Pharmacokinetics of intraperitoneally instilled aminophylline, terbutaline and tobramycin in pigs

Adenosine and γ-aminobutyric acid partially regulate metabolic and ventilatory responses of Damaraland mole-rats to acute hypoxia

Fossorial Damaraland mole-rats (Fukomys damarensis) mount a robust hypoxic metabolic response (HMR) but a blunted hypoxic ventilatory response (HVR) to acute hypoxia. Although these reflex physiological responses have been described previously, the underlying signalling pathways are entirely unknown. Of particular interest are contributions from γ-aminobutyric acid (GABA), which is the primary inhibitory neurotransmitter in the nervous system of most adult mammals, and adenosine, the accumulation of which increases during hypoxia as a breakdown product of ATP. Therefore, we hypothesized that GABAergic and/or adenosinergic signalling contributes to the blunted HVR and robust HMR in Damaraland mole-rats. To test this hypothesis, we injected adult animals with saline alone (controls), or 100 mg kg-1 aminophylline or 1 mg kg-1 bicuculline, to block adenosine or GABAA receptors, respectively. We then used respirometry, plethysmography and thermal RFID probes to non-invasively measure metabolic, ventilator and thermoregulatory responses, respectively, to acute hypoxia (1 h in 5 or 7% O2) in awake and freely behaving animals. We found that bicuculline had relatively minor effects on metabolism and thermoregulation but sensitized ventilation such that the HVR became manifest at 7% instead of 5% O2 and was greater in magnitude. Aminophylline increased metabolic rate, ventilation and body temperature in normoxia, and augmented the HMR and HVR. Taken together, these findings indicate that adenosinergic and GABAergic signalling play important roles in mediating the robust HMR and blunted HVR in Damaraland mole-rats.

Adenosine receptors mediate the hypoxic ventilatory response but not the hypoxic metabolic response in the naked mole rat during acute hypoxia

Naked mole rats are the most hypoxia-tolerant mammals identified; however, the mechanisms underlying this tolerance are poorly understood. Using whole-animal plethysmography and open-flow respirometry, we examined the hypoxic metabolic response (HMR), hypoxic ventilatory response (HVR) and hypoxic thermal response in awake, freely behaving naked mole rats exposed to 7% O₂ for 1 h. Metabolic rate and ventilation each reversibly decreased 70% in hypoxia (from 39.6 ± 2.9 to 12.1 ± 0.3 ml O₂ min(-1) kg(-1), and 1412 ± 244 to 417 ± 62 ml min(-1) kg(-1), respectively; p < 0.05), whereas body temperature was unchanged and animals remained awake and active. Subcutaneous injection of the general adenosine receptor antagonist aminophylline (AMP; 100 mg kg(-1), in saline), but not control saline injections, prevented the HVR but had no effect on the HMR. As a result, AMP-treated naked mole rats exhibited extreme hyperventilation in hypoxia. These animals were also less tolerant to hypoxia, and in some cases hypoxia was lethal following AMP injection. We conclude that in naked mole rats (i) hypoxia tolerance is partially dependent on profound hypoxic metabolic and ventilatory responses, which are equal in magnitude but occur independently of thermal changes in hypoxia, and (ii) adenosine receptors mediate the HVR but not the HMR.

Competitive antagonism of fluorescent gentamicin uptake in the cochlea

Aminoglycosides enter inner ear hair cells via apical endocytosis, or mechanoelectrical transduction channels, implying that, in vivo, aminoglycosides enter hair cells from endolymph prior to exerting their cytotoxic effect. If so, circulating aminoglycosides likely cross the strial blood-labyrinth barrier and enter marginal cells prior to clearance into endolymph. We characterized the competitive antagonism of unconjugated aminoglycosides on the uptake of fluorescent gentamicin (GTTR) in the stria vascularis and kidney cells at an early time point. In mice, uptake of GTTR by kidney proximal tubule cells was competitively antagonized by gentamicin at all doses, but only weakly by kanamycin (mimicking in vitro data). GTTR fluorescence was approximately 100-fold greater in proximal tubule cells than in the stria vascularis. Furthermore, only high molar ratios of aminoglycosides significantly reduced strial uptake of GTTR. Thus, gentamicin antagonism of GTTR uptake is more efficacious in proximal tubules than in the stria vascularis. Competitive antagonism of GTTR uptake is indicative of specific cell-regulatable uptake mechanisms (e.g., ion channels, transporters) in the kidney. Strial uptake mechanisms have lower specific affinity for gentamicin, and/or density (compared to the kidney), yet may be critical to transport gentamicin across the strial blood-labyrinth barrier into marginal cells.