Contribution of peripheral chemoreceptor drive in exercise hyperpnea in humans

ASICs and cardiovascular homeostasis

In this review we address primarily the role of ASICs in determining sensory signals from arterial baroreceptors, peripheral chemoreceptors, and cardiopulmonary and somatic afferents. Alterations in these sensory signals during acute cardiovascular stresses result in changes in sympathetic and parasympathetic activities that restore cardiovascular homeostasis. In pathological states, however, chronic dysfunctions of these afferents result in serious sympatho-vagal imbalances with significant increases in mortality and morbidity. We identified a role for ASIC2 in the mechano-sensitivity of aortic baroreceptors and of ASIC3 in the pH sensitivity of carotid bodies. In spontaneously hypertensive rats, we reported decreased expression of ASIC2 in nodose ganglia neurons and overexpression of ASIC3 in carotid bodies. This reciprocal expression of ASIC2 and ASIC3 results in reciprocal changes in sensory sensitivity of baro- and chemoreceptors and a consequential synergistic exaggeration sympathetic nerve activity. A similar reciprocal sensory dysautonomia prevails in heart failure and increases the risk of mortality. There is also evidence that ASIC heteromers in skeletal muscle afferents contribute significantly to the exercise pressor reflex. In cardiac muscle afferents of the dorsal root ganglia, they contribute to nociception and to the detrimental sympathetic activation during ischemia. Finally, we report that an inhibitory influence of ASIC2-mediated baroreceptor activity suppresses the sympatho-excitatory reflexes of the chemoreceptors and skeletal muscle afferents, as well as the ASIC1a-mediated excitation of central neurons during fear, threat, or panic. The translational potential of activation of ASIC2 in cardiovascular disease states may be a beneficial sympatho-inhibition and parasympathetic activation. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Chemoreceptor hypersensitivity, sympathetic excitation, and overexpression of ASIC and TASK channels before the onset of hypertension in SHR

RATIONALE

Increased sympathetic nerve activity has been linked to the pathogenesis of hypertension in humans and animal models. Enhanced peripheral chemoreceptor sensitivity which increases sympathetic nerve activity has been observed in established hypertension but has not been identified as a possible mechanism for initiating an increase in sympathetic nerve activity before the onset of hypertension.

OBJECTIVE

We tested this hypothesis by measuring the pH sensitivity of isolated carotid body glomus cells from young spontaneously hypertensive rats (SHR) before the onset of hypertension and their control normotensive Wistar-Kyoto (WKY) rats.

METHODS AND RESULTS

We found a significant increase in the depolarizing effect of low pH in SHR versus WKY glomus cells which was caused by overexpression of 2 acid-sensing non-voltage-gated channels. One is the amiloride-sensitive acid-sensing sodium channel (ASIC3), which is activated by low pH and the other is the 2-pore domain acid-sensing K(+) channel (TASK1), which is inhibited by low pH and blocked by quinidine. Moreover, we found that the increase in sympathetic nerve activity in response to stimulation of chemoreceptors with sodium cyanide was markedly enhanced in the still normotensive young SHR compared to control WKY rats.

CONCLUSIONS

Our results establish a novel molecular basis for increased chemotransduction that contributes to excessive sympathetic activity before the onset of hypertension.

Acid-sensing ion channels contribute to transduction of extracellular acidosis in rat carotid body glomus cells

Carotid body chemoreceptors sense hypoxemia, hypercapnia, and acidosis and play an important role in cardiorespiratory regulation. The molecular mechanism of pH sensing by chemoreceptors is not clear, although it has been proposed to be mediated by a drop in intracellular pH of carotid body glomus cells, which inhibits a K+ current. Recently, pH-sensitive ion channels have been described in glomus cells that respond directly to extracellular acidosis. In this study, we investigated the possible molecular mechanisms of carotid body pH sensing by recording the responses of glomus cells isolated from rat carotid body to rapid changes in extracellular pH using the whole-cell patch-clamping technique. Extracellular acidosis evoked transient inward current in glomus cells that was inhibited by the acid-sensing ion channel (ASIC) blocker amiloride, absent in Na+-free bathing solution, and enhanced by either Ca2+-free buffer or addition of lactate. In addition, ASIC1 and ASIC3 were shown to be expressed in rat carotid body by quantitative PCR and immunohistochemistry. In the current-clamp mode, extracellular acidosis evoked both a transient and sustained depolarizations. The initial transient component of depolarization was blocked by amiloride, whereas the sustained component was eliminated by removal of K+ from the pipette solution and partially blocked by the TASK (tandem-p-domain, acid-sensitive K+ channel) blockers anandamide and quinidine. The results provide the first evidence that ASICs may contribute to chemotransduction of low pH by carotid body chemoreceptors and that extracellular acidosis directly activates carotid body chemoreceptors through both ASIC and TASK channels.

Increased Peripheral Chemoreceptors Sensitivity and Exercise Ventilation in Heart Transplant Recipients

BACKGROUND

Heart failure is characterized by increased ventilation during exercise, which is positively related to increased peripheral and central chemoreceptor sensitivity. Heart transplantation does not normalize the ventilatory response to exercise, and its effects on the chemoreflex control of ventilation remain unknown. We tested the hypothesis that chemoreceptor sensitivity is increased in heart transplant recipients (HTRs) and linked to exercise hyperpnea.

METHODS AND RESULTS

We determined the ventilatory, muscle sympathetic nerve activity (MSNA), and circulatory responses to isocapnic hypoxia and hyperoxic hypercapnia 7+/-1 years after transplantation in 19 HTRs with a normal left ventricular ejection fraction of 60+/-2%. Results were compared with those of 11 closely matched referent subjects. Sixteen patients and 10 referent subjects also underwent cycle ergometer exercise tests. HTRs compared with referent subjects presented higher MSNA (52+/-4 versus 34+/-3 bursts/min; P<0.01) and heart rates (83+/-3 versus 68+/-3 bpm; P<0.01) during room air breathing. The ventilatory response to hypoxia was higher in HTRs than in referent subjects (P<0.01, ANOVA). The increase in MSNA also was more marked during hypoxia in the HTRs than in the referent group (P<0.05, ANOVA). Responses to hyperoxic hypercapnia did not differ between the HTRs and the referent group. The ventilatory response to exercise, characterized by the regression slope relating minute ventilation to CO2 output, was steeper in HTRs than in referent subjects (38+/-2 versus 29+/-1 L/mm Hg; P<0.01). Exercise ventilation in HTRs was related to the ventilatory response to isocapnic hypoxia (r=0.57; n=16; P<0.05) and to the ventilatory response to hyperoxic hypercapnia (r=0.50; n=16; P<0.05).

CONCLUSIONS

Peripheral chemoreceptor sensitivity is increased in HTRs and is related to exercise hyperpnea after heart transplantation.