Keely Parisian

Advisor:  Dr. Erlichman

SLU Festival of Science 2001 Oral Presentation

Ventilatory Effects of Focal Acidification in the NTS of the Conscious Rat

Abstract:

Terrestrial animals are faced with the problem of ridding the body of carbon dioxide (CO₂).  Unlike aquatic animals, CO₂ cannot diffuse across the skin and therefore a mechanism of elimination is necessary.  Breathing is one essential process of eliminating metabolically produced CO₂.  Upon hydration, CO₂ forms carbonic acid, which can denature cellular proteins and disrupt cellular functions.  Since aerobic metabolism is ongoing, CO₂ represents a continual acid load to the body that must be regulated.  Chemosensitive neurons distributed throughout the brainstem regulate systemic CO₂ levels by altering ventilation.  Although the unique stimulus to chemoreceptors is not known, changes in neuronal activity are thought to be mediated by pH rather than molecular CO₂.

Although chemoreceptive sites are widely distributed throughout the brainstem, the predominant ventilatory effects of a given site appear to be dependent on the state of arousal (i.e. wakefulness, sleep, anesthesia).  Unfortunately most of the work in the area of ventilatory control has been performed in the anesthetized rat.  In this study, I examine the ventilatory effects of focal, unilateral acidification in the nucleus tractus solitarious (NTS) of the conscious rat.  The NTS is an important dorsal medullary site involved in respiratory control.  It has been shown in vitro that intracellular pH (pH_i) changes as a function of extracellular pH (pH_o).  It is hypothesized that as pH_i becomes more acidic, CO₂ chemosensitive neurons increase firing frequency resulting in an increase in ventilation.  I found that focal acidification with 1 µM acetazolamide (AZ) perfusion increased ventilation during steady-state normocapnia and hypercapnia by ~58%.  Moreover, not all animals showed an increase in ventilatory minute expiration with AZ suggesting that CO₂ chemoreceptors may be discretely located within the brainstem.  In summary, my study suggests that there is heterogeneity in the distribution of CO₂ chemoreceptors in the brainstem and that the NTS is important in ventilatory control in the conscious animal.