Effects of propranolol, clonidine and hydrochlorothiazide treatment and abrupt discontinuation on central and peripheral noradrenergic activity in essential hypertension

Effect of diuretics on plasma renin activity in primary hypertension: A systematic review and meta-analysis

AIMS

Plasma renin activity (PRA) is regarded as a marker of sodium and fluid homeostasis in patients with primary hypertension. Whether effects of diuretics on PRA differ according to class of diuretic, whether diuretics lead to a sustained increase in PRA, and whether changes in PRA relate to those in blood pressure (BP) is unknown. We performed a systematic review and meta-analysis of trials investigating the antihypertensive effects of diuretic therapy in which PRA and/or other biomarkers of fluid homeostasis were measured before and after treatment.

METHODS

Three databases were searched: MEDLINE, EMBASE and The Cochrane Central Register of Controlled Trials. Titles were firstly screened by title and abstract for relevancy before full-text articles were assessed for eligibility according to a predefined inclusion/exclusion criteria.

RESULTS

A total of 1684 articles were retrieved of which 61 met the prespecified inclusion/exclusion criteria. PRA was measured in 30/61 studies. Diuretics led to a sustained increase in PRA which was similar for different classes of diuretic (standardised mean difference [95% confidence interval] 0.481 [0.362, 0.601], 0.729 [0.181, 1.28], 0.541 [0.253, 0.830] and 0.548 [0.159, 0.937] for thiazide, loop, mineralocorticoid receptor antagonists/potassium-sparing and combination diuretics respectively, Q = 0.897, P = .826), and did not relate to the average decrease in blood pressure.

CONCLUSION

In antihypertensive drug trials, diuretics lead to a sustained increase in average PRA, which is similar across different classes of diuretic and unrelated to the average reduction in blood pressure.

Cardiovascular effects of rilmenidine, a new alpha 2-adrenoceptor agonist, and clonidine in conscious spontaneously hypertensive rats

1. The acute and chronic effects of rilmenidine, a partial agonist of alpha 1- and alpha 2-adrenoceptors with antihypertensive properties, were compared to those of clonidine on blood pressure (BP), heart rate (HR) and the urinary excretion of catecholamines, which was used as an index of sympathetic activity. 2. As these drugs are known to interfere centrally and peripherally with the sympathetic nervous system, long-term arterial blood pressure recordings in freely moving unstressed adult spontaneously hypertensive rats (SHR) were used. 3. Acute i.v. administrations of rilmenidine (0.3 mg/kg at 1200 h, 1.2 mg/kg at 1700 and 2200 h) and clonidine (12 micrograms/kg at 1200 h, 50 micrograms/kg at 1700 and 2200 h) induced short-lasting increases in BP associated with a decrease in HR, which were followed by prolonged, dose-dependent decreases in BP without bradycardia. The pressor effect was less marked and the associated bradycardia was more marked in active SHR with physiologically high sympathetic activity than in resting SHR. 4. A 12-day oral treatment with rilmenidine (6.0 mg/kg daily) or clonidine (150 micrograms/kg daily) induced moderate decreases in BP without change in HR. Rilmenidine but not clonidine decreased normetanephrine (NMN) excretion in active but not in resting SHR. 5. Finally, during the 24 h following the cessation of the treatments, BP returned to normal, without significantly exceeding that of untreated controls. However, upswings in BP or HR were observed, more markedly and frequently after clonidine than after rilmenidine. 6. In conclusion the effects of alpha 2-adrenoceptor agonists appear to be influenced by the pre-existing sympathetic tone. The general agreement between these data and those observed in patients demonstrates that the use of conscious unstressed animals is of value to determine the cardiovascular effects of drugs which act on the sympathetic nervous system.

Effects of xylazine on cerebrospinal fluid catecholamines in the rhesus monkey

The i.v. administration of xylazine, a potent, selective alpha 2-adrenergic receptor agonist, resulted in a 76% decrease in cerebrospinal fluid (CSF) norepinephrine in chair-adapted rhesus monkeys. A significant decrease was observed within 1.5 h of administration and continued through the 3 h course of sampling. Dopamine was maximally decreased by 24% at 1.5 h. Epinephrine was not significantly decreased following xylazine administration. These data suggest that norepinephrine release into monkey CSF, as an index of central or peripheral norepinephrine turnover, is more sensitive to alpha 2-adrenergic agonists than in CSF dopamine or epinephrine.

Effect of ganglion blockade on cerebrospinal fluid norepinephrine

The source of norepinephrine (NE) in CSF has been unclear. It has been suggested that CSF NE indicates central neural noradrenergic tone and is determined differently from plasma NE. If CSF NE depended specifically on NE release in the CNS, then interference with ganglionic neurotransmission would be expected to decrease plasma NE but not CSF NE. Hypotension caused by ganglionic blockade might be expected to increase CSF NE reflexively. We infused the ganglion blocker, trimethaphan, intravenously into anesthetized dogs and measured the effects on mean arterial blood pressure (MAP) and on cisterna magna CSF levels of NE. The results were compared with those obtained on administration of saline, clonidine (2 micrograms/kg), yohimbine (0.25 mg/kg), or nitroprusside and with those obtained when hypotension during ganglion blockade was prevented by concurrent treatment with phenylephrine. Trimethaphan decreased MAP by 40%, arterial NE by 64%, and CSF NE by 61%. Nitroprusside administered intravenously to produce the same 40% depressor response increased arterial NE by 612% and CSF NE by 155%. Prevention of ganglion blockade-induced hypotension using phenylephrine did not prevent the decrease in CSF NE caused by trimethaphan, and when phenylephrine was discontinued, the resulting hypotension was not associated with increases in CSF NE. The similar decreases in plasma NE and CSF NE during ganglionic blockade, and the abolition of reflexive increases in CSF NE during hypotension in ganglion-blocked subjects, cast doubt on the hypothesis that CSF NE indicates central noradrenergic tone and are consistent instead with at least partial derivation of CSF NE from postganglionic sympathetic nerve endings.