Inhibition by hydralazine of the conversion of dopamine to noradrenaline in rat atria in vitro and in vivo

Effects of hydralazine on contractile responses to alpha 1 and alpha 2-adrenoceptor agonists in isolated rubbed rat aorta

1. Effects of hydralazine on contractile responses to noradrenaline (an alpha 1- and alpha 2-adrenoceptor agonist) to phenylephrine and methoxamine (both selective alpha 1-adrenoceptor agonists) and to clonidine and BHT-920 (both relatively selective alpha 2-adrenoceptor agonists) were examined in isolated rat aorta deprived of endothelium. Hydralazine (1 mM) produced a rightward shift with depression of the maximal tension of the concentration-response curves for all the agonists tested. The effects on curves for clonidine and BHT-920 (partial agonists) were greater than on curves for noradrenaline, phenylephrine and methoxamine (full agonists). 2. The inhibitory effect of prazosin (pA2, about 10) was much greater than that of yohimbine (pA2, about 7) for all the agonists. 3. In tissues pretreated with phenoxybenzamine, hydralazine (1 mM) inhibited the residual response to all the agonists. The inhibitory effect on residual response to full agonists was similar to that observed on response to partial agonists in tissues not treated with phenoxybenzamine. 4. The relationship between maximal response and percentage receptor occupancy was nonlinear for full agonists, but near-linear for partial agonists. 5. These results indicate that the responses induced by noradrenaline, phenylephrine, methoxamine, clonidine and BHT-920 in the rat aorta are due to the activation of alpha 1-adrenoceptors and confirm the vasorelaxant action of hydralazine. 6. These results also suggest that the differential effects of hydralazine on the responses to alpha-adrenoceptor agonists may be due to differences in the amount of receptor reserve available available in this blood vessel for full agonists (noradrenaline, phenylephrine or methoxamine) and partial agonists (clonidine or BHT-920).

Inhibition of dopamine beta-hydroxylase by bidentate chelating agents

1-2H-Phthalazine hydrazone (hydralazine; HYD), 2-1H-pyridinone hydrazone (2-hydrazinopyridine; HP), 2-quinoline-carboxylic acid (QCA), 1-isoquinolinecarboxylic acid (IQCA), 2,2'-bi-1H-imidazole (2,2'-biimidazole; BI), and 1H-imidazole-4-acetic acid (imidazole-4-acetic acid; IAA) directly and reversibly inhibit homogeneous soluble bovine dopamine beta-hydroxylase (3,4-dihydroxyphenethylamine, ascorbate:oxygen oxidoreductase (beta-hydroxylating), EC 1.14.17.1). HYD, QCA and IAA show competitive allosteric inhibition of dopamine beta-hydroxylase with respect to ascorbate (Kis = 5.7(+/- 0.9) microM, 0.14(+/- 0.03) mM, 0.80(+/- 0.20) mM; nH = 1.4(+/- 0.1), 1.8(+/- 0.4), 2.8(+/- 0.6), respectively). HYD and IAA show slope and intercept mixed-type allosteric inhibition of dopamine beta-hydroxylase with respect to tyramine. QCA shows allosteric uncompetitive inhibition of dopamine beta-hydroxylase with respect to tyramine. HP, BI and IQCA all show linear competitive inhibition (Kis = 1.9(+/- 0.3) microM, 21(+/- 6) microM, and 0.9(+/- 0.3) microM, respectively) with respect to ascorbate. HP and BI show linear mixed-type while IQCA shows linear uncompetitive inhibition of dopamine beta-hydroxylase with respect to tyramine. In the presence of HP, HYD or IAA intersecting double-reciprocal plots of the initial velocity as a function of tyramine concentration at differing fixed levels of ascorbate are observed. These findings are consistent with a uni-uni-ping-pong-ter-bi kinetic mechanism for dopamine beta-hydroxylase that involves a ternary enzyme-ascorbate-tyramine-oxygen complex. The results for HYD, QCA and IAA are the first examples of allosteric inhibitor interactions with dopamine beta-hydroxylase.

Reflex activation of renal nerves in humans: differential effects on noradrenaline, dopamine and renin overflow to renal venous plasma

Using a thermodilution technique for renal venous blood flow measurements, renal sympathetic nerve activity was evaluated in 10 healthy volunteers by measurements of noradrenaline (NA) and dopamine (DA) overflow to renal venous plasma. Renin release was measured simultaneously. At rest, arterial adrenaline (ADR) levels were 0.24 +/- 0.03 nmol-1 and NA and DA levels were higher in renal venous than in arterial plasma (1.24 vs. 0.98 and 0.14 vs. 0.09 nmol l-1, respectively, P less than 0.01 for both). The renal extraction of ADR from arterial plasma was 40 +/- 4%. ADR extractions were used to correct for the renal removal of NA or DA from arterial plasma when calculating the renal overflow of NA or DA to renal venous plasma. At rest, the thus corrected renal venous overflows of NA and DA were 228 +/- 34 and 29 +/- 3 pmol min-1, respectively. Isometric handgrip exercise (IHG) increased renal vascular resistance (RVR) by 20% and NA overflow by 123%, without altering renin release or DA overflow. Vasodilatation induced by dihydralazine (HYDR) increased NA overflow by 63% (P less than 0.05) and elevated DA overflow by 107 +/- 59%. The renal DA/NA overflow ratio was reduced from 0.15 to 0.06 (P less than 0.01) during IHG, but was not altered by HYDR. Renin release increased by 377% after HYDR (P less than 0.001) and was correlated to the reduction of mean arterial pressure but not changes in NA overflow. Thus, both IHG and HYDR increased renal sympathetic nerve activity, although differential effects on renin release and DA overflow were observed. The dissociation of renal NA and DA responses suggests that the human kidney may have a subset of dopaminergic nerves.

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.

A radionuclide method for the simultaneous study of the effects of drugs on central and peripheral haemodynamics

1 The central and peripheral cardiovascular effects of hydralazine and glyceryl trinitrate (GTN) have been contrasted using radionuclide techniques.

2 Following intravenous injection of technetium-99m labelled human serum albumin, radionuclide ventriculography was performed by the equilibrium blood pool method using a mobile gamma camera. Simultaneous measurements of `peripheral venous volume' were made using a collimated scintillation probe positioned above the patient's calf.

3 Ten patients with angina pectoris were studied at rest, after sublingual administration of 0.5 mg GTN and after intravenous administration of 10 mg hydralazine.

4 GTN caused a mean reduction in the end diastolic volume of the left ventricle of 14.6% ± 4.5% (P < 0.005) but ejection fraction increased by 0.034 ± 0.007 (P < 0.005) so that stroke volume was only reduced by 4.9% ± 5.0% (NS). There was a mean increase in heart rate of 10.8 ± 2.3 beats/min (P < 0.001) but no significant change in cardiac output. The calculated systemic vascular resistance fell by 10.0% ± 5.4% (P < 0.05). Associated with these changes there was a mean increase of 9.6% ± 1.5% (P < 0.05) in the counts from the calf.

5 Hydralazine caused a significant reduction in blood pressure and increase in heart rate. End-diastolic volume was reduced by 6.0% ± 2.7% but there was a mean increase in ejection fraction of 0.058 ± 0.010 (P < 0.001) so that in this instance stroke volume increased by 9.0% ± 3.7% (P < 0.05) and cardiac output increased by 16.4% ± 4.4% (P < 0.005). The calculated systemic vascular resistance fell by 18.9% ± 3.8% (P < 0.001). Despite these haemodynamic changes there was no significant change in counts from the calf.

6 The results confirm that GTN has a predominant venodilator effect while hydralazine acts largely on the arterial bed. These relatively simple radionuclide methods will allow a more detailed assessment of the cardiovascular effects of drugs.