Chronic Caffeine Intake in Adult Rat Inhibits Carotid Body Sensitization Produced by Chronic Sustained Hypoxia but Maintains Intact Chemoreflex Output

Abstract

Sustained hypoxia produces a carotid body (CB) sensitization, known as acclimatization, which leads to an increase in carotid sinus nerve (CSN) activity and ensuing hyperventilation greater than expected from the prevailing partial pressure of oxygen. Whether sustained hypoxia is physiological (high altitude) or pathological (lung disease), acclimatization has a homeostatic implication because it tends to minimize hypoxia. Caffeine, the most commonly ingested psychoactive drug and a nonselective adenosine receptor antagonist, alters CB function and ventilatory responses when administered acutely. Our aim was to investigate the effect of chronic caffeine intake on CB function and acclimatization using four groups of rats: normoxic, caffeine-treated normoxic, chronically hypoxic (12% O 2 , 15 days), and caffeine-treated chronically hypoxic rats. Caffeine was administered in drinking water (1 mg/ml). Caffeine ameliorated ventilatory responses to acute hypoxia in normoxic animals without altering the output of the CB (CSN neural activity). Caffeine-treated chronically hypoxic rats exhibited a decrease in the CSN response to acute hypoxia tests but maintained ventilation compared with chronically hypoxic animals. The findings related to CSN neural activity combined with the ventilatory responses indicate that caffeine alters central integration of the CB input to increase the gain of the chemoreflex and that caffeine abolishes CB acclimatization. The putative mechanisms involved in sensitization and its loss were investigated: expression of adenosine receptors in CB (A 2B ) was downregulated and that in petrosal ganglion (A 2A ) was up-regulated in caffeine-treated chronically hypoxic rats; both adenosine and dopamine release from CB chemoreceptor cells was increased in chronic hypoxia and in caffeine-treated chronic hypoxia groups.

Contribution of adenosine and ATP to the carotid body chemosensory activity in ageing

“…In agreement with what has been previously described for young rats (Conde et al . a ), in aged rats the effect of suramin, a P2X antagonist, on CSN chemosensory activity is as high as the hypoxic intensity (block of 74.71% in 0% O 2 and of 46.97% in 5% O 2 ) (Fig. D and E ).…”

“…Aiming to understand if the decreases in the amount of ATP and adenosine present extracellularly were due to altered exocytosis, we have analysed by western blot the expression of a key protein involved in the exocytotic machinery, SNAP25, a t‐SNARE protein, as well as the equilibrative nucleoside transporter‐1 (ENT1) responsible for the release of adenosine per se in moderate hypoxic conditions in the CB (Conde & Monteiro, ; Conde et al . a ), in 3‐ and 24‐month‐old animals. Surprisingly, ageing increased the SNAP‐25 and ENT‐1 expression by 83.71% and 400.52%, respectively, in the CB (Fig.…”

“…Therefore, we can postulate that in aged animals ENT1 is working to reuptake adenosine into type I cells and not to release adenosine per se as previously described (Conde & Monteiro, ; Conde et al . a ). Moreover, we found that the expression of CD73, an enzyme involved in the extracellular conversion of AMP to adenosine, doubled in aged animals (Fig.…”

“…The CB–CSN preparation was digested in collagenase type I (1 mg ml −1 ) solution, to loosen the perineurium, and maintained in ice‐cold 100% O 2 ‐equilibrated Tyrode medium until it was transferred to the recording chamber (Conde et al . , , b ). To investigate A 2A and P2X 2 receptor expression in the petrosal ganglion and the expression of equilibrative nucleoside transporter type 1 (ENT1) and synaptosomal‐associated protein of 25 kDa (SNAP25) in the carotid body, petrosal ganglions and CBs were dissected as previously described (Conde et al .…”

High fat diet blunts the effects of leptin on ventilation and on carotid body activity

“…For CB leptin receptor expression, CBs after cleaning were frozen in liquid nitrogen. The CBs were homogenized in Zurich medium containing a cocktail of protease inhibitors (Conde et al 2012b). Proteins were separated in a 10 or 12% SDS-PAGE and electroblotted on nitrocellulose membrane (0.2 mmol l −1 ; Bio-Rad, Madrid, Spain).…”