Aggravation of salt-induced hypertension in Dahl rats by 2% supplemental dietary calcium

Developmental windows and environment as important factors in the expression of genetic information: a cardiovascular physiologist's view

Genetic studies in humans and rodent models should help to identify altered genes important in the development of cardiovascular diseases, such as hypertension. Despite the considerable research effort, it is still difficult to identify all of the genes involved in altered blood pressure regulation thereby leading to essential hypertension. We should keep in mind that genetic hypertension and other cardiovascular diseases might develop as a consequence of early errors in well-co-ordinated systems regulating cardiovascular homoeostasis. If these early abnormalities in the ontogenetic cascade of expression of genetic information occur in critical periods of development (developmental windows), they can adversely modify subsequent development of the cardiovascular system. The consideration that hypertension and/or other cardiovascular diseases are late consequences of abnormal ontogeny of the cardiovascular system could explain why so many complex interactions among genes and environmental factors play such a significant role in the pathogenesis of these diseases. The detailed description and precise time resolution of major developmental events occurring during particular stages of ontogeny in healthy individuals (including advanced knowledge of gene expression) could facilitate the detection of abnormalities crucial for the development of cardiovascular alterations characteristic of the respective diseases. Transient gene switch-on or switch-off in specific developmental windows might be a useful approach for in vivo modelling of pathological processes. This should help to elucidate the mechanisms underlying cardiovascular diseases (including hypertension) and to develop strategies to prevent the development of such diseases.

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.

Cardiovascular effects of captopril and enalapril in obese Zucker rats

The effects of two weeks of oral administration of the angiotensin-converting enzyme inhibitors captopril (a sulphydryl-containing drug) and enalapril (which lacks the sulphydryl group) on skeletal muscle glucose uptake, arterial blood pressure, cardiac hypertrophy, proteinuria and aortic vascular reactivity in obese Zucker rats were evaluated. Captopril (50 mg kg(-1) once daily) and enalapril (10 mg kg(-1) did not modify body weight gain or food or water intake. Both drugs decreased systolic blood pressure (157+/-6, 133+/-4 and 136+/-3 mm Hg, in vehicle-, captopril- and enalapril-treated rats, respectively), blood glucose (172+/-8 vs. 151+/-7 and 158+/-5 mg dl(-1), respectively), proteinuria (46+/-10 vs. 17+/-2 and 18+/-2.5 mg dl(-1), respectively) and heart weight (2.17+/-0.03, 1.98+/-0.02 and 1.99+/-0.04 mg g(-1)of body weight, respectively). Plasma insulin concentration was significantly increased by enalapril (17+/-2 ng ml(-1) vs. 9+/-2) but not by captopril (12+/-1). In the absence of insulin, the diaphragms from captopril- or enalapril-treated rats showed a significantly higher glucose uptake than that of controls (31% and 30% vs. control group, respectively). The presence of insulin in the incubation medium did not stimulate peripheral glucose uptake in the control group but significantly increased glucose uptake in diaphragms from captopril- or enalapril-treated rats (enhancement of glucose uptake vs. control: 52% and 43%, respectively). Endothelium-intact aortic rings from control Zucker rats showed a poor relaxant response to acetylcholine (maximal relaxation of 38.4+/-4.7%). Captopril significantly improved the endothelium-dependent vascular relaxation responses to acetylcholine and the endothelium-independent relaxation to the nitric oxide donor sodium nitroprusside whereas enalapril did not modify these relaxant responses. Neither captopril nor enalapril significantly affected the vascular contractile responses to the vasoconstrictors noradrenaline or KCl. In conclusion, the angiotensin-converting enzyme inhibitors captopril and enalapril reversed insulin resistance and the associated cardiovascular complications (cardiac hypertrophy, hypertension and proteinuria) in the obese Zucker rat, an animal model of non-insulin-dependent (type II) diabetes mellitus. However, only captopril, but not enalapril, improved the impaired endothelium-dependent and independent relaxant responses in the isolated rat aorta.

High calcium diet prevents baroreflex impairment in salt-loaded spontaneously hypertensive rats

To investigate the role of the sympathetic control mechanism in the antihypertensive effect of dietary calcium supplementation, we examined whether a high calcium diet affected mean arterial pressure, renal sympathetic nerve activity, heart rate, and overall and central properties of the arterial baroreceptor reflex in salt-loaded young spontaneously hypertensive rats (SHR). Six-week-old SHR were fed either a normal (0.66%) or high (8.00%) salt diet with either a normal (1.17%) or high (4.07%) calcium content for 4 weeks. The arterial baroreceptor reflex was elicited with rats under halothane anesthesia by altering mean arterial pressure with nitroprusside or phenylephrine. The overall property of the arterial baroreceptor reflex was assessed by the median mean arterial pressure (MAP50) and maximal gain (Gmax) of the relation between mean arterial pressure and renal sympathetic nerve activity and between mean arterial pressure and heart rate. The central property of the arterial baroreceptor reflex was assessed by reflex inhibition of renal sympathetic nerve activity and heart rate elicited by electrical stimulation of the aortic depressor nerve. Compared with the control group fed a normal salt/normal calcium diet, the high salt/normal calcium group had significantly higher mean arterial pressure and renal sympathetic nerve activity but not heart rate. Moreover, the arterial baroreceptor reflex was impaired in the latter group, as evidenced by an increase in MAP50 and decrease in Gmax of the two relations and an attenuation of reflex inhibition of renal sympathetic nerve activity by aortic depressor nerve stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary calcium and blood pressure in experimental models of hypertension. A review

More than 80 studies have reported lowered blood pressure after dietary calcium enrichment in experimental models of hypertension. The evidence presented here suggests that dietary calcium may act concurrently through a number of physiological mechanisms to influence blood pressure. The importance of any given mechanism may vary depending on the experimental model under consideration. Supplemental dietary calcium is associated with reduced membrane permeability, increased Ca(2+)-ATPase and Na,K-ATPase, and reduced intracellular calcium. These results suggest that supplemental calcium may limit calcium influx into the cell and improve the ability of the VSMC to extrude calcium. This could be a direct effect of calcium on the VSMC or an indirect effect mediated hormonally. The calcium-regulating hormones have all been found to have vasoactive properties and therefore may influence blood pressure. Furthermore, CGRP and the proposed parathyroid hypertensive factor are both vasoactive substances that are responsive to dietary calcium. Therefore, diet-induced variations in calcium-regulating hormones may influence blood pressure. Modulation of the sympathetic nervous system is another important way that dietary calcium can influence blood pressure. There is evidence of altered norepinephrine levels in the hypothalamus as a consequence of manipulations of dietary calcium as well as changes in central sympathetic nervous system outflow. Dietary calcium has also been shown to specifically modify alpha 1-adrenergic receptor activity in the periphery. In some experimental models of hypertension, dietary calcium may alter blood pressure by changing the metabolism of other electrolytes. For example, the ability of calcium to prevent sodium chloride-induced elevations in blood pressure may be attributed to natriuresis. However, natriuresis does not account for all of the interactive effects of calcium and sodium chloride on blood pressure. Sodium chloride-induced hypertension may be due in part to calcium wasting and subsequent elevation of calcium-regulating hormones. Chloride is an important mediator of this effect because it appears that sodium does not cause calcium wasting when it is not combined with chloride. More attention to the central nervous system effects of dietary calcium is needed. Not only can calcium itself influence neural function, but many of the calcium-regulating hormones appear to affect the central nervous system. The influence of calcium and calcium-regulating hormones on central nervous system activity may have important implications for blood pressure regulation and also may extend to other aspects of physiology and behavior.

High-calcium diet in spontaneously hypertensive rats: intervention with calcium antagonist verapamil

A number of studies have shown an antihypertensive effect for high-calcium diets, but others have found no effect or, even a prohypertensive effect. Because of these disparate results, studies were conducted in spontaneously hypertensive rats (SHR) fed either a normal calcium diet (1.0% calcium) or a high-calcium diet (4.0% calcium) with or without verapamil HCl (50 mg/kg body weight) from ages 5 to 12 weeks. Systolic blood pressure (SBP) and heart rate (HR) were measured by indirect tail cuff method. During the analysis of the electrolytes and vasoactive hormones monitored in this study, it was found that rats fed high-calcium diet had significantly elevated serum ionized and total calcium and calcium excretion. Systolic blood pressure for the verapamil-normal calcium diet (week 5, 148 +/- 4 mm Hg; week 7, 162 +/- 4 mm Hg) did not differ significantly from that of normal calcium diet (week 5, 152 +/- 2 mm Hg; week 7, 160 +/- 1 mm Hg). The high-calcium diet potentiated the development of hypertension, i.e., SBP was (157 +/- 2 mm Hg) on the 5th week and (174 +/- 4 mm Hg) on the 7th week. Conversely, verapamil high-calcium diet prevented the development of hypertension (week 5, SBP was 139 +/- 4 mm Hg; week 7, SBP was 146 +/- 3 mm Hg).(ABSTRACT TRUNCATED AT 250 WORDS)

Partial protection from salt-induced stroke and mortality by high oral calcium in hypertensive rats

BACKGROUND AND PURPOSE

Repeated demonstration of an antihypertensive effect of high oral calcium in stroke-prone spontaneously hypertensive rats led us to determine whether it also protects such rats from premature mortality and stroke-related lesions.

METHODS

Female stroke-prone rats (11-13 per diet) were fed high- and low-calcium (2.0% and 0.4%, respectively) diets with both high and low salt (7.0% and 0.3%, respectively) content from age 4 weeks until spontaneous death. In addition to life span, other variables measured included blood pressures, plasma chemistries, and histological characterization of stroke-related lesions.

RESULTS

Life span was increased from 51 +/- 4 to 68 +/- 1 weeks (p less than 0.05) by high versus low oral calcium in rats fed high-salt diets; it was further increased to greater than or equal to 82 weeks (p less than 0.05) in rats fed low-salt (+/- added calcium) diets. As seen previously, high oral calcium attenuated salt-induced hypertension but did not affect blood pressure in rats fed low-salt diets. High versus low oral calcium exerted contrasting effects (p less than 0.05) on brain lesions (hemorrhages and infarctions) in rats fed high-salt diets, decreasing lesion size (242 +/- 21 versus 712 +/- 276 microns per rat [diameters seen in histological sections]) but increasing lesion number (8.9 +/- 2.4 versus 3.4 +/- 2.2 per rat); it exerted little influence on the few brain lesions that appeared in rats fed low-salt diets.

CONCLUSIONS

High oral calcium may protect stroke-prone hypertensive rats from early salt-induced mortality at least partially by decreasing severity (size) of stroke-related lesions, an effect which may relate to decreased blood pressure. However, this protection may be limited by increased number (incidence) of such lesions, an effect which suggests that high oral calcium may increase the number of brain vessels susceptible to stroke-related injury independent of change in blood pressure.

Contrasting hemodynamic effects of high oral calcium in genetic models of salt-sensitive hypertension

Interest in effects of oral calcium (Ca) on blood pressure is now generally focused on salt-induced hypertension. In this study hemodynamic effects of long-term high oral Ca were examined in two different genetic models of salt-sensitive hypertension, stroke-prone spontaneously hypertensive rats (spSHR) and Dahl salt-sensitive (DS) hypertensive rats. High vs low oral Ca (2.0 vs 0.4% Ca, 8-13 rats/diet) significantly (p less than 0.05) attenuated salt-induced hypertension (7% NaCl intake) in female spSHR (mean arterial pressure = 137 vs 175 mmHg) but aggravated such hypertension in female DS rats (141 vs 124 mmHg). Pressor responsiveness to norepinephrine (NE) and angiotensin (A) II were examined in the same rats. High oral Ca decreased pressor responses to graded intravenous injections of NE and AII in spSHR and increased such responses in DS rats. In spSHR, the decreased pressor responsiveness preceded the antihypertensive effect of high oral Ca. In summary, 2.0 vs 0.4% oral Ca produces contrasting effects on blood pressure in two genetic models of salt-sensitive hypertension (stroke-prone SHR and Dahl salt-sensitive rats). These contrasting effects on blood pressure may be related to differential effects of oral Ca on vascular responsiveness to endogenous vasoconstrictors in these two genetic models of salt-sensitive hypertension.