Central inhibitory effect of alpha-methyldopa on blood pressure, heart rate and body temperature of renal hypertensive rats

Combined Effects of Methyldopa and Baicalein or Scutellaria baicalensis Roots Extract on Blood Pressure, Heart Rate, and Expression of Inflammatory and Vascular Disease-Related Factors in Spontaneously Hypertensive Pregnant Rats

The aim of the study was to investigate the effect of baicalein or Scutellaria baicalensis root extract interaction with methyldopa in pregnant spontaneously hypertensive rats (SHR) at the pharmacodynamic, molecular, and biochemical levels. The rats, after confirming pregnancy, received baicalein (200 mg/kg/day, p.o.) and extract (1000 mg/kg/day, p.o.), in combination with methyldopa (400 mg/kg/day; p.o.), for 14 consecutive days, 1 h before blood pressure and heart rate measurements. In the heart and placenta from mothers after giving birth to their offspring, mRNA expression of factors related to inflammatory processes (TNF-α, Il-1β, IL-6) and vascular diseases (TGF-β, HIF-1α, VEGF, PlGF) was measured. Levels of markers of oxidative stress (superoxide dismutase and malondialdehyde) in the placenta and indicators of myocardial damage (troponin cTnC and cTnI, creatine kinase, myoglobin, and lactate dehydrogenase) in the heart were also assessed. Baicalein co-administered with methyldopa was associated with reduced blood pressure, especially during the first three days. The interactions were more pronounced for such factors as TGF-β, HIF-1α, VEGF, and PlGF than TNF-α, Il-1β, and IL-6. Combined application of baicalein and extract with methyldopa may be of value in the development of a new antihypertensive medication intended for patients suffering from preeclampsia or pregnancy-induced hypertension.

Phenotyping small animals as models for the human metabolic syndrome: thermoneutrality matters

It is standard practice in preclinical biomedical research to house mammalian model organisms at an ambient temperature substantially below the thermoneutral zone. These experimental studies are performed using animals that are chronically challenged by mild cold stress. This condition increases food intake, metabolic rate, sympathetic activity, blood pressure and heart rate. Furthermore, this condition alters the behavioral and physiological responses to drug administration, energy restriction and overfeeding. This paper will review these observations, which must be understood in the context of phenotyping small mammals to enhance our understanding of the biology of human disease.

Comparative pharmacokinetics of D- and L-alphamethyldopa in plasma, aqueous humor, and cerebrospinal fluid in rabbits

The 2 stereoisomers of alphamethyldopa (alpha MD) were separately injected IV at 3 different doses (3, 10, 30 mg/kg) in anesthetized rabbits. Samples of plasma, aqueous humor (AH), and cerebrospinal fluid (CSF) were collected over a 300-min period. The concentration of the aminoacid (AA) was determined by liquid chromatography and electrochemical detection. Parameters obtained from kinetic analyses of the plasma concentrations were close to the values reported in other species. Linear elimination kinetics were observed in the dose range studied. A marked dose-dependent entry of alpha MD was observed in AH. A stereospecific active transport of alpha MD was evidenced in the AH since the concentration of the L-isomer reached values above the plasma levels. CSF entry of the AA was small when compared to AH kinetics. A limited passive diffusion of the AA in the brain could account for this phenomenon. However, greater availability of the L-stereoisomer was still observed in CSF. These alpha MD kinetic analyses illustrate the adaptation of AH and CSF removal procedures to the pharmacokinetic studies of the brain and ocular entry of AA isomers.

Changes in body temperature after administration of amino acids, peptides, dopamine, neuroleptics and related agents: II

This survey begins a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published since 1978, but data from many earlier papers are also tabulated.

Effect of methyldopa on brain cholinergic neurons involved in cardiovascular regulation. A study in conscious spontaneously hypertensive rats

Chemical stimulation of brain cholinergic neurons in many species can produce hypertension. Recent studies in this laboratory have demonstrated that clonidine inhibits this central cholinergic pressor response by inhibiting the biosynthesis of brain acetylcholine. This study was designed to determine whether methyldopa, like clonidine, could inhibit brain cholinergic neurons involved in cardiovascular regulation in freely-moving spontaneously hypertensive rats (SHR). Intravenous (i.v.) injection of methyldopa (50-200 mg/kg) produced a dose-related fall in blood pressure (29/15-54/33 mm Hg) in SHR. Intracerebroventricular (i.c.v.) injection of hemicholinium-3 (HC-3) in SHR evoked a fall in arterial pressure through inhibition of acetylcholine synthesis. Doses of HC-3 (10 micrograms, or 15 micrograms, i.c.v.) and methyldopa (50 mg/kg, i.v.) were administered to produce small reductions in arterial pressure in SHR (7-14 mm Hg diastolic, respectively). When the two agents were injected simultaneously, however, a greater than additive response was obtained (p less than 0.05). Central injection of echothiophate (a long-acting cholinesterase inhibitor) to potentiate brain cholinergic activity resulted in a sustained hypertensive response (greater than 40 mm Hg) in SHR for at least 150 minutes. Simultaneous injection of or pretreatment with methyldopa (100 mg/kg, i.v.) inhibited the pressor response to echothiophate over a time course similar to its antihypertensive response in untreated SHR. Methyldopa, however, was completely ineffective in altering the hypertensive response to central injection of carbachol (1 microgram, i.c.v.). This difference in methyldopa susceptibility between the indirect-acting (echothiophate) and direct-acting (carbachol) cholinergic agonists may be related to an inhibiting effect of methyldopa on brain acetylcholine release.(ABSTRACT TRUNCATED AT 250 WORDS)

Is the hypothermic effect of alpha-methyldopa mediated by opioid peptides?

At an ambient temperature of 21 degrees C alpha-methyldopa (25-200 mg kg-1) induces a dose-dependent decrease in body temperature in rats. A relationship between adrenergic and opioid neuronal systems has been reported. Therefore, in this study the hypothesis that alpha-methyldopa produces hypothermia through release of endogenous opioid peptides has been investigated using the opiate antagonist naltrexone. The hypothermic effect of alpha-methyldopa is potentiated by naltrexone pointing to antagonism of an hyperthermic acting opioid system. However, at an ambient temperature of 6 degrees C, pretreatment with naltrexone did not significantly alter the hypothermic effect of alpha-methyldopa. Although the hypothesis proved not to be correct, it is concluded that depending on ambient temperature opioid peptides are involved in the determination of the ultimate effect of alpha-methyldopa on body temperature in rats.

Effects of acute and short-term treatment with antihypertensive drugs in sinoaortic denervated rats

Sinoaortic denervation (SAD) produced a marked increase of the systolic blood pressure (SBP). Clonidine (50 micrograms kg-1, i.p.) reduced SBP in SAD but not in sham rats. L-alpha-methyldopa (alpha-MD) (50 mg kg-1, i.p.) also induced a more effective hypotensive action in SAD than sham rats. The withdrawal of alpha-MD in SAD rats after the first treatment was not abrupt and the hypotension persisted for several days, but after the second treatment the withdrawal induced a rapid rebound hypertension. Our results suggest that SAD increases the response to the hypotensive agents. An alteration in the availability of alpha-MD to accumulate or synthesize the active metabolites was also observed after second treatment.

Inhibition of the activity of sympathetic preganglionic neurones and neurones activated by visceral afferents, by alpha-methylnoradrenaline and endogenous catecholamines

The responses of electrophysiologically identified sympathetic preganglionic neurones (SPGN) and neurones activated by visceral afferents (VA) to iontophoretic application of: (1) the intraneuronal metabolites of alpha-methyl-DOPA (alpha-MD): alpha-methylnoradrenaline (alpha-MNA) and alpha-methyldopamine (alpha-MDA); (2) noradrenaline (NA) and adrenaline (Ad); and (3) N-methyl-beta-hydroxy-phenylethylamine (NMPEA), were tested in the upper and lower segments of the thoracic spinal cord of the cat. alpha-Methylnoradrenaline, NA and Ad had an inhibitory action on the majority of spontaneously firing neurones. Inhibition of the activity of preganglionic neurones and neurones activated by visceral afferents induced by alpha-MNA was usually equal to the effect of NA, but in some neurones alpha-MNA was more potent. The transport numbers of both amines are similar. The alpha adrenoceptor antagonists, thymoxamine, piperoxan and yohimbine, antagonized the inhibitory effects of alpha-MNA in spontaneously-active preganglionic neurones and neurones activated by visceral afferents. Piperoxan antagonized also the inhibitory effects of NA. The inhibitory effect of alpha-MDA was weaker and that of NMPEA was much weaker than that of alpha-MNA and NA. The inhibitory effects of alpha-MNA and NA in the cerveau isolé preparation resembled those observed in anaesthetized animals. In reserpinized cats, with catecholamine levels in brain stem and spinal cord reduced by 98-99%, the inhibitory effects of alpha-MNA were preserved. It is postulated that alpha-MNA modulates the activity of preganglionic neurones and that this action, leading to a decrease in sympathetic output and mediated by alpha 1 and alpha 2 adrenoceptors, could be the most important factor in the antihypertensive effect of alpha-MD. The inhibition of the activity of neurones activated by visceral afferents by alpha-MNA indicates that alpha-MD may also attenuate the response of the central nervous system to the input from the heart and other organs.

Changes in body temperature after administration of adrenergic and serotonergic agents and related drugs including antidepressants

This survey, the third in a series, presents extensive tabulations of literature, primarily since 1965, on thermoregulatory effects of adrenergic and serotonergic agonists and their antagonists including ergot alkaloids, amphetamines, tryptamines, monoamine oxidase inhibitors, tricyclic and other antidepressants, a variety of other agents which alter presynaptic aminergic mechanisms including reserpine, 6-hydroxydopamine, p-chlorophenylalanine, alpha-methyltyrosines, cocaine, guanethidine and bretylium. The information listed includes the species used, route of administration and dose of drug, the environmental temperature at which the experiments were performed, the number of tests, the direction and magnitude of body temperature change and remarks on the presence of special conditions, such as age or lesions, or on the influence of other drugs, such as antagonists, on the response to the primary drug.

Acute and chronic administration of alpha-methyldopa: regional levels of endogenous and alpha-methylated catecholamines in rat brain

Acute administration of alpha-methyldopa (200 mg kg-1 s.c.) produced a short-lived decrease in dopamine levels in 5 regions of the rat brain. Maximal dopamine depletion was 55% in the corpus striatum and over 75% in other regions and occurred 4--6 h after alpha-methyldopa injection; dopamine levels recovered within 12 h. Maximal noradrenaline depletion of over 80% occurred in all regions 4--24 h after alpha-methyldopa and after 24 h noradrenaline levels were still significantly reduced from control values. In these acute studies alpha-methyldopamine accumulated rapidly in amounts equal to or greater than the depleted dopamine in all regions, reaching a peak at 4 h. By contrast, alpha-methylnoradrenaline accumulated more slowly reaching a peak at 6--24 h and was never present in amounts greater than the depleted noradrenaline. Following chronic administration of alpha-methyldopa (40 mg kg-1 s.c., twice daily 5 days) there was a similar depletion of noradrenaline and dopamine to that seen in the acute studies. The depletion was associated with a much smaller accumulation of alpha-methyldopamine. The striking feature of these results, however, was the large accumulation of alpha-methylnoradrenaline in all brain regions. This probably reflects the slow turnover and resistance to degradation by monoamine oxidase of alpha-methylnoradrenaline.

Differential development of renal, DOCA-salt, and spontaneous hypertension in the rat after neonatal sympathectomy

The development of hypertension was studied in rats after neonatal sympathectomy by s.c. injection of 6-hydroxydopamine HCl. Three different types of hypertension were investigated: renal hypertension in the two-kidney Goldblatt model, steroid hypertension produced by deoxycorticosterone (DOCA) implantation and saline as drinking fluid, and genetic hypertension in the spontaneous hypertensive rat (SHR). Blood pressure was measured directly in conscious animals via the iliac artery. Mean blood pressure of conscious sympathectomized (SX) normotensive rats was not significantly different from that of normotensive controls. Renal hypertension reached the same level in controls and SX rats four weeks after application of a renal artery clip. DOCA-salt hypertension developed faster and to a higher level in SX rats than in control rats. The hypertension in SX DOCA-salt hypertensive rats was accompanied by a marked tachycardia. In contrast hypertension did not develop in SX SHR. Up to 12 months of age mean blood pressure was markedly lower than that of control SHR, but slightly higher than that of SX normotensive Wistar Kyoto rats. These differential effects of neonatal sympathectomy on the development of hypertension in the rat may point to differences in the pathophysiological mechanisms. It is concluded that an intact sympathetic nervous system is essential for the development of hypertension in SHR. In DOCA-salt hypertension the intact sympathetic nervous system appears to protect against a rapid rise in blood pressure.

The area postrema and control of arterial blood pressure; absence of hypertension after excision of the area postrema in rats

Acute surgical excision of the area postrema (AP) in the rat failed to affect arterial blood pressure or heart rate. The was no effect on cardiovascular reflex responses during diving or on the heart rate responses to acute decreases or increases of blood pressure caused by bradykinin or angiotensin, respectively. Electrolytic lesions of the AP in acute experiments caused variable damage to the nucleus tractus solitarii (NTS). In these rats large variations in blood pressure occurred. Excision of the AP in a chronic experiment failed to change blood pressure, heart rate, water intake or plasma renin activity. In contrast, bilateral electrolytic lesions of the NTS at the level of the AP caused a severe acute hypertension and completely blocked cardiovascular reflex responses. Hypertension also existed in rats with NTS lesions studied for a longer period of time. There experiments failed to confirm the hypothesis that the AP exerts a tonic inhibitory control of basal blood pressure. Hypertension previously reported after ablation of the AP may be explained by damage to the NTS.

Evidence for an inhibitory action of methyldopa on spinal sympathetic reflexes

The antihypertensive activity of methyldopa is mediated primarily by effects on the central nervous system. A number of centrally acting hypotensive substances has recently been shown to depress spinal pathways and reflexes as well as inhibiting supraspinal areas. The present series of experiments evaluated the potential influence of methyldopa on spinal mechanisms. Stimulation of an intercostal nerve evoked a two-component response (early and late) in the splanchnic nerve of anesthetized cats. The "late" phase involves supraspinal neurons, whereas, the "early" component involves only spinal pathways. The i.v. administration of 100 mg/kg of methyldopa reduced systemic blood pressure and both components of the evoked potential progressively. These experiments indicate that methyldopa exerts an effect of spinal sympathetic mechanisms which may contribute to the antihypertensive response to the compound.

Centrally induced hypotension and bradycardia after administration of alpha-methylnoradrenaline into the area of the nucleus tractus solitarii of the rat

1 In anaesthetized rats, bilateral injections of alpha-methylnoradrenaline, noradrenaline or adrenaline into the area of the nucleus tractus solitarii (NTS) of the brain stem caused dose-dependent decreases of systemic arterial blood pressure and heart rate. The effects of alpha-methylnoradrenaline were most pronounced and lasted longest. 2 The cardiovascular effects of alpha-methylnoradrenaline appeared to be restricted to the medio-caudal part of the NTS. 3 Prior administration of the alpha-adrenoceptor blocking agent, phentolamine, reversed the fall in blood pressure and heart rate induced by alpha-methylnoradrenaline into an increase. 4 Systemic administration of atropine combined with vagotomy potentiated the inhibitory effects of alpha-methylnoradrenaline on the cardiovascular system.

Studies on the mechanism of the cardiovascualr effects of methyldopa

Oral administration of methyldopa (100 mg/kg, twice daily for 3 days) to mongrel dogs produced a significant decrease in blood pressure and heart rate. The drug treatment affected neither the resting venous tone nor the cardiac output. Thus, the hypotensive effect of the drug was predominantly due to a reduction in total peripheral resistance. Vasoconstrictor responses of the renal vasculature to sympathetic nerve stimulation were significantly impaired after methyldopa at all the frequencies, while mesenteric vasoconstrictor responses to sympathetic nerve stimulation were impaired only at the lower stimulation frequencies. In addition, methylnorepinephrine was a significantly less potent vasoconstrictor than norepinephrine in the renal vasculature, but was equipotent to norepinephrine in the mesentery. The finding of a reduction in the renal vascular resistance of methyldopa-treated dogs, with no such alteration in the mesenteric vascular resistance, is consistent with the nerve stimulation studies. Therefore, the results of the present investigation indicate that in addition to the existing evidence favoring a central site of action for methyldopa, the impairment of peripheral sympathetic neuronal function is also of importance in accounting for the hemodynamic alterations observed following treatment with methyldopa.

Distribution of the blood flow supplied by the vertebral artery in rats: anatomical, functional and pharmacological aspects

In rats, the vertebral artery make only a minor contribution to the blood perfusion of the ponto-medullary area. This was measured with radioactive microspheres and was confirmed by methylmetacrylate casts and local injection of a centrally acting hypotensive drug.

alpha-Methylnoradrenaline induced hypotension and bradycardia after administration into the area of the nucleus tractus solitarii

Bilateral injections of alpha-methylnoradrenaline into the area of the nucleus tractus solitarii of the brain stem caused a dose-dependent decrease of systemic arterial blood pressure and heart rate of anesthetized rats. The effects were prevented and even reversed by a preceding injection of the alpha-adrenoceptor blocking agent phentolamine. Pressor doses of angiotensin II and of arginine-vasopressin at the same site failed to drcrease blood pressure and heart rate.