Role of renal kallikrein in modulating the antihypertensive effect of a single oral dose of captopril in normal- and low-renin essential hypertensives.

The antihypertensive efficacy of angiotensin converting enzyme (ACE) inhibitors may result from the blockade of angiotensin II formation but also, theoretically, from the inhibition of kinin breakdown. To test whether a blunted activity of the kallikrein-kinin system might account for the failure of ACE inhibitors in lowering blood pressure (BP) in patients in whom the renin-angiotensin system (RAS) is not enhanced, 31 essential hypertensives with normal or low plasma renin activity (PRA) were evaluated before and after a single oral dose (50 mg) of captopril. A significant fall, in both systolic and diastolic BP, was obtained in the subgroup of patients who were classified as 'normal-kallikrein hypertensives' according to whether their pretreatment urinary kallikrein excretion was within the normal range, while no significant change in BP was observed in 'low-kallikrein hypertensives'. Furthermore, the mean percentage fall in mean BP, throughout the 2 h following captopril administration, was significantly related to the basal value of urinary kallikrein excretion (r = 0.47, P less than 0.05) in all the patients. Our results suggest that blunted activity of the kallikrein system might be responsible for failure of captopril to lower BP in some hypertensive patients.

The effects of aprotinin, a kallikrein inhibitor, on renin release and urinary sodium excretion in mild essential hypertensives.

The effects of aprotinin on renin release and renal function were evaluated in 24 male essential hypertensive patients, on unrestricted (n = 17) and on chronic low as well as on high sodium intake. Aprotinin (1 X 10(6) kallikrein inhibitor units) or saline (200 ml) were infused in all patients for 6 h. Blood samples were taken for plasma renin activity (PRA) and 6-h urine collections were obtained for active and inactive kallikrein, sodium and potassium excretion measurement. In patients on unrestricted sodium diet, aprotinin had no effect on blood pressure (BP), glomerular filtration rate, renal plasma flow, urinary sodium and potassium excretion. However, an inverse relationship was found between pretreatment urinary sodium excretion and the per cent reduction of the latter after aprotinin. A significant reduction in urinary sodium excretion was induced by aprotinin in patients on high sodium intake, whereas no change was observed in the same patients when on a low sodium diet. Aprotinin reduced the urinary excretion of active kallikrein by 81% and the active to total kallikrein ratio from 24 to 6%. Infusion of aprotinin induced a significant decline in active renin but did not modify inactive renin levels in patients on unrestricted sodium diet as well as in patients on low or high sodium intake. Our data suggest that the inhibition of kallikrein and/or other serine proteases by aprotinin can interfere with renal release of active renin and also support the hypothesis that the renal kallikrein system exerts a regulatory control on sodium excretion in salt replete hypertensives.

Natriuretic effect of acute nifedipine administration is not mediated by the renal kallikrein-kinin system.

Despite their vasodilating action, calcium antagonists increase renal sodium excretion. To ascertain whether renal kallikrein plays a role in the renal effects of calcium antagonists, nifedipine (N) (10 mg orally) or placebo (P) was given to 17 male patients with mild to moderate essential hypertension during a 6-h infusion of either saline (S) or aprotinin (A) (2 X 10(6) KIU in 200 ml of saline). Blood pressure (BP) and heart rate (HR) were measured every 10 min, and blood samples were taken at -10, 0, 30, 60, 120, 240, 360 min for plasma renin activity (PRA), creatinine, and osmolarity determinations. Urinary kallikrein, aldosterone, creatinine, and electrolytes were measured in 6-h urine collections. The acute administration of N induced a significant systolic BP (SBP) and diastolic (DBP) fall and a transient PRA increase that peaked at 30 min and were not modified by A infusion. Urinary volume (+47%), Na+ (+54%) and Cl- (+58%) excretion were significantly enhanced by N. There were less pronounced and statistically not significant increases in urinary excretion of Ca2+ (+38%) and K+ (+29%). Infusion of A did not interfere with the natriuretic effect of N. Our data do not support the hypothesis that the kallikrein-kinin system plays an important role in mediating the renal effects of nifedipine in humans.

The effects of aprotinin, a glandular kallikrein inhibitor, on renin release and urinary sodium excretion in mild essential hypertensives.

The effects of aprotinin on renin release and renal function were evaluated in 24 male essential hypertensive patients, on unrestricted (N = 17) and on chronic low as well as on high Na intake. Aprotinin (1 x 10(6) KIU) or Saline (200 ml) were infused in all patients for 6 hrs. Blood samples were taken for plasma renin activity (PRA) and 6-hr urine collections were obtained for active and inactive kallikrein, Na+, K+, Cl-, Ca++ excretion measurements. In patients on unrestricted sodium diet aprotinin had no effect on BP, glomerular filtration rate, renal plasma flow, urinary sodium and potassium excretion. However an inverse relationship was found between pretreatment urinary sodium excretion and the percent reduction of the latter after aprotinin. A significant reduction in urinary sodium excretion was induced by aprotinin in patients on high sodium intake, whereas no change was observed in the same patients when on low sodium diet. Aprotinin reduced the urinary excretion of active kallikrein by 81% and the active to total kallikrein ratio from 24% to 6%, suggesting that it could interfere with the in-vivo activation of inactive kallikrein. Infusion of aprotinin induced a significant decline in active renin but did not modify inactive renin levels in patients on unrestricted sodium diet as well as in patients on low or high sodium intake. Our data suggest that the inhibition of kallikrein and/or other serine proteases by aprotinin can interfere with renal release of active renin and also support the hypothesis that the renal kallikrein system exerts a regulatory control on sodium excretion in salt replete hypertensives.

Urinary kallikrein excretion can predict the blood pressure response to a single oral dose of captopril.

The antihypertensive efficacy of ACE inhibitors depends in theory from the blockade of the angiotensin II formation but also from the inhibition of kinin breakdown. To test whether a blunted activity of the kallikrein-kinin system might account for the failure of ACE inhibitors in lowering BP in patients in whom the renin-angiotensin system is not enhanced, thirty-one essential hypertensives with normal or low PRA were evaluated before and after a single oral dose (50 mg) of captopril. A significant fall both in systolic and diastolic blood pressure (BP) was obtained in the subgroup of patients who were classified as "normal kallikrein hypertensives" according to whether their pretreatment urinary kallikrein excretion was within the normal range, while no significant change in BP was observed in "low kallikrein hypertensives". Furthermore the mean percentage fall in mean BP, throughout the 2 hours following captopril administration, was significantly related to the basal value of urinary kallikrein excretion (r = 0.47, p less than 0.05) in the entire group of patients. Our results suggest that a blunted activity of the kallikrein system might be responsible for failure of captopril in lowering BP in patients in whom the renin-angiotensin system is not pathogenetically implicated.

Comparison of sublingual and oral captopril in hypertension.

The use of sublingual captopril has been recently suggested in hypertensive crisis on the assumption of a faster absorption and thus a more rapid effect on blood pressure than with the oral route. To verify this hypothesis we have compared the hypotensive effect of oral and sublingual captopril in 40 essential hypertensives who were randomly allocated to either route of administration. Captopril was administered orally dissolved in water or allowed to dissolve under the tongue. After 5, 10, 20, 30, 40, 60 and 90 minutes blood pressure, Plasma Renin Activity (PRA) and Angiotensin Converting Enzyme (ACE) were measured. No significant differences were found between the two groups in the time course of blood pressure decrease, PRA increase and ACE inhibition. The changes of the parameters studied was superimposable irrespective of the route of administration thus not supporting the hypothesis that sublingual captopril might be absorbed more rapidly.

Relation between urinary kallikrein excretion and blood pressure response to a single oral dose of captopril.

Thirty-one essential hypertensives with normal or low PRA were evaluated before and after a single oral dose (50 mg) of captopril. A significant fall both in systolic and diastolic blood pressure (BP) was obtained in the subgroup of patients who were classified as "normal kallikrein hypertensives", while no significant change in BP was observed in "low kallikrein hypertensives". Furthermore the mean percentage fall in mean BP, throughout the 2 hours following captopril administration, was significantly related to the basal value of urinary kallikrein excretion (r = 0.47 p less than 0.05) in the entire group of patients. Our results suggest that a blunted activity of the kallikrein system might be responsible for failure of captopril in lowering BP in patients in whom the renin-angiotensin system is not pathogenetically implicated.

Prokallikrein and kallikrein inhibitors in human urine.

Kallikrein activity (KA) and kallikrein inhibitors (KI) were evaluated in urine from normal subjects and essential hypertensives. Kallikrein inhibitors were almost completely removed by dialysis against water, confirming previous reports of their low molecular size. The negative relationship found between KA and KI in urine samples from 12 normal subjects suggests that KI might play a role in the modulation of KA excreted by the kidney. Heating (85 degrees C for 20 min) partially reduced (-52%) KI, indicating thermostable substances other than peptides as KI. When cations were evaluated as possible KI, by adding different salts to dialysed salt-free urine, only sodium was able to inhibit KA by 20% at a concentration not far from the physiological range. Trypsin added to dialysed urine produced a striking increase in KA without significant changes in Km and pH optimum. These findings, together with the observation that acid dialysis did not increase KA, strongly support the hypothesis that trypsin-activatable kallikrein might be a pro-enzyme. A lower urinary excretion of active kallikrein was found in 48 essential hypertensives when compared with 31 normotensive controls. However, trypsin-activatable kallikrein excretion was similar in the two groups, suggesting that low KA in hypertensives might not reflect a defective conversion of the pro-enzyme into the active form.

Active and inactive renin during acute reduction of renal perfusion pressure in essential hypertensives.

Arterial and renal venous active and inactive renin were studied in 5 patients with long established moderate hypertension following unilateral acute reductions of renal perfusion pressure (15% and 70% of control) by inflating a balloon catheter introduced into the right renal artery. This procedure failed to induce the expected release of active renin; total and inactive renin levels were also unchanged. On the contrary in a single normotensive patient smaller reductions of the renal perfusion pressure (-15% and -30%) were able to acutely increase the release of active renin with a concurrent conversion of inactive renin but without inducing blood pressure changes. These findings show that the renin pattern typical of unilateral renovascular hypertension, including the intrarenal activation of inactive renin, could be reproduced acutely in a normotensive subject. Moreover, a complete reversal of the above mentioned active and inactive renin pattern was observed in a recent onset renovascular hypertensive patient within 30 min from successful percutaneous transluminal dilation. The negative results observed in our hypertensive patients suggest that structural changes induced by the long duration of hypertension might have reduced the sensitivity of the baroceptors involved in renin release.