Influence of the renal medulla and early treatment with enalapril upon the development of hypertension in young spontaneously hypertensive rats

A comparative study of morphological changes in spontaneously hypertensive rats and normotensive Wistar Kyoto rats treated with an angiotensin-converting enzyme inhibitor or a calcium-channel blocker

It is not clear whether some pathological changes in hypertension are directly pressure-dependent, or hormonally induced, or both. The aortic arch has apparently never before been studied for those changes. The aim of this study was to compare the effects of controlling angiotensin II (Ang II) and/or blood pressure (BP), directly at the inception of hypertension, on the aortic arch, the left ventricle of the heart (LV), and the kidneys of spontaneously hypertensive rats (SHRs) and normotensive Wistar Kyoto (WKY) rats. An angiotensin-converting enzyme inhibitor (ACEI, enalapril) and a calcium-channel blocker (nifedipine) were used for 21 weeks. After treatment, rats were assessed for arterial plasma renin activity (PRA). The LV, aortic arch, and kidneys were then excised for the determination of organ and tissue weight in some of the animals, while in others the aortic arch was fixed in situ and processed for microscopic analysis. Both enalapril and nifedipine levelled BP in the SHRs to almost normal values. Enalapril was able to prevent the increase in LV and kidney weights (p=0.04 wet, p<0.001 dry; p<0.001 wet and dry, respectively) and the increase in the weight of the aortic arch and in the thickness of its media (p<0.001 wet and dry; p<0.001, respectively) seen in untreated SHRs. This was associated with a larger lumen diameter (p<0.001) and a lower media to lumen ratio (p=0.01). In contrast, nifedipine did not prevent any of the changes described. Neither nifedipine nor enalapril treatment had any effects on PRA in either rat strain. Our results support previous observations that BP is not the only factor causing some of the pathological changes in hypertension; tissue Ang II level may also play a major role.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Long-term effects of angiotensin-converting enzyme inhibition on renal medullary neutral lipid in spontaneously hypertensive rats

Short-term treatment of young spontaneously hypertensive rats (SHR) with angiotensin-converting enzyme (ACE) inhibitors reduces systolic blood pressure. Renal medullary neutral lipids (RMNLs) have vasodilator properties that may explain the effects of ACE inhibition. We measured RMNL levels of SHR treated between 6 and 10 weeks of age with (1) vehicle, (2) ramipril 1 mg. kg-1. d-1, (3) the bradykinin B2 receptor antagonist icatibant 0.5 mg. kg-1. d-1, or (4) icatibant 0.5 mg. kg-1. d-1 plus ramipril 1 mg. kg-1. d-1. RMNLs were quantified by oil red O fluorescence at 10 and 20 weeks of age. Systolic blood pressure (BP) was measured by tail-cuff plethysmography. Ramipril reduced BP at 10 weeks of age and increased RMNLs compared with controls (0.99+/-0.07% versus 0.56+/-0. 06%, P<0.01). Icatibant alone had no significant effect on RMNLs (0.55+/-0.04%) but attenuated the increase in RMNLs by ramipril (0. 81+/-0.05%). In control SHR, the increase in BP between 10 and 20 weeks of age was associated with a significant increase in RMNLs (0.79+/-0.09%). SHR that had received ramipril had significantly lower BP than controls at 20 weeks of age, but RMNL was not significantly different (0.92+/-0.10%). Therefore, in young SHR, ACE inhibition increases RMNLs and reduces blood pressure, an effect that appears to depend on bradykinin. The changes in RMNLs at the age of 10 weeks paralleled long-term BP effects and may be involved in setting the BP track in SHR.

Renal medullary blood flow and renal medullary antihypertensive mechanisms

It has long been recognised that the kidneys take part in blood pressure control via both their exocrine and endocrine functions. An endocrine antihypertensive function of the renal medulla has been proposed. The renal medullary depressor substances ("medullipins"), are released in response to increased renal perfusion pressure. It has been suggested that the release of "medullipin" is controlled via changes in renal medullary blood flow. Recent observations also suggest that renal medullary blood flow is involved in the control of the pressure/natriuretic-diuretic action of the kidney. In this review we outline a unified hypothesis for blood pressure control via a combination of the plasma volume regulating pressure-natriuresis mechanism and the powerful antihypertensive actions of the "medullipins" (i.e. vasodilatation, inhibition of sympathetic drive and a diuretic action). It is hypothesised that the activity of both these systems are under control by renal medullary blood flow.

Renal medulla and bradykinin during the development of hypertension in SHR

1. The long-term reduction in blood pressure following ACE inhibitor treatment in young spontaneously hypertensive rats (SHR) appears to depend on both the kidney and bradykinin. 2. The aim of this experiment was to examine the effects of ACE inhibition and bradykinin on renal morphology and blood pressure in SHR. 3. Between 6 and 10 weeks of age male SHR received one of four treatments: water (n = 26), ramipril (1 mg/kg per day; n = 24), ramipril (1 mg/kg per day) plus Hoe 140 (0.5 mg/kg per day; n = 25) or Hoe 140 (0.5 mg/kg per day; n = 25). 4. Renal medullary and cortical volumes were determined stereologically at 10 and 20 weeks of age. 5. After 4 weeks of treatment, ramipril reduced the size of the renal medulla while Hoe 140 increased medullary volumes compared to control. Ten weeks after treatment was stopped the renal medulla of the ramipril group had returned to normal, however, there was a persistent increase in medullary volume of both Hoe 140 treated groups. 6. Our results imply that bradykinin may influence the size of the renal medulla which may have important effects on the development of hypertension in SHR.