Thyrotropin releasing hormone and naloxone attenuate the antihypertensive action of central alpha 2-adrenoceptor stimulation through different mechanisms.

In various forms of shock, TRH is equivalent to naloxone in reversing the hypotension and improving the survival rate. The present findings indicate that in spontaneously hypertensive rats (SHR), TRH has another naloxone-like effect in antagonizing the antihypertensive response to clonidine and alpha-methyldopa. When given during the hypotensive response to alpha-methyldopa, both naloxone and TRH produce a pressor response. While this effect of naloxone is blocked by prazosin, the effect of TRH is not influenced by prazosin or hexamethonium but is inhibited by a vasopressin pressor antagonist. This suggests that the pressor response to naloxone is mediated by the sympathetic nervous system, whereas the similar action of TRH is independent of sympatho-adrenomedullary functions and it is mediated by vasopressin.

alpha-Methyldopa produces mydriasis in the rat by stimulation of CNS alpha 2-adrenoceptors.

1. The effects of i.v. administration of alpha-methyldopa (MD) on rat pupil diameter were investigated. All experiments were carried out in rats in which vagosympathetic nerve trunks were sectioned bilaterally at the cervical level. 2. In anaesthetized rats MD produced a marked dose-related increase in pupil diameter. The onset of pupillary response to MD was gradual and reached maximal levels 2-3 h after administration. 3. Pretreatment with alpha 2-adrenoceptor antagonists yohimbine (1.5 mg kg-1, i.v.) or idazoxan (0.5 mg kg-1, i.v.) blocked the pupillary response to MD. In contrast, the alpha 1-antagonists prazosin (1.0 mg kg-1, i.v.) and phenoxybenzamine (1.5 mg kg-1, i.v.) did not significantly alter the pupillary effects of MD. 4. Selective enzymatic blockade with 3-hydroxy-benzyl-hydrazine (NSD-1015; 25 mg kg-1, i.p.), a dopa-decarboxylase inhibitor, as well as bis (4-methyl-homopiperazinyl-thiocarbonyl) disulphide (FLA-63, 5.0 mg kg-1, i.p.), a dopamine-beta-hydroxylase inhibitor, prevented the mydriatic effect of MD. 5. The above findings support the hypothesis that MD produces a clonidine-like CNS mydriasis in the rat. This effect appears to be mediated primarily by the MD metabolite, alpha-methylnoradrenaline. 6. These results indicate that MD produces mydriasis in the rat by a CNS action. The mydriatic action of MD appears to be produced by its metabolite alpha-methylnoradrenaline which in turn stimulates CNS postsynaptic alpha 2-adrenoceptors.

Modification by drugs of the secretagogue effect of dopamine on the pancrease.

1 The specific stimulating action of dopamine and L-DOPA on exocrine pancreatic secretion was further investigated in the isolated blood-perfused canine pancreas.2 6-Hydroxydopamine (100 mug, i.a.) stimulated the secretion but was far less potent than dopamine. Epinine (0.3-1 mg. i.a.), alpha-methyldopamine (10-300 mug, i.a.) and octopamine (10-300 mug, i.a.) were ineffective.3 alpha-Methyldopa (30 mg, i.a.) inhibited the stimulating effect of L-DOPA (1-3 mg) on pancreatic secretion.4 Apomorphine (1 mg, i.a.) attenuated dopamine-induced (1-30 mug) pancreatic secretion but did not antagonize secretin-induced (0.03-0.3 units) or pancreozymin-induced (0.3-1 units) secretion.5 These observations suggest that L-DOPA is probably converted to dopamine to stimulate the exocrine pancreas, and that dopamine interacts with the specific receptors.6 The noradrenaline and dopamine content of the canine pancreas was estimated in controls and in dogs treated with secretin, reserpine, L-DOPA or alpha-methyldopa. The values for dopamine and noradrenaline in controls were 139 +/- 6 and 375 +/- 40 ng/g tissue (n=13), respectively. Reserpine reduced the noradrenaline content of the pancreatic tissue without affecting the dopamine content. L-DOPA or secretin caused a significant increase in the dopamine, but not in the noradrenaline content. It is suggested that dopamine has a physiological function in the pancreas which is independent of that of the noradrenaline-containing nerve fibres.

Mechanism of the hypotensive action of methyldopa in normal and immunosympathectomized rats.

1. The antihypertensive action of methyldopa was investigated in the light of the prevailing false sympathetic neurotransmitter hypothesis of Day & Rand (1963). Immunosympathectomized and normal animals were used for the investigation.2. Methyldopa in a single injection significantly decreased the blood pressure in control hypertensive, immunosympathectomized normotensive, and normal rats. Guanethidine did not decrease the blood pressure of immunosympathectomized rats; it decreased the blood pressure of control and control hypertensive rats.3. The hypotensive effect of methyldopa was not antagonized by adrenoceptor and ganglion blocking agents or by the inhibition of dopamine beta-oxidase.4. High doses of methyldopa produced less hypotension than relatively low doses. This might be due to an antagonism of the hypotensive effect by methylnoradrenaline, which is formed from methyldopa.5. The antihypertensive action of methyldopa could not be correlated with any change in aortic sodium or potassium.6. It is concluded that methyldopa does not exert its hypotensive effect by producing a weakly active false sympathetic neurotransmitter.

An investigation of alpha-methyl amino-acids and their derivatives on isolated tissue preparations.

1. The ability of alpha-methyl amino-acids and their corresponding amines to restore the sympathomimetic actions of tyramine, and the uptake of the amino-acids and the amines, were studied in isolated tissue preparations obtained from reserpine pretreated animals.2. Tyramine relaxed isolated rat ileum preparations from non-reserpinized rats but not from reserpine treated animals. alpha-Methyldopa, alpha-methylnoradrenaline and lower concentrations of metaraminol restored the responses of reserpinized preparations. alpha-Methyldopamine, alpha-methyl-m-tyrosine, alpha-methyl-p-tyrosine and higher concentrations of metaraminol did not do so. The restoring effect of alpha-methyldopa was blocked by disulphiram. alpha-Methyl-p-tyrosine or alpha-methyl-m-tyrosine blocked the restoring action of alpha-methyldopa but not of alpha-methylnoradrenaline. Cocaine blocked the restoration of responses to tyramine by alpha-methylnoradrenaline but not by alpha-methyldopa. alpha-Methyldopa and alpha-methylnoradrenaline failed to restore responses to tyramine in the presence of sodium-free Tyrode solution.3. Tyramine increased the perfusion pressure in isolated rabbit ear preparations obtained from non-reserpinized animals but was very much less active in preparations obtained from reserpine treated animals. alpha-Methyldopa, alpha-methyl-m-tyrosine, alpha-methylnoradrenaline, alpha-methyl-p-tyrosine and metaraminol restored the effects of tyramine. alpha-Methyldopamine did not do so. The restoring effect of alpha-methyldopa and alpha-methyl-m-tyrosine was blocked by disulfiram. alpha-Methyl-p-tyrosine blocked the restoring effect of alpha-methyl-m-tyrosine.4. Tyramine produced positive inotropic effects in isolated rabbit heart preparations. This was either reduced or absent in preparations obtained from reserpine pretreated animals. alpha-Methyldopa, alpha-methylnoradrenaline, alpha-methyl-m-tyrosine and metaraminol restored the responses to tyramine. alpha-Methyldopamine and alpha-methyl-p-tyrosine did not do so.

Pretreatment with quinpirole inhibits the central antihypertensive effects of rilmenidine and alpha-methyldopa in conscious rats.

Treatment of conscious spontaneously hypertensive rats (SHR) with the dopamine D2 receptor agonist quinpirole causes a short-lasting pressor response and apparent desensitisation to the effects of subsequent injections of quinpirole or central antihypertensives such as clonidine. In the present study, a number of aspects of this apparent desensitisation were investigated. Thirty minutes after intravenous injection of quinpirole into spontaneously hypertensive rats, treatment with the dopamine D2 receptors antagonist raclopride caused a significant fall in blood pressure. At this time point, circulating levels of vasopressin were not significantly different compared to controls. In Brattleboro rats, the pressor response to quinpirole was reduced in the first 15 min after injection, but not difference in blood pressure was observed at later time points. In SHR which had been treated with quinpirole, the central antihypertensive effects of rilmenidine or alpha-methyldopa were significantly inhibited. By contrast, the bradycardia induced by these drugs was similar in quinpirole-treated rats and controls. Quinpirole pretreatment caused an enhancement of the hypotension but a reduction of the reflex tachycardia after intravenous treatment with hydralazine. In SHR treated with methylatropine and quinpirole, the upper plateau of the sympathetic baroreceptor-heart rate reflex curve was reduced. These results show that treatment with quinpirole has marked effects on central sympathetic vasomotor mechanisms which are the target of antihypertensive drugs such as rilmenidine and alpha-methyldopa. At least some of these effects may occur at the level of the sympathetic baroreflex. Moreover, while the effects of quinpirole on sympathetic regulation are prolonged, the initial pressor response is counteracted by an as yet unidentified compensatory mechanism which can be unmasked when quinpirole is displaced from its receptor by dopamine D2 receptor antagonist treatment.

Reduction of the hypotensive effect of clonidine and alpha-methyldopa by various psychotropic drugs.

1. In chloralose-anaesthetized cats the centrally induced hypotensive effect of clonidine, and in some cases alpha-methyldopa, was diminished by pretreatment with a variety of tricyclic antidepressants and neuroleptic agents. 2. The interaction is assumed to occur at the central alpha-adrenoreceptors. Clonidine or alpha-methyl-noradrenaline (from alpha-methyldopa) are the agonists, and the aforementioned psychotropic drugs are the antagonists. 3. Psychotropic drugs which are not alpha-receptor blockers, like butyrophenone neuroleptics (pimozide and haloperidol) or benzodiazepine tranquillizers, do not significantly diminsh the centrally induced hypotensive effect of clonidine.

The interaction between alpha-methyl-dopa and tricyclic antidepressants.

The central hypotensive action of alpha-methyl-DOPA is antagonized by tricyclic antidepressants like desipramine and imipramine. This antagonism has been demostrated in chloralose-anaesthetized cats. The drugs to be tested were infused into the left vertebral artery. The antagonism pProbably takes place in the pons-medulla region of the CNS. Probably, the alpha-adrenolytic properties of the tricyclic antidepressants bring about the blockade of central alpha-adrenergic receptors that are stimulated by alpha-methyl-noradrenaline, the biotransformation product of alpha-methyl-DOPA.

Intestinal absorption of alpha-methyldopa: in vitro mechanistic studies in rat small intestinal segments.

The mechanism of intestinal uptake of alpha-methyldopa (Aldomet) was investigated using isolated segments of rat small intestine. Incubations were limited to 2 to 5 min as histological examination of the tissue showed significant loss of structural integrity after 10 to 20 min at 37 degrees C. alpha-Methyldopa in the tissue was extracted and assayed by high-pressure liquid chromatography. Corrections were applied for uptake into extracellular spaces using inulin (14C). Uptake was temperature- and concentration-dependent (Km congruent to 10 mM), was dependent on the location in the small intestine and was inhibited by ouabain (2 mM) or lack of sodium or glucose in the incubation medium. alpha-Methyldopa uptake also was inhibited by other neutral amino acids. The mucosal cell layer accounted for approximately 50% of the total drug accumulated in the tissue. Uptake was less when the serosal surface was exposed than when the tissue was everted. The similarity in uptake parameters between alpha-methyldopa and L-phenylalanine (parallel experiments) suggests that alpha-methyldopa is principally absorbed into rat small intestinal mucosal cells via an amino acid transport system.

In vitro study of the effect of clonidine on human umbilical artery motility.

The umbilical-placental circulation is of vital importance for fetal survival and has a dominant effect on the cardiorespiratory physiology of the fetus. The mechanisms of regulation of umbilical vessels appear to differ from those regulating other vessels. Both clonidine and alpha-methyldopa have been used in the management of hypertensive complications of pregnancy. In contrast to alpha-methyldopa, clonidine does not require previous metabolization but acts directly on the receptors. The action of the two drugs on human umbilical artery was studied in vitro. Forty-eight human umbilical artery segments were dissected and perfused as follows: 9 segments with alpha-methyldopa, 10 with clonidine, 10 with yohimbine, 9 with alpha-methyldopa in combination with yohimbine, and 10 with pure Tyrode's solution. alpha-Methyldopa had a statistically significant vasoconstrictive effect starting at 40 min (p < 0.05) and this effect was blocked by yohimbine (p < 0.05). Clonidine had no vasoconstrictive effect. The present data for clonidine do not confirm the presence of alpha 2-adrenergic receptors in the umbilico-placental circulation which had been indicated by the action of methyldopa.

CNS alpha 2-adrenoceptor induced mydriasis in conscious rats.

Clonidine (3-300 microgram/kg, i.p.) produced dose-dependent pupillary dilation in unanesthetized rats. At low levels of illumination (30 lux), this effect was similar to that seen in anesthetized animals but was diminished under higher ambient lighting conditions. alpha Methyldopa (60 mg/kg, i.p.) produced mydriasis that took several hours to develop and was prevented by pretreatment with a dopamine beta-hydroxylase inhibitor (FLA-63). Yohimbine antagonized pupillary dilation induced by both clonidine and methyldopa. These results demonstrate that mydriasis in the conscious rat can be an effective index of CNS alpha 2-adrenoceptor activation, particularly when measured under low lighting levels.

alpha-methyldopa reduces locomotor activity in rats via its metabolite, alpha-methylnorepinephrine, acting on alpha 2-adrenoceptors.

The decrease in locomotor activity in rats caused by alpha-methyldopa (alpha-MD), 400 mg/kg p.o. was antagonized by treatment with yohimbine, a selective antagonist of alpha 2-adrenoceptors. Effective doses of yohimbine ranged from 0.25--2.0 mg/kg s.c., whereas yohimbine at 0.125 mg/kg did not significantly affect the decrease in locomotor activity caused by alpha-MD. Similar results were obtained in studies on the interaction between clonidine injected intracisternally and various doses of yohimbine given s.c., except that the higher doses of yohimbine completely blocked the depression of locomotor activity caused by clonidine, but not by alpha-MD. The depression of motor activity following alpha-MD was not offset by prazosin, a preferential alpha 1-antagonist. At the same doses that failed to alter the action of alpha-MD, prazosin was effective in antagonizing the increase in motor activity resulting from intracisternally injected methoxamine, a selective agonist, at alpha 1-receptors. Treatment with FLA-63, using a regimen that was shown to inhibit dopamine-beta-hydroxylase in brain, caused a diminution in the ability of alpha-MD to depress locomotor activity. These findings indicate that alpha-MD reduces locomotor activity in the rat at least in part via the formation of alpha-methylnorepinephrine which acts on alpha 2-adrenoceptors.

Further studies on the effect of chronic alpha-methyldopa administration upon the central nervous system and sexual function in male rats.

Previous studies in our laboratory have demonstrated that alpha-methyldopa, a potent inhibitor of catecholamine synthesis, reduced both libido and erectile function in sexually mature male rats as it does in humans. In this report, those studies were extended to identify the drug's effect upon catecholamine content in the brain, spinal cord and penile erectile tissue in mature male rats. Because many men report that the adverse effects of alpha-methyldopa upon their sexual performance continue after withdrawal from the drug, a 2nd group of animals was retested after the drug was stopped. Results of the group I study showed diminished copulatory and erectile activity as well as penile and brain norepinephrine content. In the group II rats there was a continued significant reduction in erectile and copulatory activity. At the same time, brain, spinal cord and penile tissue content of norepinephrine returned to normal levels. In another experiment an alpha 2-receptor antagonist was administered with alpha-methyldopa and no inhibition of the effects of alpha-methyldopa was observed. These findings imply that the effect of alpha-methyldopa on sexual function is mediated in the central nervous system as an abnormality of receptor function.