Renal pharmacodynamics of antihypertensive drugs: clinical applications

Vasodilators, antihypertensive therapy and the kidney

Both traditional and newer treatments of essential hypertension are discussed in relation to kidney function and renal perfusion. In essential hypertension, renal vascular resistance is routinely increased and renal blood flow is often decreased. Reduced sodium intake as a form of therapy will cause a decrease in both renal blood flow and glomerular filtration, most likely due to an angiotensin-induced renal vasoconstriction caused by the reactive increase in renin release. Treatment with diuretics produces the same effects, also angiotensin-mediated. The addition of a beta-adrenergic blocking agent to prevent renin release may be a good choice, but individual agents within this class must be examined for direct renal vasoconstriction. The effects of "nonspecific" vasodilators on renal perfusion and renal sodium handling vary with the patient but may produce antinatriuresis, sodium retention and decrease in glomerular filtration. Studies with calcium antagonists have shown promising results. Nifedipine studies show a substantial increase in renal plasma flow, a well-maintained glomerular filtration rate and a brisk diuresis and natriuresis. However, patients with the lowest baseline renal flow do not show these benefits. Diltiazem has shown a potentiated renal vascular response in normotensive patients of hypertensive parents. Angiotensin converting enzyme inhibitors such as captopril and enalapril have produced increased renal blood flow and well-maintained glomerular filtration in patients with essential hypertension. The agents available for treating hypertension have improved dramatically in the past decade. A salutary effect on the kidney will remain high on the list of important characteristics to be considered in choosing one of these agents.

The kidney and antihypertensive therapy

To a large degree, modern antihypertensive therapy has evolved from the development of agents that act as vasodilators but, for one reason or another, avoid the disadvantages of the nonspecific vasodilators. This review examines the impact of antihypertensive agents on renal perfusion and function and relates it to their efficacy in reducing high blood pressure. Special attention is given to beta-adrenergic blocking agents that have a minimal impact on the kidney, converting enzyme inhibitors, calcium channel blockers and dopamine analogs. Also reviewed are the functional abnormalities involving the renal blood supply in essential hypertension, the role of newer pharmacologic agents in therapy and the nature and extent of reactive responses that often limit the response to therapeutic agents.

Strategies in antihypertensive therapy: implications of the kidney

Because we so rarely know the cause of hypertension, antihypertensive therapy remains empiric. However, certain principles of treatment are emerging; one of these concerns the critical role of the kidney in antihypertensive therapy. Whether or not the kidney is primarily responsible for hypertension in a patient, it is the patient's renal response to treatment that determines, to a major degree, an agent's efficacy. Vasodilators have been a conceptually attractive approach to the treatment of high blood pressure, because they decrease total peripheral resistance, which is considered to be the mechanism responsible for this condition in most patients. Nonspecific vasodilators exert a series of actions on the kidney--including profound sodium retention and reactive renin release--that limits therapeutic response. For reasons that are not yet clear, but are apparently related to specific action on calcium entry into vascular smooth muscle, endocrine function, and renal hemodynamics, calcium channel blocking agents, such as nifedipine, have an advantage in the treatment of hypertension. They cause little or no sodium retention; thus, the addition of a diuretic agent is not required. In fact, there is evidence that sodium loading in certain patients may potentiate the antihypertensive efficacy of these drugs. The renin-angiotensin system seems to be activated to a somewhat lesser degree by calcium channel blocking agents than it is by nonspecific vasodilators; in addition, these agents interfere with the actions of angiotensin on aldosterone release. Moreover, their dilator action on the renal blood supply favors sodium excretion. Nifedipine either has no effect on the renal blood supply or induces an increase in renal blood flow and maintains glomerular filtration rate, both of which combine to support the ensuing natriuresis.

The kidney and effective antihypertensive therapy

Whether or not the kidney is involved in the genesis of hypertension in an individual patient, it becomes a major determinant of the response to antihypertensive therapy once a treatment strategy is adopted. The major mechanisms through which the kidney influences blood pressure are renin release and sodium retention, either together or separately, but additional mechanisms may also contribute. When sodium intake is restricted or a diuretic is used, the reactive increase in plasma renin activity makes a substantial contribution to limiting the blood pressure fall. When vasodilators or agents that block the sympathetic nervous system are used, sodium retention plays an important role. Among newer agents, the effectiveness of calcium channel blockers, converting enzyme inhibitors and perhaps dopamine analogs reflects, for reasons that differ from 1 class of agent to another, a special action on the kidney that limits the reactive renal response to the reduction in blood pressure. Treatment strategies that address the problem of the renal response are more likely to be effective than approaches that avoid or ignore it.

The kidney and strategies for the treatment of hypertension

Renal compensatory mechanisms are a chief consideration when selecting an antihypertensive agent. The relationships, therefore, between renal blood flow and glomerular filtration rate, sodium handling by the kidney, and release of renin require particular attention. We shall examine therapeutic measures such as beta adrenergic blockers, vasodilators, calcium entry blockers, and converting enzyme inhibitors in light of their effects on renal blood flow.

Enhancement by diuretics of the antihypertensive action of long-term angiotensin converting enzyme blockade

Thirty-nine patients with various types of hypertension were treated by chronic blockage of the angiotensin converting enzyme, i.e. by twice daily administration of captopril, 50 to 200 mg p.o. The blood pressure reduction observed 1 hour following administration of the inhibitor was directly related to the baseline plasma renin activity (r=- 0.67, p < 0.001). Whenever blockade of the renin system alone did not lower blood pressure to normal levels additional sodium subtraction brought it under control. With the renin system neutralized, blood pressure becomes exquisitely sensitive to changes in sodium balance. Diuretics seem to preserve optimal natriuretic efficacy despite blood pressure reduction, probably because aldosterone levels are reduced and renal blood flow increases. Blockade of the renin system together with individually tailored salt subtraction provides an attractive new approach to long-term treatment of clinical hypertension.

Increased glomerular filtration rate after converting-enzyme inhibition in essential hypertension

To test the influence of an inhibitor of angiotensin-converting enzyme, teprotide (SQ 20881), we administered it to seven patients with essential hypertension and normal renal function and nine with an unequivocal reduction in creatinine clearance, caused by bilateral renal-artery stenosis in two and by essential hypertension in seven. Despite the fall in blood pressure (112.7 +/- 4.5 to 100.3 +/- 3.9 mm Hg, mean +/- S.E.M., P less than 0.01), there were prompt increases in both creatinine clearance (95.9 +/- 10.5 to 109.9 +/- 9.5 ml per minute per 1.73 m2 of body-surface area, P less than 0.01) and sodium excretion (17.0 +/- 5.9 to 31.7 +/- 7.2 mumol per minute, P less than 0.01) in patients with essential hypertension. The increase in glomerular filtration rate was most striking, averaging 33 per cent (66.0 +/- 10.3 to 88.0 +/- 9.2 ml per minute per 1.73 m2, P less than 0.001) in patients in whom an initial reduction was evident and hypertension was more severe. These observations suggest that a functional element, perhaps involving angiotensin-mediated renal vasoconstriction, frequently has a role in the reduction in glomerular filtration rate that occurs in essential hypertension. This class of agent may improve renal excretory function as it controls hypertension.