the stimulation frequency entailed a respective change in the systole frequency within the range 179.2 +/- 1.3-173.2 +/- 1.5 systoles per minute.

Co-localization of c-Fos and neurotransmitter immunoreactivities in the cat brain stem after carotid sinus nerve stimulation

To reveal neurones in the cat medulla oblongata involved in carotid baroreceptor/chemoreceptor reflexes, the distribution of c-Fos oncoprotein immunoreactivity was studied following electrical stimulation of the right carotid sinus nerve. The neurochemistry of the activated neurones was investigated using antisera to tyrosine hydroxylase, neuropeptide Y, somatostatin, and glutamate. Nitric oxide containing neurones were identified using antiserum to nitric oxide synthase (NOS) and by the histochemical localization of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase. Following sinus nerve stimulation numerous c-Fos-IR cells were detected both ipsilaterally and contralaterally in the nucleus tractus solitarii, the area postrema and throughout the ventrolateral medulla. Dual labelling studies revealed that 3.3% of c-Fos-immunoreactive cells in the nucleus tractus solitarii were also immunoreactive for tyrosine hydroxylase. The double labelled cells were scattered within the medial and ventrolateral subnuclei, predominantly rostral to obex. A higher proportion (10.3%) of c-Fos-IR cells in the ventrolateral medulla also showed tyrosine hydroxylase immunoreactivity. Caudal to obex, these were scattered in the reticular formation between the spinal trigeminal nucleus and the lateral reticular nucleus, while more rostrally they were found within the lateral reticular nucleus, the nucleus ambiguus and the lateral tegmental field. Cells expressing c-fos and reactive for glutamate, neuropeptide Y or NADPH-diaphorase (or NOS) were only rarely seen, and co-localization of c-Fos and somatostatin immunoreactivities was not seen. These results suggest that of the neurones forming pathways within the medulla activated on carotid sinus nerve stimulation, presumably mediating baro- and chemoreceptor reflexes, relatively few utilize catecholamines, glutamate, neuropeptide Y or nitric oxide as their transmitter substance.

[Carotid baroreflex in sleep apnea patients]

15 patients (11 male, 9 female) with obstructive sleep apnoea syndrome (OSAS) and 10 healthy age- and weight-matched control subjects volunteered to participate in the study to test circadian variations of cardiac response to activation of carotid baroreceptors. Activation of carotid baroreceptors was evoked by applying a negative pressure to both left and right carotid sinus regions, using two small separate neck-suction capsules. All experiments were performed under standard conditions every two hours. The cardiac response to carotid baroreceptors activation was significantly reduced in OSAS Patients, to a greater degree in the group of hypertensive patients. A characteristic circadian rhythm of baroreceptor reactivity persisted in all OSAS patients. Thus, OSAS patients constitute a high-risk group for the development of cardiovascular diseases and pathological course of blood pressure during daytime.

Postoperative hypertension after radical neck dissection

Postoperative hypertension after radical neck dissection was detected in 20.2% of 109 neck dissections in our department between 1989 and 1993. It was probably caused by carotid sinus denervation and appeared after the vasodilation generated by anesthesia had subsided. If postoperative hypertension was encountered after the first operation, the risk of such hypertension after surgery on the contralateral side significantly increased. (Otolaryngol Head Neck Surg 1997;117:91–2.)

Postoperative hypertension after radical neck dissection

Postoperative hypertension after radical neck dissection was detected in 20.2% of 109 neck dissections in our department between 1989 and 1993. It was probably caused by carotid sinus denervation and appeared after the vasodilation generated by anesthesia had subsided. If postoperative hypertension was encountered after the first operation, the risk of such hypertension after surgery on the contralateral side significantly increased.

Modeling neural mechanisms for genesis of respiratory rhythm and pattern. III. Comparison of model performances during afferent nerve stimulation

The goal of the present study was to evaluate the relative plausibility of the models of the central respiratory pattern generator (CRPG) proposed in our previous paper. To test the models, we compared changes in generated patterns with the experimentally observed alterations of the respiratory pattern induced by various stimuli applied to superior laryngeal (SLN), vagus and carotid sinus (CS) nerves. In all models, short-duration SLN simulation caused phase-resetting behavior consistent with experimental data. Relatively weak sustained SLN stimulation elicited a two-phase rhythm comprising inspiration and postinspiration whereas a stronger stimulation stopped oscillations in the postinspiratory phase ("postinspiratory apnea"). In all models, sustained vagus nerve stimulation produced postinspiratory apnea. A short vagal stimulus delivered during inspiration terminated this phase. The threshold for inspiratory termination decreased during the course of the inspiratory phase. The effects of short-duration vagal stimulation applied during expiration were different in different models. In model 1, stimuli delivered in the postinspiratory phase prolonged expiration whereas the late expiratory phase was insensitive to vagal stimulation. No insensitive period was found in model 2 because vagal stimuli delivered at any time during expiration prolonged this phase. Model 3 demonstrated a short period insensitive to vagal stimulation at the very end of expiration. When phasic CS nerve stimulation was applied during inspiration or the first half of expiration, the performances of all models were similar and consistent with experimental data: stimuli delivered at the beginning inspiration shortened this phase whereas stimuli applied in the middle or at the end of inspiration prolonged it and stimuli delivered in the first half of expiration prolonged the expiratory interval. Behavior of the models were different when CS stimuli were delivered during the late expiratory phase. In model 1, these stimuli were ineffective or shortened expiration initiating the next inspiration. Alternatively, in models 2 and 3, they caused a prolongation of expiration. Although all CRPG models demonstrated a number of plausible alterations in the respiratory pattern elicited by afferent nerve stimulation, the behavior of model 1 was most consistent with experimental data. Taking into account differences in the model architectures and employed neural mechanisms, we suggest that the concept of respiratory rhythmogenesis based on the essential role of postinspiratory neurons is more plausible than the concept employing specific functional properties of decrementing expiratory (dec-E) neurons and that the ramp firing pattern of the late expiratory neuron is more likely to reflect intrinsic properties than disinhibition from the dec-E neurons.

The effect of carotid sinus denervation on fetal heart rate variation in normoxia, hypoxia and post-hypoxia in fetal sheep

OBJECTIVE

To investigate whether carotid sinus nerve reflexes are linked to the increase in heart rate variation in acute (one hour) hypoxia in late gestation fetal sheep

DESIGN

Comparison of short term variation between intact and carotid sinus denervated fetuses in normoxia, hypoxia and post-hypoxia.

SUBJECTS

Sixteen chronically catheterised pregnant sheep in late gestation.

RESULTS

There was no significant difference in short term variation between intact and denervated fetuses in normoxia. In intact fetuses short term variation increased significantly in hypoxia. In denervated fetuses it tended to increase in hypoxia, but this was not statistically significant. During the post-hypoxia period, short term variation increased significantly in denervated fetuses, although at this time it was decreasing in intact fetuses. When the decrease in pH was small intact fetuses showed a significantly greater increase in short term variation than denervated fetuses in hypoxia. In contrast, short term variation increased similarly in both groups when the pH decrease was greater (> 0.03 in early hypoxia and > 0.05 in late hypoxia).

CONCLUSIONS

Carotid sinus nerve reflexes have an important influence on heart rate variation in hypoxia and post-hypoxia. It appears that other mechanisms (e.g. a rise in circulating catecholamines) are linked to an increase in heart rate variation when mild acidemia occurs in hypoxia.

Induction of paradoxic bradycardia in rats by inferior vena cava occlusion during the administration of isoproterenol: the essential role of augmented sympathetic tone

INTRODUCTION

Testing human susceptibility for vasodepressor reactions involves combining venous return restriction by passive upright tilting and administering isoproterenol. While sympathetic tone is usually increased by the stimuli that incite a vasodepressor reaction, it is not known if the increased sympathetic tone is an essential or passive component of the mechanism that triggers the reaction. Given that paradoxic bradycardia is a major manifestation of vasodepressor reactions and allowing for the possible extrapolation between paradoxic bradycardia in rats and vasodepressor reactions, we examined the role of sympathetic tone in the paradoxic bradycardia reaction. Paradoxic bradycardia was induced in rats by inferior vena cava occlusion during an isoproterenol infusion. To examine the role of increased sympathetic tone on this reaction, we studied whether carotid artery perfusion (80 to 100 mmHg) during inferior vena cava occlusion, a maneuver that blunts the rise in sympathetic tone, inhibits paradoxic bradycardia.

METHODS AND RESULTS

The maximum changes in R-R were measured during 60 seconds of inferior vena cava occlusion as follows: (a) in control the heart rate accelerated (delta R-R - 10.2 +/- 2.3 msec, P < 0.001); (b) during an infusion of isoproterenol, paradoxic bradycardia occurred (delta R-R + 140.6 +/- 18.2 msec, P < 0.001), and this was inhibited by common carotid artery perfusion (delta R-R - 6.6 +/- 1.5 msec, P < 0.001); and (c) following carotid sinus denervation and during an infusion of isoproterenol, paradoxic bradycardia was induced without and with carotid artery perfusion (delta R-R + 122.6 +/- 12.0 msec, P < 0.001; delta R-R + 151.8 +/- 12.7 msec, P < 0.001, respectively).

CONCLUSIONS

Since carotid artery perfusion during inferior vena cava occlusion inhibits paradoxic bradycardia only when the carotid sinus is innervated, we conclude that carotid artery perfusion blocks the reaction by increasing carotid sinus afferents, thereby limiting the increased sympathetic tone during inferior vena cava occlusion, and not as a result of cerebral perfusion. Thus, the paradoxic bradycardia resulting from inferior vena cava occlusion requires activation of sympathetic tone as a result of carotid sinus hypotension.

Identification of brainstem neurons responding to hypoxia in fetal and newborn sheep

Hypoxia causes a reversible decrease in the level of respiratory, oculomotor and postural muscle activity in fetal sheep, an effect not seen in newborn lambs. We have used Fos immunohistochemistry to identify neurons which are activated by hypoxia and which may mediate this motor inhibition in the fetus. Pregnant sheep of either 117 or 138 days gestation were made hypoxic by allowing them to breathe 8-9% O2 for 2 h. Compared to age-matched control fetuses, hypoxia caused a significant increase in Fos-immunoreactivity in several medullary nuclei including the nucleus tractus solitarius, lateral reticular nucleus and the rostral ventrolateral medulla and also in the lateral parabrachial nucleus, locus coeruleus and subcoeruleus region in the pons. Hypoxia in newborn lambs, 7-18 days old, resulted in Fos staining in the same medullary and pontine nuclei with the exception of the subcoeruleus region which was devoid of Fos-immunoreactivity. In newborn lambs in which the carotid sinus nerves had been sectioned bilaterally, Fos-immunoreactivity was increased in the nucleus tractus solitarius in the medulla and in the locus coeruleus, lateral parabrachial and Kölliker-Fuse nuclei in the pons when compared to intact control newborn lambs. When carotid sinus nerve denervated-lambs were subjected to hypoxia the pattern of Fos-ir was similar to the pattern seen in the denervated control lambs but in addition staining was present in the subcoeruleus. These results suggest that a specific set of pontine neurons are activated by low oxygen levels in the fetus but not in the newborn lamb in the presence of an intact innervation from the carotid sinus. We hypothesise that: (a) in the fetus hypoxia activates neurons in the region of the subcoeruleus and this causes cessation of breathing movements and muscle atonia; and (b) that after birth stimulation of the carotid chemoreceptors by hypoxia normally inhibits activation of these subcoeruleus neurons.