The accumulation and metabolism of (-)-noradrenaline by cells in culture

Extraneuronal noradrenaline transport (uptake2) in a human cell line (Caki-1 cells)

This study describes for the first time an experimental system for the extraneuronal transport mechanism of noradrenaline (uptake 2) which is based on a clonal cell line (Caki-1). Caki-1 cells were originally derived from a human renal cell carcinoma. The conclusion that these cells express uptake 2 is supported by several experimental findings. (1) The initial rate of 3H-noradrenaline uptake in Caki-1 cells is saturable, the Km being 450 mumol/l. (2) Inhibitors of uptake 2 such as corticosterone (1 mumol/l) and O-methyl-isoprenaline (100 mumol/l) largely inhibit 3H-noradrenaline uptake in Caki-1 cells. Whereas inhibitors of the neuronal transport mechanism for noradrenaline (uptake 1) such as desipramine (1 mumol/l) and cocaine (10 mumol/l) do not reduce it. (3) Depolarization of Caki-1 cells by the elevation of extracellular potassium inhibits 3H-noradrenaline uptake. (4) There is a highly significant correlation between the Ic50's of various compounds for the inhibition of 3H-noradrenaline uptake in Caki-1 cells and rabbit aorta known to possess uptake2. Interestingly enough, uptake 2 in Caki-1 cells and rabbit aorta is inhibited by cimetidine, quinidine and procainamide which are substrates of the renal transport mechanism for organic cations. Moreover, 3H-cimetidine is shown to be a substrate of uptake 2 in the isolated perfused rat heart. These results indicate a striking similarity between uptake 2 and the renal transport mechanism for organic cations.

Kinetic constants of isoprenaline and corticosterone for extraneuronal uptake in different cell types from various tissues

Extraneuronal uptake of isoprenaline was studied in a number of different, histologically identified, cell types: trachealis smooth muscle cells, vascular smooth muscle cells and myocardial cells. Segments of cat, rat or rabbit trachea, dog coronary artery or cat atria were incubated in isoprenaline, in the presence of U-0521 (100 mumol l-1) to inhibit catechol-O-methyltransferase. The intensities of formaldehyde-induced fluorescence (due to accumulation of isoprenaline) were measured, using microphotometry, in the appropriate cells in histological sections of the tissue. Endogenous fluorescence in adrenergic nerves was removed by pretreatment of rats with 6-hydroxydopamine and of cats and rabbits with reserpine. Segments of dog coronary artery were incubated with 6-hydroxydopamine in vitro to remove neuronal fluorescence. This treatment was also shown to be effective in guinea-pig trachea without any influence on the determination of the kinetic parameters of isoprenaline uptake in the trachealis smooth muscle cells. Fluorescence in all cell types studied was shown to represent isoprenaline which had accumulated by extraneuronal uptake, in that fluorescence was reduced by drugs which inhibit extraneuronal uptake (corticosterone, normetanephrine, metanephrine and/or phenoxybenzamine), by exposure to a K+-Krebs solution and by incubating tissues in isoprenaline at 0 degree C rather than 37 degrees C. In each cell type, isoprenaline uptake obeyed Michaelis-Menten saturation kinetics, both in the absence and presence of corticosterone. Corticosterone was a competitive inhibitor of isoprenaline uptake in all the cell types.(ABSTRACT TRUNCATED AT 250 WORDS)

The cellular location of catechol-O-methyltransferase in rat liver

Catechol-O-methyltransferase (COMT) activity on the extracellular face of the plasma membrane of isolated rat hepatocytes was assayed, and 4.3% of total COMT activity was located there in cells which satisfied our criteria of viability. However, since 1.2% of the cells' lactate dehydrogenase activity was also apparently extracellular, and this proportion increased to 3.4% under the conditions of the COMT assay the amounts of extracellular COMT may be even less. COMT in rat liver microsomes and plasma membranes represent 2.3% and 0.08% of total rat liver COMT respectively. This implies an insignificant role for plasma membrane COMT although reported altered kinetic behaviour could elevate microsomal COMT to a supporting role in the regulation of catecholamine concentration in the circulation. Since by far the largest fraction of COMT is located intracellularly in the soluble cell fraction, the physiological functions of COMT seem to be dependent on the passage of substrates through the cell membrane for their presentation to the enzyme.

Kinetic analyses of the mechanisms of action of inhibitors of the extraneuronal uptake of adrenaline in smooth muscle cells of guinea-pig trachea

1. Tracheal segments from guinea-pigs pretreated with 6-hydroxydopamine were incubated with adrenaline (50--300 microM) at 37 degrees C for 5 min in the absence or presence of an extraneuronal uptake inhibitor (normetanephrine, corticosterone or phenoxybenzamine). Catechol-O-methyl transferase and monoamine oxidase were inhibited by 100 microM U-0521 and pargyline, respectively. Tissues were prepared for fluorescence histochemistry, and accumulated adrenaline in trachealis smooth muscle cells was measured by fluorescence microphotometry. Initial rates of adrenaline uptake were calculated. 2. In the absence of an extraneuronal uptake inhibitor, adrenaline uptake obeyed Michaelis-Menten saturation kinetics. The mean Km value for adrenaline from a total of 25 guinea-pigs was 157 microM. 3. The mechanisms of action of three extraneuronal uptake inhibitors were determined by analysis of initial rate kinetic data for adrenaline uptake. The results showed that normetanephrine and corticosterone were reversible, competitive inhibitors and phenoxybenzamine was an irreversible inhibitor of extraneuronal uptake in the smooth muscle cells. 4. Two methods of analysis were used to obtain estimates of the affinities of the reversible inhibitors for extraneuronal uptake. The Ki values calculated by Marquardt non-linear least squares regression analysis and Hunter-Downs method, respectively, were normetanephrine: 2.75 microM and 3.47 microM, and corticosterone: 1.52 microM and 1.44 microM. 5. Normetanephrine and corticosterone had higher affinities than adrenaline for extraneuronal uptake in the trachealis smooth muscle cells.

The uptake kinetics and metabolism of extraneuronal noradrenaline in guinea-pig trachea as studied with quantitative fluorescence microphotometry

1 Extraneuronal uptake of noradrenaline and alpha-methylnoradrenaline into single cells of guinea-pig tracheal smooth muscle have been studied by means of quantitative fluorescence microphotometry. 2 Fluorescence brightness due to accumulation of alpha-methylnoradrenaline was dose-dependent and was increased by the catechol-O-methyltransferase inhibitor drugs, tropolone or beta-thujaplicin (200 micrometer) but not by 3,4-dimethoxy-5-hydroxybenzoic acid (200 microgram). 3 Fluorescence brightness due to accumulation of noradrenaline was increased if animals were pretreated with the monoamine oxidase inhibitor drug, nialamide. 4 The study suggests that metabolism of amines by catechol-O-methyltransferase and monoamine oxidase can occur subsequent to extraneuronal uptake is guinea-pig tracheal smooth muscle. 5 The uptake of noradrenaline into tracheal smooth muscle was concentration-dependent, saturable, and had a Km of 156 micrometer.

Initial investigations into adrenaline accumulation and adrenergic responsiveness in cultured neonatal rat heart cells

1 Accumulation of tritiated adrenaline (and/or its products) has been studied by a variety of techniques in viable, attached and free-floating myoblasts cultured from neonatal rat heart cells. 2 Accumulation increased with the number of cells used, the culture age of cells, incubation time and the concentration of adrenaline in the incubation mixture. 3 Accumulation-concentration data were not linearly correlated with dose-response data. 4 Accumulation did not correlate well with beta-adrenoceptor responsiveness in a number of situations of varying responsiveness. 5 Accumulation was not blocked by a variety of uptake blocking agents or alpha- and beta-adrenoceptor antagonists. 6 A much smaller accumulation process was observed in fibroblasts which respond to adrenaline with reversible morphological changes.