Cooperation of peripheral and central chemosensitive mechanisms in the control of the extracellular pH in brain in non-respiratory acidosis

TASK-1 (KCNK3) and TASK-3 (KCNK9) tandem pore potassium channel antagonists stimulate breathing in isoflurane-anesthetized rats

BACKGROUND

TASK-1 and TASK-3 tandem pore potassium channel subunits provide a constitutive acidic pH- and hypoxia-inhibited potassium conductance. TASK channels are expressed in a number of tissues involved in regulation of breathing, and the TASK-1/TASK-3 heterodimer provides the predominant hypoxia-sensitive potassium conductance in carotid body type 1 glomus chemosensing cells. The carotid bodies have an important role in regulation of breathing. Doxapram is a potent TASK-1 and TASK-3 potassium channel antagonist and a carotid body and breathing stimulant. PK-THPP and A1899 are potent and selective TASK-1 and TASK-3 antagonists. I hypothesized PK-THPP and A1899 are, like doxapram, breathing stimulants.

METHODS

I studied rat TASK-3 potassium channel function by Ussing chamber using Fischer rat thyroid monolayers. To quantify breathing effects, I studied male Sprague-Dawley rats spontaneously breathing 1.5% isoflurane in room air by noninvasive plethysmography and by arterial blood gas analysis.

RESULTS

PK-THPP, A1899, and doxapram inhibit rat TASK-3 potassium channel function with IC50s of 42 nM (33-52), 1.6 μM (0.8-3.3), and 22 μM (18-28) (n = 4-6; 95% confidence limits). IV PK-THPP, A1899, and doxapram stimulated breathing by plethysmography with a peak change in minute ventilation relative to baseline of 84% ± 19% and 226% ± 56% (for PK-THPP at 0.5 and 5 mg/kg; mean ± SEM; n = 3-4; P < 0.05 and P < 0.001, respectively, relative to vehicle); 46% ± 2% and 236% ± 48% (for A1899 at 5 and 25 mg/kg; n = 3-4; P > 0.05 and P < 0.001, respectively); 103% ± 20% (for doxapram at 25 mg/kg; n = 4), and 33% ± 9% (for dimethylsulfoxide vehicle at 1 mL/kg; n = 4). PK-THPP and A1899, unlike doxapram, induced a profound and lasting respiratory alkalosis by arterial blood gas analysis. Thirty minutes after IV drug administration, I observed an arterial pH and carbon dioxide partial pressure of 7.62 ± 0.02 and 23 ± 0.8 mm Hg (for PK-THPP after 5 mg/kg; n = 4; P < 0.001 for both relative to vehicle), 7.49 ± 0.02 and 31 ± 2 mm Hg (for A1899 at 25 mg/kg; n = 6; P < 0.05 and 0.001, respectively), 7.43 ± 0.03 and 39 ± 4 mm Hg (for doxapram after 25 mg/kg; n = 4; P > 0.05 for both), and 7.38 ± 0.03 and 48 ± 4 mm Hg (for dimethylsulfoxide vehicle after 1 mL/kg; n = 3).

CONCLUSIONS

PK-THPP and A1899 are potent rat TASK-3 antagonists and effective breathing stimulants. PK-THPP and A1899 effects on breathing were of greater magnitude and/or duration relative to that of doxapram. PK-THPP and A1899 or related compounds may have therapeutic potential for treating breathing disorders.

Role of acid-base balance in the chemoreflex control of breathing

This paper uses a steady-state modeling approach to describe the effects of changes in acid-base balance on the chemoreflex control of breathing. First, a mathematical model is presented, which describes the control of breathing by the respiratory chemoreflexes; equations express the dependence of pulmonary ventilation on Pco(2) and Po(2) at the central and peripheral chemoreceptors. These equations, with Pco(2) values as inputs to the chemoreceptors, are transformed to equations with hydrogen ion concentrations [H(+)] in brain interstitial fluid and arterial blood as inputs, using the Stewart approach to acid-base balance. Examples illustrate the use of the model to explain the regulation of breathing during acid-base disturbances. They include diet-induced changes in sodium and chloride, altitude acclimatization, and respiratory disturbances of acid-base balance due to chronic hyperventilation and carbon dioxide retention. The examples demonstrate that the relationship between Pco(2) and [H(+)] should not be neglected when modeling the chemoreflex control of breathing. Because pulmonary ventilation controls Pco(2) rather than the actual stimulus to the chemoreceptors, [H(+)], changes in their relationship will alter the ventilatory recruitment threshold Pco(2), and thereby the steady-state resting ventilation and Pco(2).

Highly H+-sensitive neurons in the caudal ventrolateral medulla of the rat

The ventral surface of the caudal ventrolateral medulla (cVLM) has been shown to generate intense respiratory responses after surface acid-base stimulation. With respect to their chemosensitive characteristics, cVLM neurons have been less studied than other rostral-most regions of the brainstem. The purpose of these experiments was to determine the bioelectric responses of cVLM neurons to acidic stimuli and to determine their chemosensitive properties. Using extracellular and microiontophoretic techniques, we recorded electrical activities from 117 neurons in an area close to the ventral surface of the cVLM in anaesthetised rats. All neurons were tested for their sensitivity to H+. The fluorescent probe BCECF was used to measure extracellular pH changes produced by the microiontophoretic injection of H+ in brainstem slices. This procedure provided an estimation of the local changes in pH produced by microiontophoretic H+ application in the anaesthetised rat. Neurons coupled to the respiratory cycle, R (n = 51), were not responsive to direct stimulation with H+. Sixty-six neurons that did respond to H+ stimulation were uncoupled from respiration, and identified as NR neurons. These neurons presented distinct ranges of H+ sensitivity. The neuronal sensitivity to H+ was mainly assessed by the slope of the stimulus-response curve, where the steeper the slope, the higher the H+ sensitivity. On this basis, NR neurons were classed as being either weakly or highly sensitive to H+. NR neurons with a high H+ sensitivity (n = 12) showed an average value of 34.17 +/- 7.44 spikes s-1 (100 nC)-1 (mean +/- S.D.) for maximal slope and an EC50 of 126.76 +/- 33 nC. Suprathreshold H+ stimulation of highly sensitive NR neurons elicited bursting pattern responses coupled to the respiratory cycle. The bursting responses, which were synchronised with the inspiratory phase and the early expiratory phase of the respiratory cycle, lasted for several seconds before returning to the steady state firing pattern characteristic of the pre-stimulus condition. These NR neurons, which possess the capacity to detect distinct H+ concentrations in the extracellular microenvironment, are excellent candidates to serve in a chemoreceptor capacity in the caudal medulla.

Effects of acetazolamide on medullary extracellular pH and PCO2 and on ventilation in peripherally chemodenervated cats

The responses of ventilation and of medullary extracellular fluid (ECF) pH and PCO2, to an intravenous (i.v.) infusion of 50 mg/kg acetazolamide (an inhibitor of carbonic anhydrase), were measured in cats anaesthetized with chloralose and urethane, in which both bilateral vagotomy and carotid nerve section had been performed. After 2 h, it was observed that: acetazolamide caused an acidosis in medullary ECF which was still developing after 2 h, reflected by a progressive fall in pH (mean = 0.215 pH units in 2 h), while ECF PCO2 showed an insignificant rise of about 1 kPa; acetazolamide caused a considerable rise in ventilation, which largely developed in the first 15 min after drug infusion; the direction of the ECF acid-base responses in the first 15 min varied, whereas that of the ventilatory response did not. Furthermore, the time course of the former developed quite differently from the latter. It was therefore concluded that the observed changes in medullary ECF pH and PCO2 can not explain the large and fast ventilatory response of acetazolamide.

Fast bicarbonate-chloride exchange between plasma and brain extracellular fluid at maintained PCO2

The aim of this paper was to investigate the kinetics and mechanism of bicarbonate exchange at the blood-brain ECF barrier. The experiments were performed on anaesthetized and artificially ventilated cats in such a way that acid-base parameters of the brain extracellular fluid were continuously measured while an approximately rectangular increase in the arterial plasma bicarbonate concentration was produced at maintained PCO2. The results from such a preparation were: 1. A rapid increase in the brain extracellular bicarbonate concentration was observed in response to an i.v. bolus injection of 3 ml molar NaHCO3. The bolus was followed by a slow infusion of bicarbonate solution. The brain extracellular bicarbonate reached a new steady state within a minute. 2. This increase was almost simultaneously accompanied by a decrease in the extracellular chloride concentration. The HCO-3-Cl- exchange ratio very closely approached one. 3. The extracellular bicarbonate concentration in the brain, after an initial increase, returned towards control in spite of elevated arterial bicarbonate at maintained PCO2. 4. The results are discussed in terms of a 5-compartment model, where the extracellular fluid is interposed between the glial cells and the interstitial side of the endothelial cells, similar to the blood plasma being interposed between the red cells and the luminal side of the endothelial cells. 5. A non-electrogenic carrier-mediated HCO-3-Cl- exchange at the interphase of the blood brain barrier is postulated.