Urinary kallikrein excretion in normotensive and spontaneously hypertensive rats

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.

Studies on renal and urinary kallikrein in spontaneously hypertensive rats

Urinary kallikrein excretion, active (A) and total (T), was significantly reduced in the Okamato spontaneously hypertensive rat (SHR). The renal content of A and T soluble tissue kallikrein was clearly greater in this strain. Arginine vasopressin stimulated the release of both A and T kallikrein from renal cortical slices of the normotensive (N) Wistar strain, this response was not observed in the SHR. PGE2 stimulated release of A kallikrein in the N strain and both A and T enzyme in the SHR. It is suggested that urinary kallikrein excretion values may not necessarily reflect the endogenous levels of the enzyme in the kidney. Both the level and release of renal tissue kallikrein may become altered either in response to or as a result of chronically elevated blood pressure.

The renal kallikrein-kinin system in spontaneously hypertensive rats

In male spontaneously hypertensive rats (SHRSP) of the stroke prone strain (Okamoto) and in normotensive Wistar-Kyoto rats (WKY) urinary kallikrein excretion was investigated at different age and at drug-induced diuresis. In rats of both strains from 7th till 19th week of age urinary kallikrein excretion increased with age. In SHRSP of 7th till 11th week of age kallikrein excretion was higher than in WKY rats, while it was lower in the 48-week-old SHRSP. No correlation was found between urinary kallikrein excretion and systolic blood pressure. In SHRSP and WKY rats a similar daily rhythm of kallikrein excretion in urine was found being high in the early morning and low in the afternoon. Kallikrein excretion correlated significantly with urine volume. The loop diuretic bumetanide (4 and 40 mg/kg) induced diuresis and natriuresis in both strains, however more marked in the WKY rats than in the SHRSP. Urinary kallikrein excretion, however, showed in both strains the same biphasic course with a short lasting increase and a secondary decrease. Thus, in the average urinary kallikrein excretion was not effected by the drug. Prolonged treatment with furosemide over 5 days (125 mg/kg) resulted in an increase in kallikrein excretion in urine, more pronounced in the WKY rats than in the SHRSP. The observed results suggest that renal kallikrein-kinin system is not involved in the development of spontaneous hypertension as a pathogenetic factor, but rather is influenced by other factors like hormone interactions, i.e. mineralocorticoids and catecholamines, as well as renal function and acute changes in urine flow.

Captopril and hypertension development in the SHR

Captopril (100mg/kg) administered daily by gavage to young spontaneously hypertensive rats (SHRs) from their 6th to 20th weeks of age almost completely inhibited genetic hypertension development (GHD). This effect was correlated with an early and long-lasting limitation of the progressive increase in peripheral resistance which normally develops in SHRs during their growth. Heart rate, cardiac and systolic indexes remained unchanged, plasma renin concentration was significantly increased and heart weight/body weight ratio was significantly decreased. At last, captopril's GHD preventive effect persisted up to 12 weeks after treatment discontinuation.

[Renal kallikrein-kinin system and control of blood pressure (author's transl)]

Kallikrein excreted with the urine appears to be formed in the kidney. The kallikrein-kinin system in the kidney is localized in the distal nephron from the juxtaglomerular apparatus to the collecting duct. It has been shown that intrarenal infusion of kinins produces an increase in renal blood flow as well as diuresis and natriuresis. Part of the effect of kinins appears to be mediated by the release of prostaglandins. However, the precise role of the renal kallikrein-kinin system in sodium and volume homeostasis and in blood pressure regulation still remains to be determined. Mineralocorticoids as well as the diuretics furosemide, bumetanide and bendroflumethiazide increase, spironolactone decreases kallikrein excretion. Urinary kallikrein has been shown to increase acid-as well as cryoactivation of prorenin in vitro. It is unclear as yet, however, whether the renal kallikrein-kinin system takes part in converting inactive prorenin into active renin in vivo. There are reports on subnormal, normal as well as increased kallikrein excretion in spontaneously hypertensive rats. In rats susceptible to the hypertensive effect of salt a substantially decreased excretion of kallikrein has been observed. Kallikrein excretion has been described to be increased in primary aldosteronism and to be reduced in a proportion of patients with established essential hypertension. In patients with labile hypertension, however, kallikrein excretion appears to be normal suggesting that decreased urinary kallikrein in essential hypertension is a consequence rather than a cause of hypertension. The renal kallikrein-kinin system does not appear to play a primary role in the pathogenesis of hypertension.

The renal kallikrein-kinin system in human and in experimental hypertension

The kallikrein-kinin system in the kidney is localized in the distal nephron, where it appears to be linked to processes that control water and electrolyte excretion. Some evidence exists that the effect of the renal kallikrein-kinin system is partially mediated by the release of prostaglandins. It has not yet been determined whether endogenous kinins affect the function of the nephron directly or indirectly by changes in renal blood flow distribution. Further, a number of studies in animals and humans indicates that kallikrein excretion is decreased in most types of hypertension, with the exception of hypertension caused by an excess of mineralocorticoids (where kallikrein is increased). In rats susceptible to the hypertensive effect of salt, kallikrein is conspicuously decreased. In renal diseases, urinary kallikrein excretion is also decreased. Finally, it still needs to be determined whether or not low kallikrein excretion is a pathogenetic factor in hypertension and renal diseases.

Effects on rat urinary kallikrein excretion of bumetanide, bendroflumethiazide and hydralazine

Single and/or repeated administrations of bumetanide, bendroflumethiazide and hydralazine to normotensive and spontaneously hypertensive rats resulted in an increase of urinary kallikrein excretion. No correlation was found with sodium output. The role of the increased plasma renin activity is discussed and it is suggested that the activation of the renal kallikrein-kinin system is related to the increase in renal blood flow.