The carotid bodies as thermosensors: experiments in vitro and in situ, and importance for ventilatory regulation

Cerebral oxygenation and hyperthermia

Hyperthermia is associated with marked reductions in cerebral blood flow (CBF). Increased distribution of cardiac output to the periphery, increases in alveolar ventilation and resultant hypocapnia each contribute to the fall in CBF during passive hyperthermia; however, their relative contribution remains a point of contention, and probably depends on the experimental condition (e.g., posture and degree of hyperthermia). The hyperthermia-induced hyperventilatory response reduces arterial CO₂ pressure (PaCO₂) causing cerebral vasoconstriction and subsequent reductions in flow. During supine passive hyperthermia, the majority of recent data indicate that reductions in PaCO₂ may be the primary, if not sole, culprit for reduced CBF. On the other hand, during more dynamic conditions (e.g., hemorrhage or orthostatic challenges), an inability to appropriately decrease peripheral vascular conductance presents a condition whereby adequate cerebral perfusion pressure may be compromised secondary to reductions in systemic blood pressure. Although studies have reported maintenance of pre-frontal cortex oxygenation (assessed by near-infrared spectroscopy) during exercise and severe heat stress, the influence of cutaneous blood flow is known to contaminate this measure. This review discusses the governing mechanisms associated with changes in CBF and oxygenation during moderate to severe (i.e., 1.0°C to 2.0°C increase in body core temperature) levels of hyperthermia. Future research directions are provided.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Adequate stimuli of the carotid body: more than an oxygen sensor?

The past 10-20 years has seen a significant increase in the number of studies aimed at elucidating the mechanism of action of the carotid body and this has led to an increased knowledge of how this sensory organ transduces hypoxaemia into afferent chemodischarge. Whilst hypoxia is often considered as the most significant, peripheral chemostimulus, the carotid body is able to transduce many other physico-chemical stimuli, including not only arterial P(CO2) and pH but also blood potassium concentration, temperature and osmolarity as well as, potentially, blood glucose levels and all with appropriate physiological sensitivity. Although it is difficult to be definitive, these other stimuli appear to be sensed independently of the hypoxia transduction process, albeit converging at the point of type I cell membrane depolarisation or Ca(2+) -dependent neurosecretion. We suggest, therefore, that the carotid body might better be viewed as a polymodal receptor with its multiple adequate stimuli interacting to provide additive or greater than additive effects upon chemoafferent discharge for the purpose of cardiorespiratory homeostasis during periods of stress.