Anemia up-regulates pontine A1 adenosine receptors in fetal sheep

Hypoxic regulation of the fetal cerebral circulation

Fetal cerebrovascular responses to acute hypoxia are fundamentally different from those observed in the adult cerebral circulation. The magnitude of hypoxic vasodilatation in the fetal brain increases with postnatal age although fetal cerebrovascular responses to acute hypoxia can be complicated by age-dependent depressions of blood pressure and ventilation. Acute hypoxia promotes adenosine release, which depresses fetal cerebral oxygen consumption through action of adenosine on neuronal A1 receptors and vasodilatation through activation of A2 receptors on cerebral arteries. The vascular effect of adenosine can account for approximately half the vasodilatation observed in response to hypoxia. Hypoxia-induced release of nitric oxide and opioids can account for much of the adenosine-independent cerebral vasodilatation observed in response to hypoxia in the fetus. Direct effects of hypoxia on cerebral arteries account for the remaining fraction, although the vascular endothelium contributes relatively little to hypoxic vasodilatation in the immature cerebral circulation. In contrast to acute hypoxia, fetal cerebral blood flow tends to normalize during acclimatization to chronic hypoxia even though cardiac output is depressed. However, uncompensated chronic hypoxia in the fetus can produce significant changes in brain structure and function, alteration of respiratory drive and fluid balance, and increased incidence of intracranial hemorrhage and periventricular leukomalacia. At the level of the fetal cerebral arteries, chronic hypoxia increases protein content and depresses norepinephrine release, contractility, and receptor densities associated with contraction but also attenuates endothelial vasodilator capacity and decreases the ability of ATP-sensitive and calcium-sensitive potassium channels to promote vasorelaxation. Overall, fetal cerebrovascular adaptations to chronic hypoxia appear prioritized to conserve energy while preserving basic contractility. Many gaps remain in our understanding of how the effects of acute and chronic hypoxia are mediated in fetal cerebral arteries, but studies of adult cerebral arteries have produced many powerful pharmacological and molecular tools that are simply awaiting application in studies of fetal cerebral artery responses to hypoxia.

Maintenance of adenosine A1 receptor function during long-term anoxia in the turtle brain

It has been established that adenosine has a critical role in the extraordinary ability of the turtle brain to survive anoxia. To further investigate this phenomenon we compared rat and turtle brain adenosine A1 receptors using cyclopentyl-1,3-dipropylxanthine, 8-[dipropyl-2,3-3H(N)] ([3H]DPCPX) saturation binding analyses and determined the effects of prolonged anoxia (6, 12, and 24 h) on the adenosine A1 receptor of the turtle brain. The rat brain had a 10-fold greater density of A1 receptors compared with the turtle [rat cortex receptor density (Bmax) = 1,400 +/- 134.6 fmol/mg protein, turtle forebrain Bmax = 103.2 +/- 4.60 fmol/mg protein] and a higher affinity [dissociation constant (Kd) rat cortex = 0.328 +/- 0.035 nM, Kd turtle forebrain = 1.16 +/- 0.06 nM]. However, the turtle Kd is within the reported mammalian range, and the Bmax is similar to that reported for other poikilotherms. Unlike the mammal, in which A1 receptor function is rapidly compromised in anoxia, in the turtle forebrain no significant changes in the A1 receptor population were seen during 24-h anoxia. However, in the hindbrain, whereas the Bmax remained unchanged, the Kd significantly decreased from 2.1 to 0.5 nM after 6 h anoxia and this higher affinity was maintained at 12- and 24-h anoxia. These findings indicate that, unlike the GABAA receptor, the protective effectiveness of adenosine in the anoxic turtle brain is not related to an enhanced receptor number. Protection from a hypoxia-induced compromise in A1 receptor function and an increased A1 sensitivity in the hindbrain may be important factors for maintaining the adenosine-mediated downregulation of energy demand during long-term anoxia.