Amine transport into chromaffin ghosts

Electrochemical monitoring of transport by a vesicular monoamine transporter expressed in Xenopus oocytes

Xenopus laevis oocytes were injected with synthetic mRNA coding for a rat VMAT2 mutant (rVMAT2-I483A/L484A) shown previously to be retained on the plasma membrane as a result of a presumed reduction of endocytosis. Binding of the specific VMAT inhibitor [3H]dihydrotetrabenazine indicated that expression did occur at a level of approximately 3 fmol per oocyte. To determine if rVMAT2-I483A/L484A expressed in oocytes was capable of substrate transport, oocytes were placed in buffer at pH 6.0, dopamine substrate was injected into the cell, and egress of substrate was monitored by fast scan cyclic voltammetry using a carbon fiber microelectrode. Under these conditions, transport by oocytes injected with RNA coding for rVMAT2-I483A/L484A ranged from approximately 0.5 to more than 2.5 pmol/min. Water-injected and uninjected control oocytes did not exhibit appreciable transport activity. Transport by rVMAT2-I483A/L484A-injected oocytes was reduced to control levels by tetrabenazine, a known inhibitor of VMAT transport activity. Comparison of subtracted voltammograms obtained from transport assays with those for calibration experiments confirmed that the transported species was dopamine. These results suggest that expression of VMATs in oocytes may provide a useful model system for mechanistic and regulatory studies that would not be feasible using traditional methods.

Determination of transport parameters of permeant substrates of the vesicular amine transporter

Biological transport of moderately permeant compounds is obscured by diffusion of the compounds back across the membrane, so characterization of the transport of such compounds requires correction for permeability. A relatively simple method for determining kinetic parameters for moderately permeant compounds is presented here. After evaluating a compound's apparent permeability coefficient, its steady-state uptake is measured as a function of concentration. By comparing the concentration dependence of uptake measured both in the presence and in the absence of a complete inhibitor of the transporter, K(m) and Vmax for transport of that substrate may be calculated. When used to analyze transport of tyramine and hydroxyephedrine by the vesicular amine transporter, this method yields results consistent with other methods and with values for analogous impermeant substrates. In bovine adrenal chromaffin vesicles, tyramine and (-)erythro-hydroxyephedrine have apparent permeability coefficients of 4.7 +/- 1.0 x 10(-9) and 1.1 +/- 0.4 x 10(-8) cm/s, respectively. Values for K(m) are 15 +/- 9 and 34 +/- 14 microM and for Vmax are 1.3 +/- 0.2 and 1.4 +/- 0.9 nmol/min.mg of membrane protein, respectively.

Transport of histamine by vesicular monoamine transporter-2

Histamine mediates signalling by a wide range of neural and non-neural cells including mast cells. Like other biogenic amines, histamine is released from specialized secretory vesicles and requires transport from the cytoplasm into these vesicles. Of the two vesicular monoamine transporters, histamine potently inhibits 3H-serotonin transport by one (VMAT2) but not the other (VMAT1). In addition, histamine-containing cells in both neural and non-neural cells express VMAT2. However, histamine lacks the hydroxyl groups generally considered necessary for recognition as a substrate by the vesicular monoamine transporters. Using a heterologous expression system, we now report that VMAT2 not only shows inhibition by histamine but also transports 3H-histamine. Interestingly, histamine differs from other monoamine transmitters and does not inhibit 3H-reserpine binding to VMAT2, indicating interaction at a distinct site. Surprisingly, reserpine inhibits histamine transport with much less potency than serotonin transport, suggesting a different transport mechanism. However, replacement of serines in the third transmembrane domain of VMAT2 that have been shown to be essential for recognition of other monoamines also eliminate 3H-histamine transport, suggesting that these serine residues may do more than simply recognize the hydroxyl groups on a monoamine substrate.

Cloning and functional expression of a tetrabenazine sensitive vesicular monoamine transporter from bovine chromaffin granules

Using oligonucleotide primers derived from the vesicular monoamine transporters sequences, a cDNA predicted to encode the bovine chromaffin granule amine transporter has been cloned (b-VMAT2). Surprisingly, its structure is more similar to the rat brain transporter (VMAT2), than to the rat adrenal counterpart (VMAT1). Unlike rat VMAT1, bovine VMAT2 appears to be expressed both in the adrenal medulla and the brain, as judged by Northern analysis. After modification/deletion of the seven amino acids at the N-terminus of the protein it was expressed in a functional form. The order of affinity of the bovine VMAT2 transporter to substrates is: serotonin > dopamine = norepinephrine > epinephrine. Also, the recombinant bovine adrenal transporter is highly sensitive to tetrabenazine, in sharp contrast to the rat adrenal transporter. The findings indicate, therefore, a clear species variation in which structure and function of the bovine adrenal transporter resemble the rat brain protein, while its tissue distribution is distinct from both types of rat proteins. In addition, the predicted protein sequence is identical to the experimentally determined N-terminus sequence of the purified vesicular amine transporter [Stern-Bach et al. (1992) Proc. Natl. Acad. Sci. USA 89, 9730-9733].

Disposition of endogenous adrenaline compared to noradrenaline released by cardiac sympathetic nerves in the anaesthetized dog

The fate of adrenaline released from cardiac sympathetic nerves was compared with that of noradrenaline before and during two periods of electrical stimulation of the left ansa subclavia in eight anaesthetized dogs. Cardiac spillovers and extractions of both catecholamines were estimated simultaneously using infusions of 3H-labelled adrenaline and noradrenaline. Animals were studied before and after neuronal uptake blockade with desipramine. Cardiac spillover of adrenaline, detectable at rest at 1.4 +/- 0.3 pmol/min, increased to 4.0 +/- 1.1 and 5.3 +/- 1.2 pmol/min during sympathetic stimulation. Cardiac noradrenaline spillover increased from 49 +/- 12 to 205 +/- 40 and 451 +/- 118 pmol/min. After desipramine, cardiac spillovers of adrenaline were decreased, whereas those of noradrenaline were increased so that the ratio of adrenaline to noradrenaline spillover, meaned before and during stimulation, decreased substantially from 1:42 to 1:166. The desipramine-induced decrease in cardiac extractions of 3H-labelled catecholamines indicated adrenaline was removed 60% less efficiently than noradrenaline by neuronal uptake, whereas the extractions remaining indicated adrenaline was removed 50% more efficiently by extraneuronal uptake. The differences in removal processes indicated that 35% of the adrenaline released by cardiac sympathetic nerves was recaptured compared to 88% for noradrenaline, leaving 53% to be removed extra-neuronally compared to 6.6% for noradrenaline, so that proportionally more released adrenaline than noradrenaline escaped to spillover into plasma (12% versus 5.4%). Since extra-neuronal uptake was more efficient for adrenaline than noradrenaline, proportionally less released adrenaline than noradrenaline escaped local removal to spillover into plasma when neuronal uptake was blocked (17% versus 45%). This reversed the situation before blockade so that desipramine substantially decreased the ratio of adrenaline to noradrenaline spillover. Thus, differences in the efficiencies of neuronal or extraneuronal uptake are important determinants of the amounts of locally released adrenaline and noradrenaline that escape removal processes to act at neuroeffector sites or spillover into plasma.

Characteristics of the transport of the quaternary ammonium 1-methyl-4-phenylpyridinium by chromaffin granules

1-Methyl-4-phenylpyridinium (MPP+), an active metabolite of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine which induces Parkinson's disease in man, is a substrate of the monoamine uptake system of chromaffin granules. It is accumulated without chemical modification by bovine chromaffin granule membrane vesicles in the presence of ATP. The transport is saturable and is characterized by a Km value of 0.8 microM at pH 8.0, similar to that of serotonin (5-HT). Transport occurs through the monoamine transporter since it is competitively inhibited by 5-HT and since MPP+ competitively inhibits [3H]5-HT uptake. Moreover, [3H]MPP+ uptake is blocked by the monoamine transporter inhibitors tetrabenazine and reserpine. Finally, MPP+ efficiently displaces [3H]reserpine and [3H]dihydrotetrabenazine from their binding sites on the transporter. In the pH range 6-8, the Km for [3H]MPP+ uptake and the EC50 of MPP+ for the displacement of [3H]dihydrotetrabenazine decrease logarithmically with the pH. MPP+ is the first quaternary ammonium salt shown to be a substrate of the monoamine transporter and it has the same pH-dependency as monoamines.

Inactivation of the catecholamine transporter during the preparation of chromaffin granule membrane 'ghosts'

The activity of the catecholamine transporter of chromaffin granules and the binding to these vesicles of reserpine, a transporter inhibitor, decrease during ghost preparation. In contrast, the number of binding sites of dihydrotetrabenazine, another transporter ligand, is constant. Dihydrotetrabenazine thus binds to an inactive transporter whereas reserpine binds only to active vesicles. Inactivation occurs during lysis of the granules, possibly because of an incomplete resealing. The turnover number of the transporter, determined by dividing the uptake activity by the density of dihydrotetrabenazine binding sites, has a maximal value (140 molecules/min) in intact granules. The reserpine to dihydrotetrabenazine binding ratio (10-25%) is an estimate of the proportion of correctly resealed vesicles.

Does the carrier of chromaffin granules transport the protonated or the uncharged species of catecholamines?

By osmotic lysis in the presence of urea ghosts (60-100 nmol catecholamine/mg prot.) were prepared from chromaffin granules (4-6 mumol catecholamine/mg prot.) of the bovine medulla. In the presence of 1-300 mumol/l 3H-catecholamine and ATP-Mg2+, ghosts show a net uptake of catecholamine. The net uptake is sensitive to reserpine or agents (uncouplers and ammonium) which diminish the electrochemical potential difference for protons at the granule membrane (delta p). The same uptake was found by 3H-counting or by fluorimetric measurements. At various pH-values (pH 6.2-8.2) the Km and Vmax of the ATP-stimulated rate of uptake of 3H-catecholamine into ghosts was determined (at 30 degrees C) to identify the species of catecholamine (protonated, uncharged, or anionic) which is the substrate for the granule carrier. The pH difference (delta pH = pHout - pHin) and the electrical potential difference (delta psi) were determined to calculate delta p under conditions of 3H-catecholamine uptake. When the pHout was increased (pH 6.2, 7.4, 8.2), the apparent Km of uptake decreased (50, 5, 1-2 mumol/l), showing a linear relation between pH and logarithm of Km. The Km was calculated for the uncharged catecholamine (with pK1 = 8.8 and pK2 = 10.0); it was nearly pH-independent and amounted to about 0.2 mumol/l. The Vmax declined only in the extreme pH-range. Between pH 6.6 and 7.8 Vmax and delta p showed a slight increase from 16 to 20 nmoles/(mg prot. X min) and from 110 to 140 mV, resp. In the same pH-range the pHin inside ghosts increased from pH 5.2 to 5.7, whereas delta psi was constant (30 mV). At constant pHout (= 7.3) ammonium (0-30 mmol/l) caused an increase of pHin from 5.5 to 6.6. The increase of pHin was accompanied by an increase of Km from 5 to 20 mumol/l 3H-catecholamine and by a decrease of both Vmax and delta p from 20 to 5 nmoles/(mg prot. X min) and from 123 to 85 mV, respectively. From the dependence of the Km of uptake on pHout is concluded that the uncharged species of catecholamine is transported, whereas the dependence of Km on pHin suggests that the translocation of the catecholamine-carrier complex across the granule membrane is not the rate-limiting step of catecholamine uptake.