Stereoselectivity of catecholamine uptake in noradrenergic and dopaminergic peripheral organs

Effects of Levodopa on Retina in Parkinson Disease

Purpose

To evaluate the effects of levodopa on retina, we assessed retina with optical coherence tomography (OCT) in Parkinson disease (PD).

Methods

Thirty-five patients with PD (17 with levodopa monotherapy, 18 untreated) and 11 healthy controls were included in this cross-sectional study. Unified Parkinson's Disease Rating Scale (UPDRS) and Hoehn & Yahr (H&Y) staging scale were used for the evaluation of disease severity. All retinal scans were performed using OCT.

Results

Total UPDRS and motor subscores were lower in untreated patients compared to patients with treated PD (p = 0.013, p = 0.033, respectively). There was no significant difference in the range of H&Y stages between the untreated and treated PD groups (p = 0.342). The average retinal nerve fiber layer thickness of the untreated (106.76 ± 10.55 μm) and treated (104.62 ± 8.23 μm) patients with PD were significantly thinner than those of controls (115.60 ± 9.11 μm) (p<0.005). However, there was no significant difference between the untreated and patients with treated PD (p = 0.780). No significant difference in mean values of average thickness of ganglion cell complex layer was observed among controls, patients with untreated PD, and patients with treated PD (p = 0.304).

Conclusions

Although the disease in patients with treated PD was more severe than in the untreated group, no significant difference in the thickness of retina was found between the 2 groups. Therefore, we thought that levodopa might have a protective effect on retina in patients with PD.

ATP-regulated neuronal catecholamine uptake: a new mechanism

Uptake of the catecholamines (CA), dopamine (DA) and norepinephrine (NE) into synaptosomes prepared from rat and bovine brains was potentiated by ATP (from 0.1 to 5.0 mM) in a dose-dependent manner. Other nucleotides, particularly the nonhydrolyzable ATP analogs beta,gamma-imidoadenosine-5'-triphosphate (AMP-PNP) and beta,gamma-methyladenosine-5'-triphosphate (AMP-PCP) also potentiated [3H]DA and [3H]NE uptake. Several endogenous 5'-nucleotide triphosphates (e.g. GTP, UTP and CTP) potentiated [3H]CA uptake, but were less effective than ATP. Among the ATP metabolites, only ADP potentiated uptake whereas AMP and adenosine did not. [3H]Dopamine uptake measured in Krebs bicarbonate buffer had a Km of 2.1 microM and a Vmax of 163.9 pmol/mg prot./min. In presence of ATP, [3H]DA uptake had much higher affinity (Km = 0.56 microM) and larger capacity (Vmax = 333 pmol/mg prot./min) than uptake in absence of added ATP. Furthermore, [3H]DA uptake in presence of ATP had faster rate of uptake, and was independent of temperature while in absence of added ATP it was temperature-dependent. This ATP-dependent [3H]DA uptake was retained by synaptosomal ghosts that were obtained after lysing the striatal synaptosomes and removing their contents of synaptic vesicles and mitochondria. It is proposed that, in addition to the carrier-mediated (neuronal) uptake of CA, there is neuronal uptake that is regulated by ATP and inhibited by cocaine, which may be more relevant for terminating the synaptic action of CA because of its faster rate of uptake and larger capacity.

The saturability of a site of loss and the degree of supersensitivity to agonists which are substrates of this site of loss

The present investigation was undertaken to test the hypothesis that the experimentally determined degree of supersensitivity to an agonist caused by inhibition of a site of loss depends on the ratio "Km of the site of loss/ED50 of the agonist". The influence of inhibition of neuronal uptake by cocaine on the alpha-adrenoceptor-mediated effect of noradrenaline was studied on the dog saphenous vein; the influence of inhibition of COMT by U-0521 on the beta-adrenoceptor-mediated effect of isoprenaline was studied on the dog saphenous vein and on the guinea-pig trachea; the influence of inhibition of acetylcholinesterase by physostigmine on the effect of acetylcholine was studied on the guinea-pig ileum. To further extend the range of values of the ratio "Km/ED50", several concentrations of phentolamine, propranolol or atropine were used to "increase the ED50" of the respective agonist. It was thus possible to determine the degree of supersensitivity caused by inhibition of a site of loss over a range of "Km/ED50" values of 0.02 to 2,307. In seven situations the ratio "Km/ED50" was higher than 10. In all of these cases the supersensitivity caused by inhibition of the site of loss was maximal. In eleven situations the ratio "Km/ED50" was less than 10 and higher than 0.1 and the supersensitivity obtained was sub-maximal but was closer to the maximum, the closer the ratio was to 10. In two situations the ratio was less than 0.1 and no supersensitivity was obtained. The results confirm the hypothesis.

Kinetic constants for uptake and metabolism of 3H-(-)noradrenaline in rabbit aorta. Possible falsification of the constants by diffusion barriers within the vessel wall

1. The neuronal uptake of 3H-(-)noradrenaline into aortic rings from reserpine-pretreated animals was a saturable process with a Km of 2.3 mumol X l-1 and a Vmax of 0.5 nmol X g-1 X min-1. Similar constants were obtained when the neuronal deamination of noradrenaline was taken as an index for neuronal uptake. When the tissue was incubated in the usual way, i.e., when the amine was allowed to enter the aortic rings via both the intimal and the adventitial surface, then there was no initial delay (tlag) for the neuronal uptake of noradrenaline (MAO and COMT inhibited). On the other hand, when the amine entry was restricted to the intimal surface, there was a tlag of 2 min, probably due to the slow equilibration of the extracellular space of the media with the incubation medium. Furthermore, for low amine concentrations the rate of uptake in the latter situation was about 10 times lower than that in the former one. Thus, the rate of uptake clearly depended on the way the amine entered the tissue. 2. The corticosterone-sensitive extraneural uptake of 3H-(-)noradrenaline determined in nerve-free aortic rings was characterised by a high Km (490 mumol X l-1) and Vmax (35 nmol X g-1 X min-1). For uptake2 a tlag of 1 min was observed. 3. The analysis of the extraneuronal O-methylating system in nerve-free aortic rings yielded a Km of 3.6 mumol X l-1 and a Vmax of 0.6 nmol X g-1 X min-1. The tlag for the O-methylation of noradrenaline was in the same order of magnitude as that for uptake2. At low amine concentrations the corticosterone-sensitive accumulation of noradrenaline was prevented by COMT, but not by MAO. The latter enzyme reduced the steady-state accumulation of noradrenaline by about 50%. When the amine entry was restricted to one surface only, the results indicated that the extraneuronal O-methylation of noradrenaline generated a steep concentration gradient of the parent amine within the extracellular space of the aorta. 4. All saturable processes fitted the Michaelis-Menten equation. However, kinetic constants determined in incubated organs may be falsified by poor diffusion of the substrate through the extracellular space.

Inhibition of [3H]norepinephrine uptake in peripheral organs of some mammalian species by ouabain

In order to demonstrate the possible involvement of (Na+ + K+)-ATPase in the high affinity uptake of [3H]-norepinephrine in the sympathetic nerve endings, the effect of ouabain on [3H]norepinephrine uptake in spleen and heart slices of five mammalian species was examined. The ouabain sensitivity of [3H]norepinephrine uptake in the heart slices form various species, as determined by the estimation of IC52, was, in increasing order, lamb (2,3 muM) less than calf (2.5 muM) less than guinea pig (4 muM) less than rabbit (10muM) less than rat (greater than 500 muM). The IC50 values in the spleen slices were: lamb (1 muM) less than calf (3.2 muM) less than rabbit (9.5 muM) less than guinea pig (25 muM) less than rat (greater than 500 muM). The IC50 values for the inhibition of specific [3H]ouabain binding in the microsomal fractions of spleen and heart of the five mammalian species by ouabain were similar to the IC50 values for the inhibition of [3H]norepinephrine uptake by the cardiac glycoside. Since ouabain is known to bind exclusively to (Na+ + K+)-ATPase of a microsomal fraction, these results suggest that the inhibition of [3H]norepinephrine uptake in the sympathetic nerve endings by ouabain is mediated by the inhibition of (Na+ + K+)-ATPase.

Relationship between the uptake of 3H-(+/-)metaraminol and the density of adrenergic innervation in isolated rat tissues

1. The uptake of 3H-(+/-)metaraminol (MA) by tissue slices or pieces was studied in vitro in several peripheral rat organs of varying density of sympathetic innervation (the tissue level of endogenous noradrenaline ranging from 1.7-99.1 nmoles/g). In each individual tissue preparation amine uptake was corrected for entry into the 14C-D-sorbitol space. 2. When the tissues were incubated with 1.4 muM MA, the rate of total amine uptake (i.e., neuronal plus extraneuronal uptake of MA) remained virtually constant for up to 7 min. Therefore, rates of uptake were determined after 2 min of incubation with substrate concentrations ranging from 0.25-12.2 muM. In all tissues the total uptake of MA was saturable. 3. Under the condition of inhibition of neuronal uptake by the presence of 100 muM cocaine, the uptake of MA (considered as extraneuronal amine uptake) was no longer saturable. When tissues were exposed to 1.4 muM MA, the relative contribution by extraneuronal (measured in the presence of cocaine) to total amine uptake (measured in the absence of cocaine) was inversely correlated with the log endogenous noradrenaline content. 4. After correction of the rates of total MA uptake for the cocaine-resistant distribution of the amine, a saturable component of uptake was obtained for each tissue. This uptake was considered to be neuronal; it was subjected to kinetic analysis. 5. Apparent Km values for the neuronal uptake of MA were very similar in all tissues and did not show any dependence on the tissue level of endogenous noradrenaline (average Km = 1.2 muM). 6. V max values for the neuronal uptake of MA were linearly correlated with the endogenous noradrenaline content of the tissues (r = 0.976; P less than 0.001), the V max for the vas deferens being excluded. When related to the content of endogenous noradrenaline, the V max obtained in the vas deferens was lower than that for all other tissues. 7. The results presented here strongly suggest that the membrane site involved in neuronal amine uptake (operationally characterized by the Km of MA) is likely to be identical in all rat tissues and that the number of uptake sites available per nerve terminal does not vary greatly between tissues.