Dihydralazine and catecholamine-synthesizing enzymes in spontaneous hypertension

Regional specificity of long-term regulation of tyrosine hydroxylase in some catecholaminergic rat brainstem areas. II. Effect of a chronic dihydralazine treatment

Dihydralazine, which is used in the treatment of hypertension, causes a long-lasting hypotensive action by a direct vasodilator effect on arteriolar smooth muscle. The present study was carried out to investigate the effect of a daily single injection of dihydralazine (20 mg/kg, s.c.) for 14 days on the tyrosine hydroxylase (TH) protein quantity in some catecholaminergic rat brainstem areas such as the dorsomedial medulla (DMM), the ventrolateral medulla (VLM) and the locus coeruleus (LC). This study demonstrates that the dihydralazine produced (1) an 85% increase in TH protein quantity exclusively in the rostral part of DMM, (2) a 58% increase of TH protein content exclusively in the rostral part of the LC, and (3) a 37% increase of the TH protein quantity in VLM catecholaminergic area. To determine whether the increase in TH protein quantity could be related to a change in norepinephrine (NE) content, the rate constant of disappearance (k) of NE was measured in the catecholaminergic regions of the same rats treated with dihydralazine. Our results show that dihydralazine causes an increase of the TH protein, in addition to an elevation of NE content, within the subpopulations of catecholaminergic structures. These data suggest a selective response of the TH regulation to dihydralazine within the rostral DMM area which receives barosensory inputs.

Inhibitory action of hydralazine on catecholamine secretion from cultured bovine adrenal chromaffin cells

Hydralazine caused a concentration-dependent inhibition of the secretion of catecholamines induced by carbamylcholine or high K+ from cultured bovine adrenal chromaffin cells, and also caused the significant inhibition of radioactive calcium uptake induced by carbamylcholine into the cells. However, hydralazine failed to inhibit the secretion of catecholamines evoked by the calcium-ionophore, A23187. The inhibitory action of hydralazine on catecholamine secretion induced by carbamylcholine was not affected by increasing the concentration of calcium ion in the reaction mixture. These observations therefore seem to indicate that the inhibitory action of hydralazine is not due to either the blocking of receptors for carbamylcholine or the disruption of the secretory machinery, and suggest that the drug may cause the inhibition of catecholamine secretion through its blocking action on calcium influx into the cells.

Inhibitory action of hydralazine on catecholamine-synthesizing enzymes prepared from bovine adrenal medulla

The direct effect of hydralazine on catecholamine-synthesizing enzymes was investigated. Hydralazine caused a concentration-dependent inhibition of tyrosine hydroxylase (TH) prepared from bovine adrenal medulla, and a more pronounced effect was obtained by incubating the enzyme with the drug prior to the enzyme assay. Kinetic studies showed that hydralazine increased the apparent Km value of the enzyme for tyrosine and cofactor, 6,7-dimethyl-5,6,7,8-tetrahydropterin (DMPH4), without any change in the Vmax. The inhibitory effect of the drug was irreversible, and an excess amount of FeSO4 failed to restore the enzyme activity inhibited by this drug. Furthermore, hydralazine also inhibited the dopamine beta-hydroxylase (DBH) in chromaffin granule membranes. Hydralazine increased the apparent Km value of DBH for ascorbic acid without any change in the Vmax, and it decreased the Vmax of the enzyme for tyramine with no change in the apparent Km value. The observations described here suggest the possibility that hydralazine presumably causes the inhibition of catecholamine-synthesizing enzymes as a result of allosteric alterations in the molecular structures of these enzymes. It thus seems unlikely that the inhibitory action of hydralazine on these enzymes may totally be based on its metal-chelating activity.

Increase in noradrenaline-synthesizing enzyme activity in medulla oblongata in Parkinson's disease

Dopamine beta hydroxylase (DBH), The noradrenaline-synthesizing enzyme, and phenyl-ethanolamine-N-methyltransferase (PNMT), the adrenaline-synthesizing enzyme, were assayed in 18 areas of brain stem in eight cases of parkinsonian syndromes and of four age- and postmortem delay-matched controls. Dissection was performed by the "punch" technique and enzyme activities assayed by radiometric methods. No significant change was found for PNMT activity. DBH activity was significantly increased in the A2-C2 area of the medulla oblongata (including the nucleus tractus solitarius) in the cases of Parkinson's disease. The A2-C2 area is known to be implicated in the control of blood pressure in rats. These findings are discussed in relation to orthostatic hypotension and the influence of L-dopa therapy.

Central dopaminergic neurons during development of genetic and DOCA-salt hypertension in the rat

The in vivo binding of [3H]spiroperidol was measured in discrete areas of the brain in 7-, 9- and 16-week-old spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto (WKY) controls. An increase in the [3H]spiroperidol binding in the striatum, tuberculum olfactorium and frontal cortex but not in the cerebellum was detected at all ages in SHR. The increase was more pronounced in 7- than in 9- or 16-week-old SHR. In vitro data indicated an increase in Bmax but no variation in Kd in the striatum of 7-week-old SHR. Moreover no difference was detectable in the dopaminergic cell bodies (A9, A10). This increase was specific to [3H]spiroperidol binding sites since no difference was observed in the in vivo binding of [3H]QNB and [3H]LSD in the same brain regions. No variation in dopamine level or dopamine utilization, as estimated by measuring the disappearance of the amine induced by alpha-methyl-p-tyrosine, was observed. The DOPA accumulation after injection of the DOPA decarboxylase inhibitor NSD 1015 was greater in the tuberculum olfactorium from 7-week-old SHR. An increase in [3H]spiroperidol binding sites was also observed in the striatum and tuberculum olfactorium after 7 weeks of DOCA-salt treatment. These results suggest that dopaminergic neurons might be implicated in the onset of hypertension in the rat.