Influence of propofol on isolated neonatal rat carotid body glomus cell response to hypoxia and hypercapnia

The Modulation by Anesthetics and Analgesics of Respiratory Rhythm in the Nervous System

Rhythmic eupneic breathing in mammals depends on the coordinated activities of the neural system that sends cranial and spinal motor outputs to respiratory muscles. These outputs modulate lung ventilation and adjust respiratory airflow, which depends on the upper airway patency and ventilatory musculature. Anesthetics are widely used in clinical practice worldwide. In addition to clinically necessary pharmacological effects, respiratory depression is a critical side effect induced by most general anesthetics. Therefore, understanding how general anesthetics modulate the respiratory system is important for the development of safer general anesthetics. Currently used volatile anesthetics and most intravenous anesthetics induce inhibitory effects on respiratory outputs. Various general anesthetics produce differential effects on respiratory characteristics, including the respiratory rate, tidal volume, airway resistance, and ventilatory response. At the cellular and molecular levels, the mechanisms underlying anesthetic-induced breathing depression mainly include modulation of synaptic transmission of ligand-gated ionotropic receptors (e.g., γ-aminobutyric acid, N-methyl-D-aspartate, and nicotinic acetylcholine receptors) and ion channels (e.g., voltage-gated sodium, calcium, and potassium channels, two-pore domain potassium channels, and sodium leak channels), which affect neuronal firing in brainstem respiratory and peripheral chemoreceptor areas. The present review comprehensively summarizes the modulation of the respiratory system by clinically used general anesthetics, including the effects at the molecular, cellular, anatomic, and behavioral levels. Specifically, analgesics, such as opioids, which cause respiratory depression and the "opioid crisis", are discussed. Finally, underlying strategies of respiratory stimulation that target general anesthetics and/or analgesics are summarized.

Cholinergic Chemotransmission and Anesthetic Drug Effects at the Carotid Bodies

General anesthesia is obtained by administration of potent hypnotics, analgesics and muscle relaxants. Apart from their intended effects (loss of consciousness, pain relief and muscle relaxation), these agents profoundly affect the control of breathing, in part by an effect within the peripheral chemoreflex loop that originates at the carotid bodies. This review assesses the role of cholinergic chemotransmission in the peripheral chemoreflex loop and the mechanisms through which muscle relaxants and hypnotics interfere with peripheral chemosensitivity. Additionally, consequences for clinical practice are discussed.

Competitive Interactions between Halothane and Isoflurane at the Carotid Body and TASK Channels

Background

The degree to which different volatile anesthetics depress carotid body hypoxic response relates to their ability to activate TASK potassium channels. Most commonly, volatile anesthetic pairs act additively at their molecular targets. We examined whether this applied to carotid body TASK channels.

Methods

We studied halothane and isoflurane effects on hypoxia-evoked rise in intracellular calcium (Ca2+i, using the indicator Indo-1) in isolated neonatal rat glomus cells, and TASK single-channel activity (patch clamping) in native glomus cells and HEK293 cell line cells transiently expressing TASK-1.

Results

Halothane (5%) depressed glomus cell Ca2+i hypoxic response (mean ± SD, 94 ± 4% depression; P < 0.001 vs. control). Isoflurane (5%) had a less pronounced effect (53 ± 10% depression; P < 0.001 vs. halothane). A mix of 3% isoflurane/1.5% halothane depressed cell Ca2+i response (51 ± 17% depression) to a lesser degree than 1.5% halothane alone (79 ± 15%; P = 0.001), but similar to 3% isoflurane alone (44 ± 22%; P = 0.224), indicating subadditivity. Halothane and isoflurane increased glomus cell TASK-1/TASK-3 activity, but mixes had a lesser effect than that seen with halothane alone: 4% halothane/4% isoflurane yielded channel open probabilities 127 ± 55% above control, versus 226 ± 12% for 4% halothane alone (P = 0.009). Finally, in HEK293 cell line cells, progressively adding isoflurane (1.5 to 5%) to halothane (2.5%) reduced TASK-1 channel activity from 120 ± 38% above control, to 88 ± 48% (P = 0.034).

Conclusions

In all three experimental models, the effects of isoflurane and halothane combinations were quantitatively consistent with the modeling of weak and strong agonists competing at a common receptor on the TASK channel.

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Reversal of Partial Neuromuscular Block and the Ventilatory Response to Hypoxia

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Background

The ventilatory response to hypoxia is a life-saving chemoreflex originating at the carotid bodies that is impaired by nondepolarizing neuromuscular blocking agents. This study evaluated the effect of three strategies for reversal of a partial neuromuscular block on ventilatory control in 34 healthy male volunteers on the chemoreflex. The hypothesis was that the hypoxic ventilatory response is fully restored following the return to a train-of-four ratio of 1.

Methods

In this single-center, experimental, randomized, controlled trial, ventilatory responses to 5-min hypoxia (oxygen saturation, 80 ± 2%) and ventilation at hyperoxic isohypercapnia (end-tidal carbon dioxide concentration, 55 mmHg) were obtained at baseline, during rocuronium-induced partial neuromuscular block (train-of-four ratio of 0.7 measured at the adductor pollicis muscle by electromyography), and following reversal until the train-of-four ratio reached unity with placebo (n = 12), 1 mg neostigmine/0.5 mg atropine (n = 11), or 2 mg/kg sugammadex (n = 11).

Results

This study confirmed that low-dose rocuronium reduced the ventilatory response to hypoxia from 0.55 ± 0.22 (baseline) to 0.31 ± 0.21 l · min−1 · %−1 (train-of-four ratio, 0.7; P < 0.001). Following full reversal as measured at the thumb, there was persistent residual blunting of the hypoxic ventilatory response (0.45 ± 0.16 l · min−1 · %−1; train-of-four ratio, 1.0; P < 0.001). Treatment effect was not significant (analysis of covariance, P = 0.299) with chemoreflex impairment in 5 (45%) subjects following sugammadex reversal, in 7 subjects (64%) following neostigmine reversal, and in 10 subjects (83%) after spontaneous reversal to a train-of-four ratio of 1.

Conclusions

Despite full reversal of partial neuromuscular block at the thumb, impairment of the peripheral chemoreflex may persist at train-of-four ratios greater than 0.9 following reversal with neostigmine and sugammadex or spontaneous recovery of the neuromuscular block.