Chemotransduction in the carotid body: K+ current modulated by PO2 in type I chemoreceptor cells

Effects of doxapram on ionic currents recorded in isolated type I cells of the neonatal rat carotid body

Whole-cell patch-clamp recordings were used to investigate the effects of the respiratory stimulant doxapram on K+ and Ca2+ currents in isolated type I cells of the neonatal rat carotid body. Doxapram (1-100 microM) caused rapid, reversible and dose-dependent inhibitions of K+ currents recorded in type I cells (IC50 approximately 13 microM). Inhibition was voltage-dependent, in that the effects of doxapram were maximal at test potentials where a shoulder in the current-voltage relationship was maximal. These K+ currents were composed of both Ca(2+)-activated and Ca(2+)-independent components. Using high [Mg2+], low [Ca2+] solutions to inhibit Ca(2+)-activated K+ currents, doxapram was also seen to directly inhibit Ca(2+)-independent K+ currents. This effect was voltage-independent and was less potent (IC50 approximately 20 microM) than under control conditions, suggesting that doxapram was a more potent inhibitor of the Ca(2+)-activated K+ currents recorded under control conditions. Doxapram (10 microM) was without effect on L-type Ca2+ channel currents recorded under conditions where K+ channel activity was minimized and was also without significant effect on K+ currents recorded in the neuronal cell line NG-108 15, suggesting a selective effect on carotid body type I cells. The effects of doxapram on type I cells show similarities to those of the physiological stimuli of the carotid body, suggesting that doxapram may share a similar mechanism of action in stimulating the intact organ.

Cyclic AMP modulates differentially the release of dopamine induced by hypoxia and other stimuli and increases dopamine synthesis in the rabbit carotid body

We have investigated the effects of different treatments that increase cyclic AMP levels on the in vitro synthesis and release of catecholamines in the rabbit carotid body. We also measured the rate of 45Ca2+ efflux from previously loaded carotid bodies under different conditions. Forskolin produced a dose-dependent increase in the release of [3H]dopamine elicited by a hypoxic stimulus of medium intensity (PO2 = 33 mm Hg) without altering basal [3H]dopamine release (100% O2-equilibrated medium). At a concentration of 5 x 10(-6) M, forskolin increased the release of [3H]dopamine induced by hypoxic stimuli of different intensities; the increase was maximal (498%) at the lowest intensity of hypoxic stimuli (PO2 = 66 mm Hg), averaged 260% for hypoxic stimuli of intermediate intensity and 2 x 10(-4) M cyanide, and was 150% under anoxia. Dibutyryl cyclic AMP (2 mM) and 3-isobutyl-1-methylxanthine (0.5 mM) mimicked forskolin effects under hypoxic stimulation. Forskolin (5 x 10(-6) M) also increased (180%) the release of [3H]dopamine induced by 20% CO2/pH 6.6, 2.5 x 10(-4) M dinitrophenol, and 3 x 10(-5) M ionomycin. Forskolin and 3-isobutyl-1-methylxanthine were without effect on the release of [3H]dopamine elicited by 30 mM extracellular K+. Forskolin (5 x 10(-6) M) augmented significantly the rate of 45Ca2+ efflux induced by hypoxic stimuli (PO2 of 33 and 66 mm Hg) and 2 x 10(-4) M cyanide and showed a tendency to increase (20%) the 45Ca2+ efflux induced by dinitrophenol and low pH and to decrease (21%) the efflux induced by 30 mM K+ without altering the rate of efflux under basal conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Contrasting effects of HEPES vs HCO3(-)-buffered media on whole-cell currents in cultured chemoreceptors of the rat carotid body

In this study we compared the effects of physiological bicarbonate/CO2-buffered media (BBM) with the commonly used N-2-hydroxyethylpiperazine-N'-2-ethane-sulfonic acid (HEPES)-buffered media (HBM) on whole-cell currents in cultured rat arterial chemoreceptors (i.e. glomus cells) using the perforated-patch technique. Two separate effects were observed on switching from HBM to BBM. First, in the majority of cells tested (31 of 36) there was an increase in the leakage conductance (ca. 5 fold) and a concomitant increase in channel noise, which in preliminary studies appears to arise from the opening of large-conductance anion channels. Second, there was a reversible decrease in voltage-activated outward K+ current which we attribute to cytoplasmic acidification, catalysed by carbonic anhydrase in glomus cells.

Hypoxia increases the cyclic AMP content of the cat carotid body in vitro

The cyclic AMP content of cat carotid bodies in vitro measured with a radioimmunoassay under control conditions (PO2: 230 torr) was 0.79 +/- 0.10 pmol/carotid body (n = 10). Lowering medium PO2 to 20 torr for 2 min significantly increased cyclic AMP content to 1.13 +/- 0.14 pmol/carotid body (n = 10). This increase was inhibited neither by propranolol (34 microM) nor by propranolol plus haloperidol (27 microM). Inhibition of the cyclic nucleotide phosphodiesterase with 1-methyl-3-isobutylxanthine (0.8 mM) provoked a fast and large increase in cyclic AMP during both control and hypoxic conditions. The cyclic AMP increase induced by hypoxia was still observed when extracellular Ca2+ was absent. Inhibition of the adenylate cyclase by N-(cis-2-phenylcyclopentyl)azacyclotridecan-2-imine hydrochloride (MDL 12330A; 20-1,000 microM) under zero-Ca2+ conditions irreversibly inhibited the cyclic AMP increase produced by hypoxia. Similarly, inhibition of the Ca2(+)-calmodulin complex by trifluoperazine (0.2 mM) or calmidazolium (R 24571; 50-200 microM) prevented the cyclic AMP response. These results suggest that cyclic AMP may be involved in the PO2-sensing mechanism of the carotid body. Hypoxia appears to activate adenylate cyclase directly and independent of any hormone-receptor interactions.

Response of cytosolic calcium to anoxia and cyanide in cultured glomus cells of newborn rabbit carotid body

Microscopic fluorometry was used to examine the effects of anoxia and cyanide (CN-) on cytosolic calcium [Ca2+]i of cultured carotid body (CB) glomus cells from newborn rabbits. Applications of high K+ and veratridine (VRT), a sodium channel activator, induced rapid and marked increases in [Ca2+]i. These effects were inhibited by D600 a calcium channel blocker. [Ca2+]i changes induced by VRT were also blocked by tetrodotoxin (TTX). Glomus cells exhibited a slow increase in [Ca2+]i in response to anoxia and CN-, and a slight decrease during hyperoxia. The effects of anoxia and CN- were blocked by D600 but not by TTX. We conclude that these stimuli induce calcium entry into glomus cells via voltage-dependent Ca2+ channels. Voltage-dependent Na+ channels were not involved.

Inhibition of Ca(2+)-activated K+ currents by intracellular acidosis in isolated type I cells of the neonatal rat carotid body

1. K+ and Ca2+ currents were recorded from enzymatically isolated type I cells of the neonatal rat carotid body, using the whole-cell configuration of the patch-clamp technique. The effects of intracellular acidosis, caused by bath application of anions of weak acids (propionate and acetate), were tested on these currents. 2. Bath application of propionate or acetate (10 or 20 mM) caused reversible reductions in K+ current amplitudes. These effects were maximal at low, positive test potentials where a shoulder in the current-voltage relationship occurs due to the activation of Ca(2+)-activated K+ currents. 3. Time-course studies showed propionate to cause a rapid initial reduction of K+ currents which recovered partially during its continued application. Removal of propionate produced small, transient overshoots of K+ current amplitudes. In the absence of propionate or acetate, bath application of the Na(+)-H+ exchange inhibitor amiloride caused slowly developing inhibition of K+ current amplitudes. 4. Changing extracellular pH from 7.4 to 8.0 increased K+ current amplitudes, but at this pHo propionate caused smaller reductions in K+ currents than at a pHo of 7.4. 5. In the presence of 0.1 mM-Cd2+, or in high-Mg2+ (6 mM), low-Ca2+ (0.1 mM) solutions, the residual, Ca(2+)-independent K+ currents were unaffected by 20 mM-propionate or acetate. 6. Ca2+ channel currents were also recorded, using 10 mM-Ba2+ as the charge carrier. These sustained currents were completely abolished by 0.1 mM-Cd2+ and were enlarged in the presence of 5 microM-Bay K 8644, suggesting that the currents passed through L-type Ca2+ channels. 7. Ca2+ channel currents were not significantly affected by intracellular acidosis caused by bath application of 10 mM-propionate or acetate. They were also unaffected by a reduction of the extracellular pH from 7.4 to 7.0. 8. It is concluded that intracellular acidosis selectively inhibits Ca(2+)-activated K+ currents in type I carotid body cells. The possible significance of this effect on chemotransduction in the intact carotid body is discussed.

The presence of CO2/HCO3- is essential for hypoxic chemotransduction in the in vivo perfused carotid body

Carotid chemoreceptor activity was increased by the perfusion of the carotid body in vivo with hypoxic HEPES-buffered solution (HBS) containing CO2/HCO3- (HBA+), but not with hypoxic HBS without CO2/HCO3- (HBS-). When the perfusate was switched to hypoxic HBS+ during hypoxic HBS-perfusions, chemoreceptor activity increased immediately. Thus, CO2/HCO3- played a critical role in the hypoxic chemotransduction of the in vivo perfused carotid body.

Single K+ channels in membrane patches of arterial chemoreceptor cells are modulated by O2 tension

Type I cells of the carotid body are known to participate in the detection of O2 tension in arterial blood but the primary chemotransduction mechanisms are not well understood. Here we report the existence in excised membrane patches of type I cells of a single K+ channel type modulated by changes in PO2. Open probability of the O2-sensitive K+ channel reversibly decreased by at least 50% on exposure to hypoxia but single-channel conductance (approximately 20 pS) was unaltered. In the range between 70 and 150 mmHg (1 mmHg = 133 Pa) the decrease of single-channel open probability was proportional to the PO2 measured in the vicinity of the membrane patch. The inhibition of K+ channel activity by low PO2 was independent of the presence of non-hydrolyzable guanine triphosphate analogues at the internal face of the membrane. The results indicate that the O2 sensor of type I cells is in the plasma membrane and suggest that environmental O2 interacts directly with the K+ channels.

Whole-cell and perforated-patch recordings from O2-sensitive rat carotid body cells grown in short- and long-term culture

We are investigating transduction mechanisms in a major peripheral chemosensory organ, the rat carotid body, using short- and long-term dissociated cell cultures and patch-clamp, whole-cell recording. In this study membrane properties of cultured glomus or type I cells were characterized with conventional whole-cell recording and the new perforated-patch technique during control (160 Torr) and low-PO2 (20 Torr) conditions. These cells contained voltage-gated channels typical of electrically excitable cells and had large input resistances (approx. 2 G omega). Under whole-cell voltage clamp the cells produced brief inactivating inward currents, which were largely abolished by 0.2-2.0 microM tetrodotoxin, followed by prolonged outward currents, which were reduced by 5 mM tetraethylammonium or abolished by the substitution of Cs+ ions for K+ ions in the pipette. On exposure to hypoxia the outward K+ current was reduced typically by 15%-20% with both conventional whole-cell and perforated-patch recording, which minimizes washout of the cell's cytoplasm. This effect persisted in long-term culture and was specific, since the inward current was unaffected and, moreover, it did not occur in cultured small intensely fluorescent cells, which are closely related to glomus cells. These properties of cultured rat glomus cells are contrasted with those recently reported for freshly isolated rabbit glomus cells.

Release of dopamine and chemoreceptor discharge induced by low pH and high PCO2 stimulation of the cat carotid body

1. Cat carotid bodies were incubated with the precursor [3H]tyrosine to label the catecholamine deposits and then mounted in a superfusion chamber which allowed simultaneous collection of the released [3H]dopamine (DA) and recording of action potentials from the carotid sinus nerve. 2. Low pH (7.2-6.6) superfusion of the carotid bodies for periods of 10 min produced a parallel increase in the release of [3H]DA and chemoreceptor discharge. 3. Carotid sinus nerve denervation of the carotid body 12-15 days prior to the experiments did not modify the release of [3H]DA elicited by low pH. 4. Superfusion of the carotid bodies with Ca(2+)-free, high-Mg2+ (1.6 mM) media reduced basal release of [3H]DA and chemoreceptor discharge by about 30%. Release evoked by low pH was reduced by 82%. Peak and average chemoreceptor discharge recorded in response to low pH were reduced by 28%. 5. Solutions containing weak acids (sodium acetate, 10 mM), adjusted at pH 7.4, elicited release of [3H]DA and increased chemoreceptor discharge. 6. With HCO3-CO2-buffered superfusion media, a reduction of bicarbonate to 5.6 mM (pH 6.8), an increase in CO2 to 20% (pH 6.8), or a simultaneous increase in CO2 to 20% and bicarbonate to 90 mM (pH 7.4), resulted in all cases in a corresponding increase in [3H]DA release and chemoreceptor discharge. The most effective stimulus was 20% CO2-pH 6.8 and the least effective 5% CO2-5.6 mM-HCO3-pH 6.8. 7. Inhibition of carbonic anhydrase with acetazolamide while perfusing the carotid bodies with a 20% CO2-equilibrated (pH 7.4) solution resulted in comparable reductions in the release of [3H]DA and chemoreceptor discharge. 8. It is concluded that the effective acidic stimulus at the carotid body chemoreceptors is an increase in hydrogen ion concentration in type I cells. It is also concluded that DA plays a critical role in the genesis of carotid sinus nerve discharges.

The role of cyclic AMP in chemoreception in the rabbit carotid body

The present study identified physiological factors which influence the generation (and degradation) of cyclic AMP (cAMP) in the arterial chemoreceptor tissue of the mammalian carotid body. Experiments established a 3-way correlation between cAMP generation, neurotransmitter release from chemoreceptor cells, and carotid sinus nerve (CSN) activity. Incubation of carotid bodies in vitro for 10 min in media equilibrated with different low O2 ('hypoxic') gas mixtures (5% O2 or 10% O2, balance N2) elevated basal cAMP levels (100% O2 media) in proportion to the stimulus intensity. Similar experiments using nodose sensory ganglia showed that low O2 stimulation did not alter cAMP levels in this non-chemosensory tissue. However, the adenylate cyclase (AC) activator, forskolin (10 microM), evoked large increases in the cyclic nucleotide content in both carotid bodies and nodose ganglia. After chronic (10 days) CSN denervation or sympathectomy, the basal levels of cAMP in the carotid body were elevated; the cAMP response to low O2 media (stimulus minus control) was increased after CSN denervation but remained unaltered after sympathectomy. The effects of zero Ca2+ media on cAMP generation was examined in order to assess whether feedback from released neurotransmitters acting on known (presynaptic) type I cell receptors could have contributed to the observed changes in cAMP. Basal levels of cAMP were increased 2.8-fold, and the response to hypoxic stimulation was elevated 5-fold, in the absence of extracellular Ca2+. Forskolin (10 microM) did not alter basal release of [3H]-catecholamines ([3H]CA; synthesized from [3H]tyrosine), or resting CSN discharge; however, stimulus-evoked [3H]CA release and CSN discharge were potentiated in the presence of forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)

Ionic mechanisms for the transduction of acidic stimuli in rabbit carotid body glomus cells

1. The release of [3H]dopamine (DA) in response to inhibition of the Na+ pump or to intracellular acid load was studied in rabbit carotid bodies (CB) previously incubated with the precursor [3H]tyrosine. The ionic requirements of the release response and the involvement of specific ion transport systems were investigated. 2. Inhibition of the Na+ pump, by incubating the CB with ouabain or in K(+)-free medium, evokes a DA release response which requires the presence of Na+ and Ca2+ in the medium and is insensitive to nisoldipine. This suggests that the response is triggered by entry of external Ca2+ through Na(+)-Ca2+ exchange, a consequence of the increase in intracellular Na+ resulting from inhibition of the pump. 3. Incubation of the CB in medium equilibrated with 20% CO2 at pH 6.6, or in medium containing the protonophore dinitrophenol (DNP) or the weak acid propionate, elicits a DA release response which requires also the presence of Na+ and Ca2+ in the medium and is insensitive to dihydropyridines. 4. Ethylisopropylamiloride (EIPA), an inhibitor of the Na(+)-H+ exchanger, markedly decreases the release response elicited by DNP or propionate in bicarbonate-free medium, but has not any effect in bicarbonate-buffered medium. In the latter condition, the EIPA-insensitive release of DA is inhibited by reducing the HCO3- concentration in the medium to 2 mM or by removal of Cl-, suggesting that in bicarbonate-buffered medium a Na(+)-dependent HCO3(-)-Cl- exchanger is involved in the release response. 5. It is concluded that the release of DA by the chemoreceptor cells in response to acidic stimulation is triggered by entry of external Ca2+ through Na(+)-Ca2+ exchange. This exchange is promoted by the increase of intracellular Na+ that results from the operation of Na(+)-coupled H(+)-extruding mechanisms activated by the acid load.

Electrophysiological evidence for the reconstitution of chemosensory units in co-cultures of carotid body and nodose ganglion neurons

The electrophysiological characteristics of nodose ganglion sensory neurons, cultured alone or co-cultured with carotid body tissue, were compared. Some properties of the neurons and their response to acid (a carotid body 'natural' stimulus) changed in the presence of this tissue. (a) The evoked action potential after-hyperpolarization was smaller and longer whereas spike amplitude and duration, and the passive membrane properties remained unaltered. (b) Spontaneously occurring action potentials happened more frequently (16% vs 3%). (c) Acid solutions induced appreciable depolarization, an increased discharge, or both, only in a population of co-cultured neurons. These changes probably arose because of synaptic and/or trophic interactions between neurons and glomus cells.

Hypoxic suppression of K+ currents in type I carotid body cells: selective effect on the Ca2(+)-activated K+ current

Whole-cell K+ currents were recorded in isolated type I carotid body cells using the patch-clamp technique. Hypoxia (pO2 25 torr) reversibly suppressed K+ currents in a voltage-dependent manner: maximal effects were seen at low, positive test potentials, where the Ca2(+)-activated component of K+ currents was greatest. Enhancing this component with 5 microM BAY K 8644 exaggerated the effects of hypoxia, and when the component was inhibited (100 microM Cd2+ or 5 microM nifedipine) hypoxic effects were abolished. As hypoxia does not affect Ca2+ currents directly, these data indicate the suppressive effect of hypoxia is selective for the Ca2(+)-activated component of K+ currents in type I cells.

Effects of D600 on hypoxic suppression of K+ currents in isolated type I carotid body cells of the neonatal rat

K+ currents recorded from isolated type I carotid body cells were reversibly suppressed by hypoxia (pO2 25 Torr). Inhibition of Ca2+ influx using high Mg2+, low Ca2+ solutions abolished this effect, indicating hypoxia selectively inhibits the Ca2(+)-activated component of K+ currents. The Ca2+ channel blockers D600 and verapamil reversed the suppressive effects of hypoxia. These drugs also partially inhibited Ca2(+)-independent K+ currents, but this effect was relieved by hypoxia, causing apparent reversal of hypoxic suppression of the K+ currents seen under control conditions.

Effects of different types of stimulation on cyclic AMP content in the rabbit carotid body: functional significance

Cyclic AMP levels in rabbit carotid bodies incubated under control conditions, 100% O2- or 95% O2/5% CO2- equilibrated medium, are close to 1 pmol/mg wet tissue (range 0.4-2.43 pmol/mg). Isobutylmethylxanthine (0.5 mM) increases cyclic AMP levels by a factor of 14 and 8 in HEPES- and CO2/CH3O(-)-buffered medium, respectively. Forskolin (0.5-10 microM) applied during 30 min increases cyclic AMP levels in a dose-dependent manner. Incubation of carotid bodies at low O2 tensions resulted in an elevation of cyclic AMP levels both in the absence and in the presence of isobutymethylxanthine. In the latter conditions cyclic AMP increase was maximum at an O2 tension of 46 mm Hg and tended to decrease at extremely low PO2. In isobutylmethylxanthine-containing Ca2(+)-free medium, cyclic AMP increased linearly with decreasing PO2 from 66 to 13 mm Hg; the absolute cyclic AMP levels attained in Ca2(+)-free medium were smaller than those observed in Ca2(+)-containing medium at any PO2. The differences between Ca2(+)-free and Ca2(+)-containing media appear to be due to the action of released neurotransmitters in the latter conditions, because dopamine and norepinephrine, which are known to be released by hypoxia in a Ca2(+)-dependent manner, increase cyclic AMP in the carotid body. Low pH/high PCO2 and high [K+]e increase cyclic AMP levels only in Ca2(+)-containing medium. Forskolin potentiates the release of catecholamines induced by low PO2. These results suggest that cyclic AMP plays an important role in the modulation of the chemoreception process.

Responses of type I cells dissociated from the rabbit carotid body to hypoxia

1. The carotid body chemoreceptors are stimulated in situ by hypoxia. We have studied type I cells freshly dissociated from the carotid body of the rabbit. We have used microfluorimetric and patch clamp techniques to examine the responses to hypoxia, to anoxia, and to metabolic inhibition. 2. NADH autofluorescence measured at both 400 and 500 nm increased rapidly and reversibly in response to anoxia or to cyanide (CN-), reflecting a change in mitochondrial metabolism. 3. Indo-1 was used to measure changes in intracellular calcium, [Ca2+]i. Anoxia reversibly increased [Ca2+]i from approximately 50-100 to approximately 200-450 nM in all cells tested. The response showed a striking temperature sensitivity. Responses to hypoxic stimuli were barely detectable at 17-20 degrees C, and were dramatically increased on warming to 36 degrees C. In contrast, responses to K(+)-induced depolarization were only slightly increased in rate of onset and recovery by warming. 4. The rise in [Ca2+]i originated largely from an intracellular store which was slowly depleted by exposure to nominally Ca2(+)-free solutions. Responses were unaffected by blockade of Ca2+ channels with organic (D600, verapamil) or inorganic (Co2+) blockers, by blockade of Na+ channels with tetrodotoxin (TTX), or by increasing action potential duration with tetraethylammonium (TEA). Responses to anoxia were increased by the increased [Ca2+]i loading that follows prior exposure to Ca2(+)-free solutions. 5. Responses to anoxia, to blockade of electron transport by CN-, and to the mitochondrial uncoupler, carbonyl cyanide p-trifluoromethoxy-phenylhydrazone (FCCP), were equivalent in amplitude. The response to anoxia was occluded by concurrent application of FCCP, suggesting that the Ca2+ originates from the same pool in each case. 6. At 35-36 degrees C, responses to graded levels of PO2 were also graded. Thresholds varied between cells, but were typically 30-50 mmHg. Stimulus-responses curves were essentially hyperbolic, increasing dramatically as the PO2 approached 0 mmHg. 7. The sensitivity of cells to hypoxic solutions was increased by acidification of the superfusate over the pH range from 7.3 to 6.85. 8. Cell-attached patch clamp recordings showed depression of spontaneous action potentials associated with a rise in [Ca2+]i during exposure to anoxic solutions. Whole-cell recordings showed that anoxia increased a voltage-gated gK as described previously for CN-, while producing no change in resting conductance. 9. These data suggest that the rise in [Ca2+]i originates largely from Ca2+ efflux from a mitochondrial pool.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential stimulus coupling to dopamine and norepinephrine stores in rabbit carotid body type I cells

Recent studies suggest that preneural type I (glomus) cells in the arterial chemoreceptor tissue of the carotid body act as primary transducer elements which respond to natural stimuli (low O2, pH or increased CO2) by releasing chemical transmitter agents capable of exciting the closely apposed afferent nerve terminals. These type I cells contain multiple putative transmitters, but the identity of the natural excitatory agents remains an unresolved problem in carotid body physiology. Characterization of putative transmitter involvement in the response to natural and pharmacological stimuli has therefore become fundamental to further understanding of chemotransmission in this organ. The present study demonstrates that a natural stimulus (hypoxia) evokes the release of dopamine (DA) and norepinephrine (NE) in approximate proportion to their unequal stores in rabbit carotid body (DA release/NE release = 8.2). In contrast, nicotine (100 microM), a cholinomimetic agent thought to act on the nicotinic receptors present on the type I cells, evokes the preferential release of NE (DA release/NE release = 0.17). These findings suggest that distinct mechanisms are involved in a differential mobilization of these two catecholamines from the rabbit carotid body.

Effects of denervation on the glomus cell membrane

Trophic influences of the carotid nerve (CN) on carotid body (CB) glomus cells were studied by comparing the membrane potential (Em), input resistance (Ro) and voltage noise (Erms) of normal and 3-31-day denervated cells had more negative Ems. Higher Ros were recorded at 3 and 6 days. Erms sharply increased at 3 days, returned to normal at 6-15 days and was below normal at 31 days. A transmitter (ACh) and NaCN, producing histotoxic anoxia, were used for stimulation. These substances either depolarized or hyperpolarized innervated cells and increased or decreased voltage noise. Denervation selectively changed these patterns but only for a short time. ACh preferentially depolarized the cells, only at 3 days, whereas its effects on noise did not change. The Em responses to NaCN remained unaltered although at 3-6 days noise increases were smaller and depressions exaggerated. Possible reasons for these effects are discussed.

Potassium currents recorded in type I carotid body cells from the neonatal rat and their modulation by chemoexcitatory agents

Whole cell patch clamp recordings were made from type I cells of the neonatal rat carotid body, isolated and maintained in primary culture for up to 48 h. Depolarizing voltage steps applied from a holding potential of -70 mV evoked outward currents positive to approximately -30 mV. Currents were strongly blocked by extracellular tetraethylammonium (25 mM), and were therefore attributed to activation of voltage-dependent K+ channels. Currents were also suppressed by 4-aminopyridine, removal of extracellular Ca2+, and replacement of extracellular Ca2+ with Ba2+. These results suggest there are Ca2(+)-dependent and Ca2(+)-independent components of the K+ currents. No evidence was found to suggest that ATP-sensitive K+ channels were present. The effects of 3 chemoexcitatory agents (NaCN, almitrine and reduced extracellular pH) on K+ currents in isolated type I cells were investigated. All three agents suppressed K+ currents to similar degrees. The effects of lowered pH and NaCN were reversible, and NaCN-induced reductions occurred regardless of the presence of intracellular ATP. The effect of almitrine was irreversible for up to 30 min of recording. It is concluded that the reduction of K+ currents by chemoexcitants may play a role in the mechanism of chemotransduction in the carotid body.

Neurons sensitive to pH in slices of the rat ventral medulla oblongata

The effects of extracellular pH changes on neurons in slices of the rat ventral medulla oblongata were investigated by extracellular recording. Changes in discharge rate were correlated with pH changes in the tissue next to the recorded cell, as measured by H(+)-selective microelectrodes. pH was altered by varying the bicarbonate concentration ([HCO3-]) in the superfusion solution. In 136 out of 316 neurons, the number of spontaneous or electrically evoked discharges per unit time increased with decreasing pH and decreased with increasing pH. Changes of only 0.01-0.04 pH unit were effective in these pH-sensitive neurons. The response was transient; the discharge rate returned to the control value within a few minutes. The pH sensitivity persisted in the presence of 0.5 microM atropine, 20 microM bicuculline and after replacing Ca2+ by Mg2+ in the superfusion solution to reduce synaptic transmission. The response to the same pH decrease was stronger when increasing PCO2 than when reducing [HCO3-]0. The pH-induced response significantly increased during hypoxia. The results show that in the ventral medulla oblongata neurons exist that transiently respond to small decreases and increases of pH. The pH sensitivity is an intrinsic property of these neurons; it is not due to a synaptic mechanism but is modulated by PCO2 and PO2.

Effect of lowered extracellular pH on Ca2(+)-dependent K+ currents in type I cells from the neonatal rat carotid body

1. The whole-cell configuration of the patch-clamp technique was used to record K+ currents from type I cells enzymatically dispersed from the neonatal rat carotid body. The current-voltage (I-V) relationship for the K+ currents showed a prominent, outward shoulder at test potentials of between +10 and +30 mV. 2. The shoulder of the I-V curve could be enhanced by raising extracellular Ca2+ concentration or by bath application of 5 microM-Bay K 8644. It could also be suppressed by bath application of 100 microM-Cd2+ or 5 microM-methoxyverapamil (D600), indicating that a large component of the K+ current in these cells was activated by an influx of Ca2+ through its own channels during cell depolarization. 3. Potassium currents were also reversibly suppressed by 8 nM-charybdotoxin but unaffected by 100 nM-apamin, suggesting that the Ca2(+)-dependent K+ current was carried through large or intermediate conductance Ca2(+)-activated K+ channels. 4. Lowering the pH of the bathing medium from 7.40 to 7.00 reversibly reduced the K+ current amplitudes, and suppressed the shoulder normally seen in the I-V relationship. This effect was enhanced in the presence of 5 microM-Bay K 8644 and abolished in the presence of 5 microM-D600. 5. It is concluded that the Ca2(+)-dependent K+ channels of type I carotid body cells are selectively suppressed by extracellular acidity. Possible mechanisms underlying this effect, and its role in excitation of the carotid body are discussed.

Effects of Putative Neurotransmitters of the Carotid Body on its Own Glomus Cells

Carotid body glomus cells produce and release acetylcholine (ACh), catecholamines, and neuropeptides, and there is biochemical evidence that these cells possess receptors for these substances. Thus, we studied the effects of cholinergics [ACh, nicotine (Nic), bethanechol (BN)] and peptides [met-enkephalin (ME), substance P (SP)] on the membrane potential (Em), voltage noise (Erms), and input resistance (Ro) of glomus cells. Sliced carotid bodies (for cell visualization) of cats, rabbits, and mice were used. The mean Em and Ro of rabbit glomus cells were lower than those of cat and mouse. Ro of mouse cells was the largest, whereas Erms was similar in all species. The various agents had qualitatively similar effects on the cells of the three species although some quantitative differences were sometimes observed. But, for simplicity, results were pooled. ACh depolarized most cells (effect depressed by zero [Ca2+]o and Mn2+), reduced their resistance, and induced variable changes in Erms. Different ACh doses produced non-linear effects on DeltaEm. Nic and BN also depolarized most cells, reducing Ro and Erms. Atropine depressed the cell responses to BN; alpha-bungarotoxin the depolarizing response to Nic. ME and SP depolarized most cells, but only ME significantly reduced Ro. Neither peptide significantly changed voltage noise. Comparing the effects of all drugs showed that BN was the most effective depolarizing agent, producing the largest reductions in Ro. There were negative correlations between DeltaEm and DeltaRo with the cholinergics and SP; correlations between DeltaErms and DeltaRo were significant and positive only with the cholinergics. These results confirm the presence of nicotinic, muscarinic, and peptidergic receptors in glomus cells. The similar effects of cholinergics and peptides and those of flow interruption and anoxia suggest that the latter may partly act via autoreceptors for the released transmitters.

Measurements of intracellular Ca2+ in dissociated type I cells of the rabbit carotid body

1. The carotid body chemoreceptors are stimulated in situ by cyanide (CN-), which mimics the effect of hypoxia. We have shown that CN- increases a calcium-dependent potassium conductance (gK(Ca)) in single type I cells dissociated from the carotid body of the rabbit. We have now used the Ca2(+)-sensitive fluorophore, Fura-2, to measure intracellular Ca2+ directly in single type I cells. 2. CN- reversibly increased [Ca2+]i from approximately 90 nM to a mean of approximately 200 nM. Some of this Ca2+ originated from an intracellular store, which was depleted by exposure to Ca2(+)-free solutions. Prolonged application of CN- caused a sustained increase in [Ca2+]i, suggesting that CN- impairs the removal or sequestration of Ca2+. 3. pHi measured with the dye BCECF (2,7-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein) did not change consistently in response to CN-, although pHi changed predictably in response to both ammonium chloride and to acidification of the superfusate with CO2. 4. Potassium-induced depolarization (35 mM-K+) caused a large, cadmium-sensitive rise in [Ca2+]i. The K(+)-induced Ca2+ load was used to study the regulation of [Ca2+]i. 5. The clearance of a Ca2+ load was slowed either by removal of [Na+]o or by application of CN-. This shows that both a Na+-Ca2+ exchange and an energy-dependent process or processes contribute to the regulation of [Ca2+]i. 6. Carbachol (CCh, 10-100 microM), which also hyperpolarizes type I cells, caused a small transient rise in [Ca2+]i, indicating release from an exhaustible intracellular pool. The response to CN- was unaffected by prior or continued exposure to CCh, suggesting that the two stimuli operate by distinct mechanisms. 7. The increased gK(Ca) seen in type I cells in response to CN- thus reflects a change in cellular Ca2+ homeostasis. The rise in [Ca2+]i presumably underlies the documented increase in transmitter release from the carotid body in response to CN-. If chemotransduction is a consequence of the release of transmitters from the type I cell, the response of the carotid body to CN-, and possibly also to hypoxia, is thus a direct consequence of the energy dependence of Ca2+ homeostasis in the type I cell.

Evidence for a PO2-sensitive K+ channel in the type-I cell of the rabbit carotid body

Type-I cells of rabbit carotid bodies were studied with the patch-clamp technique in the whole-cell and on the cell-attached configuration. Cells exhibiting resting potentials of about -40 mV under normoxic conditions (PO2: 20 kPa), depolarized during hypoxia (PO2: 3.7 kPa). Hypoxia did not affect inward Ca2+ currents but inactivated outward K+ currents in voltage-clamp experiments. Single-channel currents recorded for the cell-attached mode showed a slope conductance of about 137 pS and a 0 mV reversal potential under symmetrical K+ concentration (140 mM). The open-probability (PO) of the single channel was dependent on the extracellular PO2. These data demonstrate the existence of a PO2-sensitive K+ channel in type-I cells, which may account for cell depolarization and the resulting chemosensory response.

Electrophysiological responses of dissociated type I cells of the rabbit carotid body to cyanide

1. The carotid body is the major peripheral sensor of arterial PO2 in the mammal and is excited by cyanide (CN-). Type I cells, the presumed sites for transduction, were freshly dissociated from the carotid body of the adult rabbit and studied with the whole-cell patch clamp technique. 2. Type I cells were hyperpolarized by CN-, the action potential was shortened, and there was an increased after-hyperpolarization. 3. Under voltage clamp control, CN- increased a voltage-dependent outward current, which showed pronounced outward rectification. Tail currents increased by CN- reversed close to the predicted EK, the reversal potential of the CN--induced current depended on extracellular [K+], and the current was blocked by intracellular TEA+ and Cs+. 4. The i-V relation of the CN--induced conductance strongly mirrored that of voltage-gated Ca2+ entry, and the response was abolished by removal of extracellular Ca2+. We conclude that the increased gK is Ca2+ -dependent (gK(Ca]. 5. The Ca2+ current was attenuated by CN-, and showed an increased rate of inactivation. Thus, the increased gK(Ca) must result from an alteration in Ca2+ homeostasis independent of the Ca2+ current, and not an increased Ca2+ entry through voltage-activated channels. 6. Carbachol also hyperpolarized cells and increased a K+ conductance. 7. At depolarized holding potentials a steady-state outward current was increased by CN-. The current reversed close to EK, and was associated with increased current fluctuations. Noise analysis showed that a channel conductance of 3 pS carries the current. 8. The response to CN- was not impaired by the inclusion of 5 mM-MgATP in the patch pipette. 9. If signals to the CNS are initiated by the calcium-dependent release of transmitters from type I cells, transduction would appear to be the direct consequence of the energy dependence of Ca2+ homeostasis.

Ionic currents in dispersed chemoreceptor cells of the mammalian carotid body

Ionic currents of enzymatically dispersed type I and type II cells of the carotid body have been studied using the whole cell variant of the patch-clamp technique. Type II cells only have a tiny, slowly activating outward potassium current. By contrast, in every type I chemoreceptor cell studied we found (a) sodium, (b) calcium, and (c) potassium currents. (a) The sodium current has a fast activation time course and an activation threshold at approximately -40 mV. At all voltages inactivation follows a single exponential time course. The time constant of inactivation is 0.67 ms at 0 mV. Half steady state inactivation occurs at a membrane potential of approximately -50 mV. (b) The calcium current is almost totally abolished when most of the external calcium is replaced by magnesium. The activation threshold of this current is at approximately -40 mV and at 0 mV it reaches a peak amplitude in 6-8 ms. The calcium current inactivates very slowly and only decreases to 27% of the maximal value at the end of 300-ms pulses to 40 mV. The calcium current was about two times larger when barium ions were used as charge carriers instead of calcium ions. Barium ions also shifted 15-20 mV toward negative voltages the conductance vs. voltage curve. Deactivation kinetics of the calcium current follows a biphasic time course well fitted by the sum of two exponentials. At -80 mV the slow component has a time constant of 1.3 +/- 0.4 ms whereas the fast component, with an amplitude about 20 times larger than the slow component, has a time constant of 0.16 +/- 0.03 ms. These results suggest that type I cells have predominantly fast deactivating calcium channels. The slow component of the tails may represent the activity of a small population of slowly deactivating calcium channels, although other possibilities are considered. (c) Potassium current seems to be mainly due to the activity of voltage-dependent potassium channels, but a small percentage of calcium-activated channels may also exist. This current activates slowly, reaches a peak amplitude in 5-10 ms, and thereafter slowly inactivates. Inactivation is almost complete in 250-300 ms. The potassium current is reversibly blocked by tetraethylammonium. Under current-clamp conditions type I cells can spontaneously fire large action potentials. These results indicate that type I cells are excitable and have a variety of ionic conductances. We suggest a possible participation of these conductances in chemoreception.