Enhancement of In Vivo Tyrosine Hydroxylation in the Rat Adrenal Gland Under Hypoxic Conditions

Abstract

We examined the effects of hypoxia (8% O2) on in vivo tyrosine hydroxylation, a rate-limiting step for catecholamine synthesis, in the rat adrenal gland. The hydroxylation rate was determined by measuring the rate of accumulation of 3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition. One hour after hypoxic exposure, DOPA accumulation decreased to 60% of control values, but within 2 h it doubled. At 2 h, the apparent Km values for tyrosine and for biopterin cofactor of tyrosine hydroxylase (TH) in the soluble fraction were unchanged, whereas the Vmax value increased by 30%. The content of total or reduced biopterin was unchanged, but the content of tyrosine increased by 80%. Tyrosine administration had little effect on DOPA accumulation under room air conditions but enhanced DOPA accumulation under hypoxia. After denervation of the adrenal gland, the hypoxia-induced increase in DOPA accumulation and in the Vmax value was abolished, whereas the hypoxia-induced increase in tyrosine content was persistent. These results suggest that in vivo tyrosine hydroxylation is enhanced under hypoxia, although availability of oxygen is reduced. The enhancement is the result of both an increase in tyrosine content coupled with increased sensitivity of TH to changes in tyrosine tissue content and of an increase in dependence of TH on tyrosine levels. The increase in the sensitivity of TH and in the Vmax value is neurally induced, whereas the increase in tyrosine content is regulated by a different mechanism.

Long-lasting adverse effects of prenatal hypoxia on developing autonomic nervous system and cardiovascular parameters in rats

“…20 min before sacrifice. DOPA accumulation under these conditions is considered to be an index of the in vivo TH activity and of the synthesis of catecholamines [5,29]. The DOPA accumulation in ganglia, central areas, adrenals, heart and lungs was determined using HPLC-ED as previously described [3,8,31].…”

Effect of acute hypoxia on resistance to hypoxia in rats

“…A stress-syndrome has several phases, which determines phasic changes in the resistance to hypoxia. This dynamics corresponds to phasic changes in the catecholamine content in the adrenal medulla observed from the first hours of hypoxia [14] with massive mobilization of endogenous catecholamines [17]. The latter seems to occur also after short-term exposure to hypoxia, being accompanied by an increased fluorescence of myocardial adrenergic terminals not only in the early (6 h) but also in the late periods (3 weeks) after acute hypoxia [11].…”

6R-L-erythro-5,6,7,8-Tetrahydrobiopterin and Dopamine Release

“…Assay of DA collected in dialysates was, in principle, similar to the method described previously (28,29) but was modified to be suitable for a small volume (27,30,31). After addition of ioo J.lI of 0.1 M perchloric acid containing I mg/ml of sodium bisulfite, I mM EDTA and 20 mg of activated alumina to the dialysate, the pH of the mixture was adjusted to 8.3-8.4 with I M Tris-HCI buffer (pH 8.6) and the mixture was vigorously shaken for 20 min.…”

“…Biopterins in the striatum were extracted as described (15,29). For determination of reduced biopterin levels in the extracellular fluid, the striatum was dialyzed with a double-barrelled probe (BDP-I-8-03W, ElCOM) instead of the U-shaped probe.…”

“…After implantation of the probe complex in the striatum, each probe was independently perfused: one probe was perfused with Ringer's solution containing pteridines for their administration, and the other with Ringer's solution alone for sampling of extracellular reduced biopterins. The biopterins in the striatal extract and the dialysates ware determined using HPLC with fluorescence detection as described elsewhere (15,29) The reduced biopterin (mainly consisting of 6R-BH 4 ) was calculated by subtracting oxidized bipopterin from total (reduced+oxidized) biopterin. (32,33).…”