ISSN:
1432-1912
Keywords:
Isoprenaline
;
Submaxillary gland
;
Extraneuronal catecholamine uptake
;
“O-methylating systems”
;
Corticosteroids
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary 1. The uptake and O-methylation of 3H-(±)isoprenaline was studied in slices of the rat submaxillary gland. 2. The initial uptake of 3H-isoprenaline after inhibition of catechol-O-methyl transferase (COMT) was described by a single saturable process with relatively high K m (311 μM) and V max (101 nmoles·g−1·min−1). Both corticosterone and normetanephrine were competitive inhibitors of uptake. 3. When examined at substrate concentrations lower than the K m for uptake (and after block of COMT), 3H-isoprenaline distributed into two compartments in the tissue which approached equilibrium with half times of 2.4 and 15.8 min. The filling of both compartments was inhibited by corticosterone or phenoxybenzamine and also by high-K+ medium (in which 118 mM NaCl of the incubation medium had been replaced by KCl), but remained unaffected on substituting 118 mM NaCl with Tris-HCl. 4. In tissues in which COMT was not inhibited, the metabolism of 3H-isoprenaline to 3H-O-methylisoprenaline proceeded at a constant rate from the beginning of the incubation with the amine. When the substrate concentration was very low, little unchanged 3H-isoprenaline was found in the tissue. On the other hand, at high substrate concentrations the parent amine accumulated in the tissue, and at a time when 0-methylation had reached a steady state, the accumulation of 3H-isoprenaline was continuing. 5. The formation of 3H-O-methylisoprenaline was impaired by the presence of corticosterone, normetanephrine, phenoxybenzamine or 17-β-oestradiol with no indication of inhibition of COMT. While lowering the external Na+ concentration (on replacing 118 mM NaCl by 236 mM sucrose) did not affect the formation of 3H-O-methylisoprenaline, replacement of 118 mM NaCl by KCl reduced it. 6. The dependence of the steady-state rate of formation of 3H-O-methylisoprenaline on the substrate concentration in the incubation medium showed that two saturable components participated in the O-methylation of 3H-isoprenaline (low K m system: K m =7.2 μM and V max=1.2 nmoles·g−1·min−1; high-K m system: K m =339 μM and V max=4.6 nmoles·g−1·min−1). Corticosterone and normetanephrine competitively inhibited both the low-K m and the high-K m O-methylation. 7. The results indicate that the submaxillary gland of the rat resembles other tissues in having a low-K m (high-affinity) “O-methylating system” as well as a high-K m (low-affinity) extraneuronal uptake mechanism for catecholamines.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00497006
