Quantitative studies of the vasculature of the carotid body in the chronically hypoxic rat. (25/641)

The carotid bodies of rats made chronically hypoxic by breathing 12% O2 in a normobaric chamber (inspired PO2 91 mmHg) were compared with those of controls. Serial 5-microm sections of the organs were examined using an interactive image analysis system. The total volume of the carotid bodies was increased by 64%. The total vascular volume rose by 103% and was likely due to an increase in size of the large vessels (>12 microm lumen diameter) because the small vessel (5-12 microm lumen diameter) volume did not increase significantly while the small vessel density tended to decrease. The extravascular volume was increased by 57%. Expressed as a percentage of the total volume of the organ, the total vascular volume did not change, but the small vessel volume was significantly decreased from 7.83 to 6.06%. The large vessel volume must therefore have been increased. The proportion occupied by the extravascular volume was virtually unchanged (84 vs 82%). In accordance with these findings, the small vessel endothelial surface area per unit carotid body volume was diminished from 95.2 to 76.5 mm-1, while the extravascular area per small vessel was increased from 493 to 641 microm(2) or by 30%. In conclusion, the enlargement of the carotid body in chronic hypoxia is most likely due to an increase in total vascular volume, mainly involving the "large" vessels, and to an increase in extravascular volume. This is in contrast to our previously published findings indicating that in the spontaneous insulin-dependent diabetic rat the enlargement of the carotid body is due solely to an increase in extravascular volume.  (+info)

Single-unit recordings of arterial chemoreceptors from mouse petrosal ganglia in vitro. (26/641)

A preparation was developed that allows for the recording of single-unit chemoreceptor activity from mouse carotid body in vitro. An anesthetized mouse was decapitated, and each carotid body was harvested, along with the sinus nerve, glossopharyngeal nerve, and petrosal ganglia. After exposure to collagenase/trypsin, the cleaned complex was transferred to a recording chamber where it was superfused with oxygenated saline. The ganglia was searched for evoked or spontaneous unit activity by using a glass suction electrode. Single-unit action potentials were 57 +/- 10 (SE) (n = 16) standard deviations above the recording noise, and spontaneous spikes were generated as a random process. Decreasing superfusate PO(2) to near 20 Torr caused an increase in spiking activity from 1. 3 +/- 0.4 to 14.1 +/- 1.9 Hz (n = 16). The use of mice for chemoreceptor studies may be advantageous because targeted gene deletions are well developed in the mouse model and may be useful in addressing unresolved questions regarding the mechanism of chemotransduction.  (+info)

Blunted respiratory responses to hypoxia in mutant mice deficient in nitric oxide synthase-3. (27/641)

In the present study, the role of nitric oxide (NO) generated by endothelial nitric oxide synthase (NOS-3) in the control of respiration during hypoxia and hypercapnia was assessed using mutant mice deficient in NOS-3. Experiments were performed on awake and anesthetized mutant and wild-type (WT) control mice. Respiratory responses to 100, 21, and 12% O(2) and 3 and 5% CO(2)-balance O(2) were analyzed. In awake animals, respiration was monitored by body plethysmography along with O(2) consumption (VO(2)) and CO(2) production (VCO(2)). In anesthetized, spontaneously breathing mice, integrated efferent phrenic nerve activity was monitored as an index of neural respiration along with arterial blood pressure and blood gases. Under both experimental conditions, WT mice responded with greater increases in respiration during 12% O(2) than mutant mice. Respiratory responses to hyperoxic hypercapnia were comparable between both groups of mice. Arterial blood gases, changes in blood pressure, VO(2), and VCO(2) during hypoxia were comparable between both groups of mice. Respiratory responses to cyanide and brief hyperoxia were attenuated in mutant compared with WT mice, indicating reduced peripheral chemoreceptor sensitivity. cGMP levels in the brain stem during 12% O(2), taken as an index of NO production, were greater in mutant compared with WT mice. These observations demonstrate that NOS-3 mutant mice exhibit selective blunting of the respiratory responses to hypoxia but not to hypercapnia, which in part is due to reduced peripheral chemosensitivity. These results support the idea that NO generated by NOS-3 is an important physiological modulator of respiration during hypoxia.  (+info)

Cellular mechanisms involved in carotid body inhibition produced by atrial natriuretic peptide. (28/641)

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.  (+info)

Cardiovascular responses to carotid chemoreceptor stimulation in the dog: their modulation by urinary bladder distension. (29/641)

Respiratory, heart rate and hindlimb vascular responses were studied in response to increasing levels of stimulation of the carotid body chemoreceptors, together with an examination of the modulation of their effects by distension of the urinary bladder in the dog anaesthetized with a mixture of chloralose and urethane. The vascularly isolated carotid bifurcation regions were perfused with blood, stimulation of the carotid bodies being carried out by three different levels of hypoxic isocapnic blood (PO2 approximately 58, 40 and 22 mmHg) obtained from a donor animal. A vascularly isolated hindlimb was autoperfused at constant blood flow through its femoral artery. In spontaneously breathing animals, increasingly intense hypoxic stimulation of the carotid bodies caused a progressive augmentation of respiratory minute volume. Superimposition of distension of the bladder increased ventilation further, by the same amount during hypoxic as during normoxic blood perfusion of the chemoreceptors. Prevention of the effects of lung stretch afferent stimulation by artificial ventilation modified the heart rate and hindlimb vascular responses to excitation of the carotid bodies by revealing or accentuating the primary cardiovascular responses, bradycardia and vasoconstriction. In contrast, no such respiratory modulation was apparent in the cardiovascular responses to bladder distension. When, under conditions of artificial ventilation and in the absence of changes in the arterial baroreceptor input, the primary cardio-inhibitory and vasoconstrictor responses to carotid chemoreceptor stimulation predominated, the heart slowed progressively as the stimulus was increased. At the same time the cardio-accelerator effects of bladder distension progressively diminished, indicating an interaction between the cardiac reflex responses evoked by the two inputs. In contrast, the reflex vascular responses resulting from stimulation of the two inputs were additive, at least for PO2 levels of carotid body perfusate down to approximately 40 mmHg. In conclusion these experiments demonstrate the differential nature of the integration of respiratory and cardiovascular responses evoked by stimulation of the carotid chemoreceptors and bladder distension.  (+info)

Cholinergic dopamine release from the in vitro rabbit carotid body. (30/641)

The aim of this study was to test whether cholinergic mechanisms regulate dopamine (DA) release from the carotid body (CB) and interact with DA D(2) autoreceptors. One hundred forty-two CBs from adult rabbits were infused in vitro in a surviving medium bubbled with O(2) (Bairam A, Marchal F, Cottet-Emard JM, Basson H, Pequignot JM, Hascoet JM, and Lahiri S. J Appl Physiol 80: 20-24, 1996). CB DA content and release were measured after 1 h of exposure to various treatments: control, cholinergic agonist (0.1-50 microM carbachol), full muscarinic antagonist (1 and 10 microM atropine), antagonists of M(1) and M(2) muscarinic receptors (1 and 10 microM pirenzepine and 10 microM AFDX-116, respectively), and the DA D(2) receptor antagonist domperidone (1 microM), alone and with carbachol (1 microM). Compared with control, the release of DA was significantly increased by carbachol (1-50 microM), AFDX-116, and domperidone and decreased by atropine (10 microM) and pirenzepine (10 microM). The effects of domperidone and carbachol were not significantly different but were clearly additive. It is concluded that, in the rabbit CB, M(1) and M(2) muscarinic receptor subtypes may be involved in the control of DA release, in addition to the DA D(2) autoreceptors.  (+info)

Ventilatory responses to specific CNS hypoxia in sleeping dogs. (31/641)

Our study was concerned with the effect of brain hypoxia on cardiorespiratory control in the sleeping dog. Eleven unanesthetized dogs were studied; seven were prepared for vascular isolation and extracorporeal perfusion of the carotid body to assess the effects of systemic [and, therefore, central nervous system (CNS)] hypoxia (arterial PO(2) = 52, 45, and 38 Torr) in the presence of a normocapnic, normoxic, and normohydric carotid body during non-rapid eye movement sleep. A lack of ventilatory response to systemic boluses of sodium cyanide during carotid body perfusion demonstrated isolation of the perfused carotid body and lack of other significant peripheral chemosensitivity. Four additional dogs were carotid body denervated and exposed to whole body hypoxia for comparison. In the sleeping dog with an intact and perfused carotid body exposed to specific CNS hypoxia, we found the following. 1) CNS hypoxia for 5-25 min resulted in modest but significant hyperventilation and hypocapnia (minute ventilation increased 29 +/- 7% at arterial PO(2) = 38 Torr); carotid body-denervated dogs showed no ventilatory response to hypoxia. 2) The hyperventilation was caused by increased breathing frequency. 3) The hyperventilatory response developed rapidly (<30 s). 4) Most dogs maintained hyperventilation for up to 25 min of hypoxic exposure. 5) There were no significant changes in blood pressure or heart rate. We conclude that specific CNS hypoxia, in the presence of an intact carotid body maintained normoxic and normocapnic, does not depress and usually stimulates breathing during non-rapid eye movement sleep. The rapidity of the response suggests a chemoreflex meditated by hypoxia-sensitive respiratory-related neurons in the CNS.  (+info)

Co-release of ATP and ACh mediates hypoxic signalling at rat carotid body chemoreceptors. (32/641)

Using functional co-cultures of rat carotid body (CB) O2 chemoreceptors and 'juxtaposed' petrosal neurones (JPNs), we tested whether ATP and ACh acted as co-transmitters. Perforated-patch recordings from JPNs often revealed spontaneous and hypoxia-evoked (PO2 approximately 5 mmHg) excitatory postsynaptic responses. The P2X purinoceptor blocker, suramin (50 microM) or a nicotinic ACh receptor (nAChR) blocker (hexamethonium, 100 microM; mecamylamine, 1 microM) only partially inhibited these responses, but together, blocked almost all activity. Under voltage clamp (-60 mV), fast perfusion of 100 microM ATP over hypoxia-responsive JPNs induced suramin-sensitive (IC50 = 73 microM), slowly-desensitizing, inward currents (IATP) with time constant of activation tauon = 30.6 +/- 4. 8 ms (n = 7). IATP reversed at 0.33 +/- 3.7 mV (n = 4), and the dose-response curve was fitted by the Hill equation (EC50 = 2.7 microM; Hill coefficient approximately 0.9). These purinoceptors contained immunoreactive P2X2 subunits, but their activation by alpha,beta-methylene ATP (alpha,beta-meATP; EC50 = 2.1 microM) suggests they are P2X2/P2X3 heteromultimers. Suramin and nAChR blockers inhibited the extracellular chemosensory discharge in the intact rat carotid body-sinus nerve preparation in vitro. Further, P2X2 immunoreactivity was widespread in rat petrosal ganglia in situ, and co-localized in neurones expressing the CB chemo-afferent marker, tyrosine hydroxylase (TH). P2X2 labelling in the CB co-localized with nerve-terminal markers, and was intimately associated with TH-positive type 1 cells. Thus ATP and ACh are co-transmitters during chemotransduction in the rat carotid body.  (+info)