Long-term thiazide therapy in essential hypertension. Evidence for persistent alteration in plasma volume and renin activity

Effects of thiazides and new findings on kidney stones and dysglycemic side effects

Thiazide and thiazide-like diuretics (thiazides) belong to the most frequently prescribed drugs worldwide. By virtue of their natriuretic and vasodilating properties, thiazides effectively lower blood pressure and prevent adverse cardiovascular outcomes. In addition, through their unique characteristic of reducing urine calcium, thiazides are also widely employed for the prevention of kidney stone recurrence and reduction of bone fracture risk. Since their introduction into clinical medicine in the early 1960s, thiazides have been recognized for their association with metabolic side effects, particularly impaired glucose tolerance, and new-onset diabetes mellitus. Numerous hypotheses have been advanced to explain thiazide-induced glucose intolerance, yet underlying mechanisms remain poorly defined. Regrettably, the lack of understanding and unpredictability of these side effects has prompted numerous physicians to refrain from prescribing these effective, inexpensive, and widely accessible drugs. In this review, we outline the pharmacology and mechanism of action of thiazides, highlight recent advances in the understanding of thiazide-induced glucose intolerance, and provide an up-to-date discussion on the role of thiazides in kidney stone prevention.

Techniques to Assess the Effect of Sodium-Glucose Cotransporter 2 Inhibitors on Blood Volume in Patients with Diabetic Kidney Disease

BACKGROUND

Sodium-glucose cotransporter 2 (SGLT2) inhibitors exert a kidney protective effect in patients with diabetic kidney disease. Several mechanisms have been proposed, but why precisely SGLT2 inhibition has a kidney protective effect is incompletely understood. Clinical trials using SGLT2 inhibitors have found them to induce a rapid weight loss likely due to loss of sodium and subsequently fluid. While SGLT2 inhibitors are reported to increase hematocrit, it remains unknown whether the natriuretic and aquaretic effect reduces patient's blood volume and whether this could partly explain its kidney protective effects. A blood volume reduction could induce several beneficial effects with reduction in arterial and venous blood pressure as two central mechanisms. The aim of this paper was to review current techniques for assessing patient blood volume that could enhance our understanding of SGLT2 inhibitors' physiological effects.

SUMMARY

Changes induced by SGLT2 inhibitors on erythrocyte volume and plasma volume can be assessed by tracer dilution techniques that include radioisotopes, indocyanine green (ICG) dye, or carbon monoxide (CO). Techniques with radioisotopes can provide direct estimates of both erythrocyte volume and plasma volume but are cumbersome procedures and the radiation exposure is a limitation for repeated measures in clinical studies. Methods more suitable for repeated assessment of erythrocyte and plasma volume include dilution of injected ICG dye or dilution of inhaled CO. ICG dye requires higher precision with timed blood samples and provides only a direct estimate of plasma volume wherefrom erythrocyte volume is estimated. Inhalation of CO is a time-effective and automated method that provides measure of the total hemoglobin mass wherefrom erythrocyte and plasma volumes are estimated.

KEY MESSAGES

A kidney protective effect has been observed in clinical trials with SGLT2 inhibitors, but the underlying mechanisms are not fully understood. Significant weight loss within weeks has been reported in the SGLT2 inhibitor trials and could be related to a reduction in blood volume secondary to increased natriuresis and aquaresis. Alterations in blood volume compartments can be quantified by tracer dilution techniques and further improve our understanding of kidney protection from SGLT2 inhibitors.

Thiazide and the Thiazide-Like Diuretics: Review of Hydrochlorothiazide, Chlorthalidone, and Indapamide

The term thiazide is universally understood to refer to diuretics that exert their principal action in the distal tubule. The thiazide class is heterogenous and can be further subdivided into compounds containing the benzothiadiazine ring structure-the thiazide-type (e.g., hydrochlorothiazide)-and those lacking the benzothiadiazine ring-the thiazide-like (e.g., chlorthalidone and indapamide) drugs. Thiazide-like agents are longer acting and constitute the diuretics used in most of the cardiovascular outcome trials that established benefits of treatment with diuretics, but pragmatic aspects, such as lack of availability in convenient formulations, limit their use. Regardless of class heterogeneity, thiazides have retained importance in the management of hypertension for over 60 years. They are reliably effective as monotherapy in a majority of hypertensive patients, and augment the efficacy of other classes of antihypertensives when used in combination. Importantly, a thiazide-based treatment regimen lowers cardiovascular events, and their sturdy effect reinforces their place among the recommended first-line agents to treat hypertension in major domestic and international hypertension guidelines. There are few head-to-head comparisons within the class, but potential differences have been explored indirectly as well as in non-blood pressure mechanisms and potential pleiotropic properties. Until proven otherwise, the importance of these differences remains speculative, and clinicians should assume that cardiovascular events will be lowered similarly across agents when equivalent blood pressure reduction occurs. Thiazides remain underutilized, with only about one-third of hypertensive patients receiving them. For many patients, however, a thiazide is an indispensable component of their regimen to achieve adequate blood pressure control.

Mechanism of Thiazide Diuretic Arterial Pressure Reduction: The Search Continues

Thiazide diuretic (TZD)-mediated chronic reduction of arterial pressure is thought to occur through decreased total peripheral vascular resistance. Further, the decreased peripheral vascular resistance is accomplished through TZD activation of an extrarenal target, resulting in inhibition of vascular constriction. However, despite greater than five decades of investigation, little progress has been made into the identification of the TZD extrarenal target. Proposed mechanisms range from direct inhibition of constrictor and activation of relaxant signaling pathways in the vascular smooth muscle to indirect inhibition through decreased neurogenic and hormonal regulatory pathways. Surprisingly, particularly in view of this lack of progress, comprehensive reviews of the subject are absent. Moreover, even though it is well recognized that 1) several types of hypertension are insensitive to TZD reduction of arterial pressure and, further, TZD fail to reduce arterial pressure in normotensive subjects and animals, and 2) different mechanisms underlie acute and chronic TZD, findings derived from these models and parameters remain largely undifferentiated. This review 1) comprehensively describes findings associated with TZD reduction of arterial pressure; 2) differentiates between observations in TZD-sensitive and TZD-insensitive hypertension, normotensive subjects/animals, and acute and chronic effects of TZD; 3) critically evaluates proposed TZD extrarenal targets; 4) proposes guiding parameters for relevant investigations into extrarenal TZD target identification; and 5) proposes a working model for TZD chronic reduction of arterial pressure through vascular dilation.

The non-diuretic hypotensive effects of thiazides are enhanced during volume depletion states

Thiazide derivatives including Hydrochlorothiazide (HCTZ) represent the most common treatment of mild to moderate hypertension. Thiazides initially enhance diuresis via inhibition of the kidney Na+-Cl- Cotransporter (NCC). However, chronic volume depletion and diuresis are minimal while lowered blood pressure (BP) is maintained on thiazides. Thus, a vasodilator action of thiazides is proposed, likely via Ca2+-activated K+ (BK) channels in vascular smooth muscles. This study ascertains the role of volume depletion induced by salt restriction or salt wasting in NCC KO mice on the non-diuretic hypotensive action of HCTZ. HCTZ (20mg/kg s.c.) lowered BP in 1) NCC KO on a salt restricted diet but not with normal diet; 2) in volume depleted but not in volume resuscitated pendrin/NCC dKO mice; the BP reduction occurs without any enhancement in salt excretion or reduction in cardiac output. HCTZ still lowered BP following treatment of NCC KO on salt restricted diet with paxilline (8 mg/kg, i.p.), a BK channel blocker, and in BK KO and BK/NCC dKO mice on salt restricted diet. In aortic rings from NCC KO mice on normal and low salt diet, HCTZ did not alter and minimally decreased maximal phenylephrine contraction, respectively, while contractile sensitivity remained unchanged. These results demonstrate 1) the non-diuretic hypotensive effects of thiazides are augmented with volume depletion and 2) that the BP reduction is likely the result of HCTZ inhibition of vasoconstriction through a pathway dependent on factors present in vivo, is unrelated to BK channel activation, and involves processes associated with intravascular volume depletion.

Diuretics in the treatment of hypertension

Diuretics have long been used for the treatment of hypertension. Thiazide diuretics are the most commonly prescribed diuretics for hypertension, but other classes of diuretics may be useful in alternative circumstances. Although diuretics are no longer considered the preferred agent for treatment of hypertension in adults and children, they remain acceptable first-line options. Diuretics effectively decrease blood pressure in hypertensive patients, and in adults with hypertension reduce the risk of adverse cardiovascular outcomes. Because of varied pharmacokinetic and pharmacodynamic differences, chlorthalidone may be the preferred thiazide diuretic in the treatment of primary hypertension. Other types of diuretics (e.g., loop, potassium sparing) may be useful for the treatment of hypertension related to chronic kidney disease (CKD) and other varied conditions. Common side effects of thiazides are mostly dose-related and involve electrolyte and metabolic abnormalities.

How do thiazide and thiazide-like diuretics lower blood pressure?

Thiazide diuretics are widely used for the treatment of hypertension, but the mechanism by which these drugs lower blood pressure in the long term remains unknown. This article reviews current knowledge about the hypotensive actions of thiazides and thiazide-like diuretics and discusses possible mechanisms of action.

Approach to Antihypertensive Therapy

Antihypertensive agents are commonly prescribed by physicians to prevent the long-term mortality from chronic hypertension. They are also given to improve survival in a number of conditions (eg, heart failure, coronary artery disease), independent of the effect on blood pressure. Several classes of antihypertensives are available with unique pharmacologic characteristics and adverse effects. Not all agents in the same class have identical effects, and careful selection of drugs based on the comorbid conditions is recommended.

Mechanisms and pharmacogenetic signals underlying thiazide diuretics blood pressure response

Thiazide (TZD) diuretics are among the most commonly prescribed antihypertensives globally; however their chronic blood pressure (BP) lowering mechanism remains unclear. Herein we discuss the current evidence regarding specific mechanisms regulating the antihypertensive effects of TZDs, suggesting that TZDs act via multiple complex and interacting mechanisms, including natriuresis with short term use and direct vasodilatory effects chronically. Additionally, we review pharmacogenomics signals that have been associated with TZDs BP-response in several cohorts (i.e. NEDD4L, PRKCA, EDNRA-GNAS, and YEATS4) and discuss how these genes might be related to TZD BP-response mechanism. Understanding the association between these genes and TZD BP mechanism might facilitate the development of new drugs and therapeutic approaches based on a deeper understanding of the determinants of BP-response.

Role of proximal tubule in the hypocalciuric response to thiazide of patients with idiopathic hypercalciuria

The most common metabolic abnormality found in calcium (Ca) kidney stone formers is idiopathic hypercalciuria (IH). Using endogenous lithium (Li) clearance, we previously showed that in IH, there is decreased proximal tubule sodium absorption, and increased delivery of Ca into the distal nephron. Distal Ca reabsorption may facilitate the formation of Randall's plaque (RP) by washdown of excess Ca through the vasa recta toward the papillary tip. Elevated Ca excretion leads to increased urinary supersaturation (SS) with respect to calcium oxalate (CaOx) and calcium phosphate (CaP), providing the driving force for stone growth on RP. Thiazide (TZ) diuretics reduce Ca excretion and prevent stone recurrence, but the mechanism in humans is unknown. We studied the effect of chronic TZ administration on renal mineral handling in four male IH patients using a fixed three meal day in the General Clinical Research Center. Each subject was studied twice: once before treatment and once after 4-7 mo of daily chlorthalidone treatment. As expected, urine Ca fell with TZ, along with fraction of filtered Ca excreted. Fraction of filtered Li excreted also fell sharply with TZ, as did distal delivery of Ca. Unexpectedly, TZ lowered urine pH. Together with reduced urine Ca, this led to a marked fall in CaP SS, but not CaOx SS. Since CaOx stone formation begins with an initial CaP overlay on RP, by lowering urine pH and decreasing distal nephron Ca delivery, TZ might diminish stone risk both by reducing CaP SS, as well as slowing progression of RP.

Repeated assessment of physical biomeasures or blood biomarkers for the definition of volume status and cardiac loading in LVSD

The application of biomarker technology can be usefully implemented in areas where current techniques are inadequate and where a clinical issue, which affects outcome, can be defined. The definition of the loading status of the heart where there is pre-existent impairment of contractile function is a key target. Heart failure is a complex clinical presentation with many varied etiologies, but at the essence of its successful management is the reliable definition of cardiac volume loading. Traditional and many current technological measures are applied to define this relationship, yet their accuracy and performance in individual patients is either basically inadequate or poorly understood and applied. There is a wide range of both physical measurements and blood biomarkers that can be considered to better define this key issue in patients with ventricular systolic impairment. Their performance is considered in detail in this review.

Mechanisms for blood pressure lowering and metabolic effects of thiazide and thiazide-like diuretics

Thiazide and thiazide-like diuretics are among the most commonly used antihypertensives and have been available for over 50 years. However, the mechanism by which these drugs chronically lower blood pressure is poorly understood. Possible mechanisms include direct endothelial- or vascular smooth muscle-mediated vasodilation and indirect compensation to acute decreases in cardiac output. In addition, thiazides are associated with adverse metabolic effects, particularly hyperglycemia, and the mechanistic underpinnings of these effects are also poorly understood. Thiazide-induced hypokalemia, as well as other theories to explain these metabolic disturbances, including increased visceral adiposity, hyperuricemia, decreased glucose metabolism and pancreatic beta-cell hyperpolarization, may play a role. Understanding genetic variants with differential responses to thiazides could reveal new mechanistic candidates for future research to provide a more complete understanding of the blood pressure and metabolic response to thiazide diuretics.

Body fluid volumes and the response of renin and aldosterone to short- and long-term thiazide therapy of essential hypertension

Plasma volume (PV), extracellular fluid volume (ECV) serum electrolytes, renin and aldosterone were measured before and after 1 week and 4 months of hydrochlorothiazide (HCT) treatment, 50 mg twice daily, in nine male patients with uncomplicated essential hypertension. All studies were carried out under strictly standardized conditions in a metabolic ward. After 1 week of HCT treatment, significant reductions were found in PV and ECV, but after 4 months only ECV was significantly reduced. During HCT therapy, renin and aldosterone were permanently elevated whereas serum sodium and potassium were lowered. After 1 week, renin was inversely correlated with PV and ECV and directly correlated with heart rate. After 4 months, renin was inversely correlated with serum sodium. These results indicate a permanent decrease in ECV during long-term HCT therapy. It is further suggested that the mechanisms responsible for the renin response during short- and long-term HCT treatment are different, changes in body fluid volumes and increased neural activity being responsible for the initial rise in renin, and serum sodium being the predominant factor during chronic treatment.

Sympatho-adrenal mechanisms and the antihypertensive response to thiazide diuretics

Thiazide diuretics lower the blood pressure during long-term treatment by reducing the peripheral vascular resistance. This seems to be related to the natriuresis rather than to a direct vasodilating effect of the thiazides. There is evidence that the volume depletion caused by thiazide treatment initially triggers an increase in sympathetic nerve activity, which may account for initial increases in vascular resistance. Although both responses subside during long-term treatment, there is no evidence showing a causal relationship between these changes. The long-term hemodynamic adaptation to thiazide treatment may be related to altered cardiovascular reflexes. Changes in sympathetic nerve activity and reduced vascular sensitivity to noradrenaline may contribute to the adaptation. An analysis of the relative importance of these mechanisms requires further studies comparing the time courses for various effects in responders and non-responders to the treatment.

Investigations on the antihypertensive activity of timolol and bendroflumethiazide and the combination in dogs and rats

The effects of timolol and bendroflumethiazide, either alone or combined in a fixed ratio of 4:1, on blood pressure, plasma renin activity, and plasma potassium concentration, have been investigated in normotensive and renal hypertensive dogs, and in normotensive and spontaneously hypertensive rats. In addition, the urinary kallikrein excretion has been measured in normotensive and hypertensive rats. When administered to hypertensive dogs and rats, the drug combination significantly reduced the blood pressure. Marginal reductions were observed in normotensive animals or after the administration of the single drugs. The thiazide-induced hypokalaemia and hyperreninaemia were almost completely antagonised by the concomitant administration of timolol in both animal species. A highly significant elevation of urinary kallikrein excretion was also observed in rats treated with the drug combination. A less marked increase of kallikrein excretion was noted in the bendroflumethiazide treated rats. The possibility that renal haemodynamic changes, in addition to the inhibition of the increase in plasma renin, play a role in the observed antihypertensive effects is discussed.

Hemodynamic effects of diuretic therapy in hypertension

The antihypertensive effect of diuretics is well documented. At least some preserved kidney function seems to be necessary to achieve a demonstrable effect. The importance of fluid volume reduction for the antihypertensive effect of these drugs during long-term treatment is however uncertain. The effects on central hemodynamics vary according to the literature, but the blood pressure reduction in responders (delta MAP greater than 10%) may primarily be explained by a reduction of the total peripheral vascular resistance. The peripheral circulation is influenced in the same way. Sometimes a flow reduction is observed, and sometimes a resistance reduction. In spite of a lasting blood pressure reduction after treatment with diuretics, the structural hypertensive vascular changes do not seem to be influenced.

Aliskiren fails to lower blood pressure in patients who have either low PRA levels or whose PRA falls insufficiently or reactively rises

BACKGROUND

Suppressed baseline plasma renin activity (PRA) levels or large reactive increases in renin secretion are two possible reasons for treatment failure with antirenin system drugs.

METHODS

To investigate their prevalence we reanalyzed data from three published clinical trials of the renin inhibitor aliskiren.

RESULTS

Aliskiren failed to lower systolic blood pressure (SBP) by at least 10 mm Hg in half of all patients. It was very effective in two-thirds of medium- to high-renin patients (-19 mm Hg). But BP did not fall in most low-renin patients, or in 30% of medium- to high-renin patients. BP actually rose by >10 mm Hg in 5% of patients taking aliskiren and in >10% of patients when aliskiren was added to an angiotensin receptor blocker (ARB) or angiotensin converting enzyme inhibitor (ACEI). PRA changed in parallel with BP. Although PRA fell in most patients, it actually rose in 5% and in up to 30% when aliskiren was added to an ARB or ACEI.

CONCLUSIONS

There are two reasons for treatment failure with aliskiren. Many hypertensive patients have large BP falls. But, BP does not fall in most low-renin patients or in medium- to high-renin patients whose PRA levels do not fall sufficiently. If the concept of that treatment resistance is caused by excessive reactive increases in renin secretion is confirmed, then a PRA determination during treatment could be used to guide subsequent addition or subtraction of either natriuretic or antirenin drug types, to thereby correct BP and reduce cardiovascular risk.

Action of thiazide on renal interstitial calcium

BACKGROUND

Although thiazides increase urinary sodium excretion, they also decrease urinary calcium excretion. Recent studies in our laboratory have shown that increased dietary salt significantly reduces interstitial fluid calcium in Dahl salt-sensitive (DS) rats, and this was associated with a rise in blood pressure and increased urinary calcium excretion. Owing to the vasorelaxant actions of increased extracellular fluid calcium, we reasoned that the antihypertensive action of hydrochlorothiazide (HCTZ), a commonly used thiazide, may be the result of increased interstitial fluid calcium as a consequence of decreased urinary calcium excretion.

METHODS

To test this hypothesis, DS and Dahl salt-resistant (DR) rats were given high salt alone or in combination with HCTZ for 1 week. Renal cortical interstitial fluid calcium was determined by the zero net flux method.

RESULTS

High salt decreased cortical interstitial fluid calcium (1.69 +/- 0.25 vs. 1.13 +/- 0.05 mmol/l; P < 0.05) in DS rats as previously reported; thiazide treatment had no effect on the high salt interstitial fluid calcium response in salt-sensitive animals. However, thiazide decreased interstitial fluid calcium in DS on a normal salt diet. Cortical interstitial fluid calcium was unchanged by dietary salt in DR rats, and thiazide did not alter this interstitial fluid calcium response.

CONCLUSION

We interpret these data to mean that (i) short-term thiazide treatment does not reduce blood pressure by restoring renal cortical interstitial fluid calcium concentration and (ii) a decrease in renal cortical interstitial fluid calcium may not contribute to the increased renal vasoconstriction seen in salt-sensitivity.

Thiazide‐Type Diuretics: Ongoing Considerations on Mechanism of Action

Thiazide-type diuretics have enjoyed a considerable success in the management of hypertension. These drugs have assumed a standard-of-care position in the minds of many health care providers; however, a number of questions remain unresolved in relation to their use. Such questions include issues of mechanism of action, comparability to loop-diuretics in their actions, class-effect, and the basis for their additivity with non-diuretic antihypertensive medication classes. Understanding these issues is important to the effective use of these compounds.

Clinical Pharmacology of Antihypertensive Therapy

Adequate control of blood pressure poses challenges for hypertensive patients and their physicians. Success rates of greater than 80% in reducing blood pressure to target values among high-risk hypertensive patients reported by several recent clinical trials argue that effective medications currently are available. Yet, only 34% of hypertensive patients in the United States are at their goal blood pressure according to the most recent national survey. Rational selection of antihypertensive drugs that target both the patient's blood pressure and comorbid conditions coupled with more frequent use of low-dose drug combinations that have additive efficacy and low adverse-effect profiles could improve significantly US blood pressure control rates and have a positive impact on hypertension-related cardiovascular and renal mortality and morbidity. This article reviews the pharmacokinetic and pharmacodynamic principles that underlie the actions of drugs in each of the classes of antihypertensive agents when used alone and in combination, provides practical pharmacologic information about the drugs most frequently prescribed for treatment of hypertension in the outpatient setting, and summarizes the current data influencing the selection of drugs that might be used most effectively in combination for the majority of hypertensive patients whose blood pressures are not controlled adequately by single-drug therapy.

Continued importance of diuretics and beta-adrenergic blockers in the management of hypertension

The use of thiazide diuretics as one of the preferred antihypertensive medications has stood the test of time. Since the introduction of orally effective agents in 1957 to 1958, these drugs have continued to prove their usefulness. Numerous clinical trials have confirmed that these medications are as or more effective in reducing blood pressure and cardiovascular events than any of the other effective antihypertensive agents. Although some metabolic changes may occur with higher dosages of these medications, they seem to be of limited clinical significance. In addition to being well tolerated and effective. thiazide-type diuretics are less expensive than other agents. This should be considered in view of the fact that quality of care is not compromised when the less expensive medication is used. beta-Blockers should also be considered as a major drug class in the management of hypertensive patients. There are few studies directly comparing these drugs with other agents. Although some data suggest that they are less effective in reducing CHD events in the elderly, there are numerous situations where beta-blockers are clearly indicated. Combination therapy with a beta-blocker and diuretic has been shown to be highly effective in reducing cardiovascular events in both diabetic and nondiabetic patients.

The regulation and measurement of plasma volume in heart failure

Plasma volume, the intravascular portion of the extracellular fluid volume, can be measured using standard dilution techniques with radiolabeled tracer molecules. In healthy persons, plasma volume remains relatively constant as a result of tight regulation by the complex interaction between neurohormonal systems involved in sodium and water homeostasis. Although chronic heart failure (CHF) is characterized by activation of many of these neurohormonal systems, few studies have evaluated plasma volume in this condition under treatment. Untreated edematous decompensated heart failure (HF) is associated with a significant expansion of plasma volume. Patients with stable CHF, receiving conventional therapy, appear to have a contracted plasma volume, a concept that is in contrast to the widely held belief that CHF is associated with long-term hypervolemia. It is likely that significant changes in plasma volume occur during intensification of medical therapy or during transition from the edematous to the stable state. Clinical assessment of plasma volume may be of particular value during treatment in patients with decompensated HF, in whom the plasma volume is contracted despite an increase in total extracellular fluid volume. Under these circumstances, treatment with inotropes or renal vasodilators may be more appropriate than intravenous diuretics alone. Further studies evaluating plasma volume in HF may help to improve our understanding of the pathophysiologic mechanisms occurring in the development and progression of this complex condition.

Relaxation of vascular smooth muscle by cicletanine in aged wistar aorta under stress conditions: importance of nitric oxide

The vascular mechanism of action of cicletanine, an antihypertensive agent, was studied on isolated Wistar rat aortas (24-months-old) in presence and in absence of endothelium in two different stress conditions, normoxic and hypoxic, in presence of norepinephrine (NE). Under normoxic conditions, in presence of endothelium, cicletanine (10(-9)-10(-5)M) induced a concentration-dependent relaxation, whereas in absence of endothelium, cicletanine (10(-9)-10(-5)M) was ineffective although it relaxed the smooth muscle at higher concentrations (10(-4)M). At pharmacologic concentrations (below or equal 10(-5)M), relaxation induced by cicletanine, in presence of endothelium, was prevented by N(omega)-nitro-L-arginine (L-NNA) (P <.005) and relaxation induced by the highest concentration (10(-4)M) was reversed by BaCl2 (P <.005). Under hypoxic conditions, in presence of NE and endothelium, the aorta displayed an increased developed tension that was significantly (P <.05) attenuated by cicletanine (10(-5)M) and insensitive to indomethacine (10(-7)M). When the two compounds were added together, the relaxation induced by cicletanine was significantly improved (P <.005). These results indicated that cicletanine, under stress conditions, relaxes vascular smooth muscle through an endothelium-dependent action mediated by the nitric oxide (NO) synthase pathway. We proposed that the observed vascular effects could be associated with the counter-regulation mechanisms linked to the antihypertensive action of cicletanine.

Diuretics and beta-blockers: is there a risk for dyslipidemia?

BACKGROUND

Hypertension is a major risk factor for cardiovascular disease, particularly in combination with other risk factors such as obesity, smoking, diabetes, and dyslipidemia. Effective management of hypertension, even a modest reduction in blood pressure, results in reduced mortality and morbidity from cerebrovascular and cardiovascular disease.

METHODS

Data from clinical trials were examined to assess the effect of low-dose diuretics or beta-blockers on lipoprotein profile.

RESULTS

Results of numerous clinical trials demonstrate that low doses of thiazides or cardioselective beta-blockers alone or in combination result in minimal changes in lipid profile.

CONCLUSIONS

Low doses of thiazide diuretics or beta-blockers are a safe and effective approach to the management of uncomplicated essential hypertension and have the advantage of an association with an evidence-based reduction in morbidity and mortality in long-term clinical trials. There is little or no evidence that the minimal changes in serum lipid profile associated with use of these drugs have resulted in a negation of the beneficial effects of these agents on blood pressure.

Old antihypertensive agents-diuretics and beta-blockers: do we know how and in whom they lower blood pressure?

Morbidity and mortality data that increasingly favor the use of diuretics and beta-blockers continue to accumulate. Nonetheless, the use of these agents particularly diuretics is viewed as a dated practice. The issue of which antihypertensive drug to use will remain complex and confusing as the marketing juggernauts of the pharmaceutical industry vigorously promote drug classes such as angiotensin- converting enzyme inhibitors, angiotensin-receptor antagonists, and calcium-channel blockers. A pharmacologic middle ground will probably be reached in the form of fixed-dose combination therapy. Diuretics and b-blockers are mechanistically rational when combined with numerous other drugs; thus, the issue of which drug should be used to begin treatment will be settled by a truce, albeit an uneasy one, favoring the increased use of fixed-dose combination antihypertensive therapy.

Hydrochlorothiazide Increases Efferent Glomerular Arteriolar Resistance in Spontaneously Hypertensive Rats

BACKGROUND: Micropuncture studies were performed to determine the intrarenal hemodynamic effects of two conventional antihypertensive agents, hydrochlorothiazide (HCTZ) and hydralazine (HYDR) alone and in combination. METHODS AND RESULTS: Male spontaneously hypertensive and normotensive Wistar Kyoto rats (19 weeks old) were treated for 3 weeks with vehicle (control), HCTZ (80 mg/kg/d), HYDR (5 mg/kg/d), or combined therapy (HCTZ 30 mg/kg/d and HYDR 2 mg/kg/d). Each treatment significantly reduced arterial pressure while effective renal plasma flow, glomerular filtration rate and single nephron glomerular filtration rate were unaffected by any treatment in either strain. In spontaneously hypertensive rats HCTZ decreased single nephron plasma flow (111 +/- 8 to 84 +/- 4 nL/min; P <.05) but, despite this reduction, glomerular pressure remained unchanged (51.4 +/- 0.7 to 52.1 +/- 0.8 mmHg) attributable to increased efferent glomerular resistance (1.58 +/- 0.14 to 2.11 +/- 0.12 10 U; P <.05). By contrast, HYDR increased single nephron plasma flow (to 147 +/- 8 nL/min; P <.01) and decreased efferent glomerular resistance (to 1.09 +/- 0.09 U; P <.05). Combined treatment produced responses similar to HCTZ when used alone, thereby nullifying the beneficial efferent glomerular resistance effects: single nephron plasma flow +/- fell (to 89 +/- 7 nL/min; P <.05) and efferent glomerular resistance increased (to 2.05 +/- 0.17 U; P <.05). In Wistar Kyoto rats, HCTZ and combined treatment had no effect. HCTZ alone induced glomerular ischemia that was associated with efferent glomerular arteriolar constriction in these spontaneously hypertensive rats. CONCLUSIONS: These findings provide a possible explanation for the lack of improved renal target-organ damage in controlled multicenter trials employing thiazide diuretics.

The role of potassium in postural hypotension: electrolytes and neurohumoral factors in elderly hypertensive patients using diuretics

OBJECTIVE

To study the association between postural hypotension and (i) electrolyte levels and (ii) neurohumoral factors in elderly hypertensive patients using diuretics.

DESIGN

Cross-sectional study of patients and controls.

SETTING

The subjects were gathered from senior citizen clubs or they were referred to the study by general practitioners. The subjects were examined on a geriatric ward in Turku City Hospital.

SUBJECTS

Seven subjects with postural hypotension and 13 controls.

MEASUREMENTS

Plasma electrolyte levels and neurohumoral response to head-up tilt.

RESULTS

There were significantly more hypokalaemic subjects in the postural hypotension group (5/7) than in the control group (1/13) (P < 0.01). The plasma potassium level was negatively correlated to plasma aldosterone (r = -0.57; P < 0.01) and renin activity (r = -0.69; P < 0.001). Subjects with postural hypotension had higher levels of noradrenaline, both supine (P < 0.05) and during tilt (P < 0.05). There were no significant differences in supine or tilt levels of plasma adrenaline, vasopressin, atrial natriuretic peptide, aldosterone and renin activity between the groups.

CONCLUSION

The results suggest that potassium depletion is associated with postural hypotension in elderly hypertensive patients using diuretics. However, it is unclear whether there is a causative link between potassium depletion and postural hypotension or whether they are both caused by some other factor, e.g. volume contraction.

Apparent polycythaemia: diagnosis, pathogenesis and management

The term "apparent polycythaemia" is applied to a group of patients who have a raised PCV (> 0.51 in males, > 0.48 in females) but a normal red cell mass (less than 25% above their predicted mean normal value). Some have additionally a marked reduction in plasma volume and can be defined as a subgroup: relative polycythaemia. Smoking, hypertension and to a lesser extent obesity, excessive alcohol, low-dose diuretic therapy and hypoxaemia have all been associated with apparent polycythaemia but the mechanism is both uncertain and likely to be complex. This group of patients is unlikely to be uniform in pathogenesis and may well include some normal individuals. Investigation requires exclusion of factors associated with other types of polycythaemia. The possibility of an increased vascular occlusive risk is uncertain in these patients except at the higher PCV values. Reduction of PCV by venesection is sensible at PCV > 0.54 or where there is perceived to be an increased risk of vascular occlusion. The remaining patients should be managed by regular observation to detect further rise in PCV or evolution to absolute polycythaemia (raised red cell mass). In some, the PCV returns to normal.

Pharmacologic management of childhood hypertension

Antihypertensive drug therapy is used in children primarily to treat secondary forms of hypertension, because the prevalence of essential hypertension in the first decade of life is considerably less than 1% of the childhood population. This prevalence increases during the second decade of life, but the percentage of teenagers with essential hypertension continues to be low. Pharmaceutical companies have been able to target drug development to specific physiologic and biochemical systems. The converting enzyme inhibitors and calcium-channel blockers have greatly improved the success of therapy concomitant with a reduction in the incidence of adverse effects. The result has been a major change during the past decade in the recommendations for antihypertensive drug therapy.

Central hemodynamic effects of diuretic therapy in chronic heart failure

In chronic heart failure diuretic drugs improve central hemodynamic variables and cardiac pumping secondary to altered plasma and extracellular volumes; humoral markers of these changes include increased plasma renin and aldosterone levels. The latter increases are maximal over the first week but decline with chronic therapy. The plasma alpha-ANP levels show a reciprocal effect; these data are compatible with a rapid contraction of the plasma volume which is sustained during chronic therapy. The acute hemodynamic actions of diuretic agents reflect both immediate and direct vascular actions and also effects secondary to diuresis (volume redistribution). At rest substantial reductions in pulmonary "wedge" pressure (-29%), with a consequent fall in cardiac output (-10%), are described. Total systemic vascular resistance initially increases but "reverse autoregulation" over subsequent weeks returns this elevation gradually towards control values. Tolerance to these initial hemodynamic effects does not occur with maintained therapy; moreover, echocardiographic markers of contractility and exercise capacity may increase. The early venodilator effects of diuretic drugs can be attributed to prostaglandin release and the initial pressor actions to activation of the renin angiotensin system; these vascular actions may have limited relevance to long-term beneficial effects on hemodynamics. Direct pulmonary vasodilation and improved pulmonary compliance remain an interesting finding. Although most patients are both symptomatically and hemodynamically improved at rest, the actions during exercise are more varied. Some individuals with severely impaired left ventricular function show little hemodynamic improvement, whereas those with milder dysfunction usually benefit; in the main this is probably related to the latter being on a steeper cardiac function curve.(ABSTRACT TRUNCATED AT 250 WORDS)

The place of diuretics in the treatment of hypertension: a historical review of classical experience over 30 years

Thiazide diuretics have been the mainstay of antihypertensive therapy for over 30 years. Their precise mechanism of antihypertensive action is still incompletely understood. They reduce arterial pressure initially through a fall in plasma volume and cardiac output. However, with chronic administration cardiac output tends to return toward pretreatment levels, suggesting that the long-term pressure reduction is mediated through a reduction in vascular resistance. Although multiple lines of evidence suggest that salt and water loss is an essential part of the mechanism, at least in some cases an indirect vasodilator effect may play a role as well. The antihypertensive efficacy of diuretics is proven; they are at least as effective as other classes of antihypertensive drugs. They have been shown to protect against stroke, but not against mortality from myocardial infarction. There is some concern about the metabolic side effects, such as hypokalemia, hyperglycemia, and hyperlipidemia. In order to minimize these side effects the lowest effective dose should be used. Diuretics are likely to remain first-line antihypertensive agents, but they should be considered as one of several possible choices for the initial therapy among other classes, such as beta-blockers, ACE inhibitors, or calcium entry blockers.

Relation of regression of left ventricular hypertrophy to changes in ambulatory blood pressure after long-term therapy with perindopril versus nifedipine

Casual as well as ambulatory 24-hour blood pressure (BP) and echocardiographic parameters were studied in 40 patients with untreated or insufficiently treated mild to moderate essential hypertension. Left ventricular (LV) hypertrophy was assessed before and after 24 weeks of therapy with either the converting enzyme inhibitor perindopril or the calcium antagonist nifedipine. The design was a double-blind parallel study with a placebo run-in period. Patients received a daily oral dosage of either 4 to 8 mg of perindopril or 40 to 80 mg of nifedipine in slow-release form. A diuretic (25 mg/day of hydrochlorothiazide) was added in nonresponders (greater than 90 mm Hg casual diastolic BP). Once-daily perindopril and twice-daily nifedipine comparably reduced both casual and ambulatory BP throughout 24 hours (p less than 0.01) without affecting 24-hour heart rate. Six subjects withdrew from the nifedipine group and 4 from the perindopril group. After 12 and 24 weeks of therapy, LV hypertrophy was significantly reduced by both agents. Before active treatment was begun, LV mass index was more closely correlated to 24-hour (p less than 0.001) than to casual BP. This correlation disappeared after treatment with both agents. The correlation between ambulatory systolic day-time BP and LV mass was only still present (r = 0.54; p less than 0.05) after 24 weeks of treatment with nifedipine. It is concluded that regression of LV hypertrophy during converting enzyme inhibition or calcium antagonism may be partly independent of dosage and magnitude of 24-hour BP decrease.

Chlorthalidone reduces vascular hyperresponsiveness in DOCA-salt hypertensive rats

The mechanisms of anti-hypertensive effect of diuretics remain unknown. The purpose of this study was to test the hypothesis that long-term treatment with chlorthalidone decreases the responsiveness of resistance vessels to neurohormones. The study was performed in deoxycorticosterone acetate (DOCA)-salt hypertensive rats with and without treatment with chlorthalidone (Chlor. 8 mg/day, for 20 days). Resting mean arterial pressure in freely moving state was significantly reduced in DOCA-salt-Chlor rats when compared to DOCA-salt rats (116 +/- 3 vs 147 +/- 7 mmHg, respectively). Chlorthalidone treatment reduced the high plasma sodium content observed in DOCA-salt rats to the same levels observed in normotensive control groups. Results obtained in isolated perfused mesenteric arteries showed: a) the increase in perfusion pressure elicited by norepinephrine (NE), serotonin (SE) and vasopressin (VP) was significantly greater in DOCA-salt than in DOCA-salt + Chlor rats or control normotensive rats; b) the endothelium removal increased the pressor responses to NE, SE and VP in a similar way in all groups. These data provide evidence that long-term chlorthalidone treatment reduces vascular hyperresponsiveness to these neurohormones. In addition, these results indicate that this reduction in vascular hyperresponsiveness, associated with a decrease in extracellular sodium level, could be a possible mechanism by which the diuretics reduce the high blood pressure.