Structurally based changes of renal vascular reactivity in spontaneously hypertensive and two-kidney, one-clip renal hypertensive rats, as compared with kidneys from uninephrectomized and intact normotensive rats

Thromboxane-B2, prostaglandin-E2 and hypertension in the rat 2-kidney 1-clip model: a possible mechanism of the garlic induced hypotension

Serum collected from unilaterally clipped and unclipped rats before and after treatment with water, garlic or cilazapril and subsequent to measuring blood pressure was assayed for thromboxane-B2 and prostaglandin-E2. The unclipped rats' thromboxane-B2 and prostaglandin-E2 levels were about 23 ng/ml and 2 ng/ml, respectively, and blood pressure was 126+/-3 mmHg. These values were not affected by either water or garlic administration. The clipped rats' thromboxane-B2 and prostaglandin-E2 concentrations were close to 34 ng/ml and 4 ng/ml, respectively, and declined only in response to garlic (by 15 ng/ml and 3 ng/ml) and cilazapril (by 12 ng/ml and 1.5 ng/ml). The blood pressure of these rats was 196+/-7 mmHg and again was reduced only by garlic to 169+/-14 mmHg and cilazapril to 137+/-5 mmHg. The no-treatment and water-treatment readings were significantly higher in the clipped rats. The data suggest that prostanoid system activity in the 2-kidney 1-clip rat is enhanced and mostly toward maintaining the hypertension. Furthermore, the blood pressure lowering effects of garlic and cilazapril might have been induced partially by a greater reduction in the synthesis of vasoconstrictor prostanoids.

Distention of the lateral intercellular spaces (LIS) in the proximal tubule cells of the non-stenosed kidney of the 2K-1C Goldblatt model of hypertension as evidence of pressure diuresis

This study shows the development of two major deformities in the non-stenosed kidney of the 2K-1C Goldblatt model; namely the widening of the LIS and the enlargement of the basilar interdigitations of the proximal tubule cells. These deformities were much less in the 2K-1C animals treated with the angiotensin I converting enzyme inhibitor (AICEI) cilazapril. From these findings it is suggested that the non-stenosed kidney is operating under the diuretic effect of the elevated systemic blood pressure (SBP) via an increase in the renal interstitial hydrostatic pressure (RIHP). Therefore, the AII antidiuretic effect is masked by the diuretic effect of the elevated SBP. The suggested rise in urine output fits well with the idea that kidneys lose water and sodium when SBP increases enormously. Therefore, in this model of hypertension, the non-stenosed kidney tries to lower SBP by losing water and sodium, an excretion behavior which is opposite to that of the stenosed kidney. Thus, the rise in SBP in this model is probably due to an increase in the vascular peripheral resistance rather than fluid accumulation.

Structure and mechanics of growing arterial microvessels from hypertrophied urinary bladder in the rat

Rat bladder hypertrophy, induced by a partial ligation of the urethra, was used to study the accompanying changes of microvascular smooth muscle mechanics, pharmacology and morphology. A segment of a microarterial vessel to the bladder was taken from a defined anatomical location and studied in a wire myograph in vitro at the length for maximal isometric force development (Lmax). After 10 days of ligation, bladder hypertrophy resulted in a microvascular growth response compared to non-operated controls which was characterized by (i) an increase of the calculated diameter at Lmax from 134 +/- 5 microns to 222 +/- 19 microns; (ii) an increase of the media thickness from 22.4 +/- 1.9 microns to 32.2 +2- 3.0 microns; (iii) an increase of the active tension from 1.42 +/- 0.28 mN/mm to 3.06 +/- 0.33 mN/mm; (iv) no change of the wall/lumen ratio (from 0.83 +/- 0.10 to 0.79 +/- 0.15). Normalized length/force relations (active, passive and total) did not differ significantly between microarteries from control and hypertrophic bladders. Microvascular smooth muscle growth was also associated with a decreased sensitivity to K(+)-induced depolarization and an increased sensitivity to alpha 1-adrenergic stimulation. No differences were noted regarding the Ca2+ sensitivity of force during K(+)-induced depolarization. The results suggest that microvascular growth (1) is immediately and positively influenced by the organ growth; (2) results in a functional resetting of the microvascular segments towards larger diameters without gross morphological or mechanical alterations; and (3) is accompanied by pharmacological alterations of the smooth muscle reactivity.

"Structural factor" in primary and secondary hypertension

The history of research on the "structural factor" in primary hypertension is briefly reviewed, and the gradual realization of its important influence on the hemodynamics of hypertension is outlined, as seen from a "personal angle." Experiences from previous studies of normal vascular function in animals were decisive for our first hemodynamic demonstration concerning the "structural upward resetting" of the systemic resistance vessels in human primary hypertension. Subsequent quantitative studies in rats with primary and secondary hypertension complemented these studies, confirming that the critical structural changes are a rapid increase in precapillary resistance at full dilatation associated with an increase in wall/lumen ratio due mainly to media hypertrophy and occurring in both primary and renal hypertension. Analyses were also performed concerning cardiac, barostat, and venous structural resettings, which are briefly mentioned. In our first studies of human primary hypertension, we suggested that the structural factor might itself be genetically reinforced, and increasing evidence in favor of this view is now accumulating. It is further discussed how antihypertensive therapy should be directed primarily against the structural upward resetting, as dependent on the local pressure and "trophic" influences, and some of our results in rat models are outlined. Finally, as the structural factor at the systemic resistance level also invites positive feedback interactions with functional "pressor" influences, it is, in a way, more difficult to explain why 85-90% of people remain normotensive than how hypertension gradually develops in 10-15% of people. This points to some powerful and durable negative feedbacks, which are still poorly understood, because most so far known barostats are readily reset upward in hypertension. It is here that the Muirhead renomedullary depressor system, and perhaps also the unmyelinated baroreceptor-volume receptor afferents, may be of particular importance.

Vascular remodeling in the growth hormone transgenic mouse

Using mice transgenic for the growth hormone gene (TGHM), we have studied the effects of a systemic elevation of growth hormone on vascular growth with the aim of investigating the role of vascular mass changes in producing hypertension. In contrast to human acromegaly or gigantism, there was no elevation of blood pressure in TGHM, but there were significant increases in vascular wall mass. In accordance with a presumably increased perfusion of larger organs, the medial cross-sectional areas of thoracic aorta and mesenteric resistance vessels were greater in the TGHM. These differences could be normalized in the aorta by body weight and in the mesenteric vessel by small intestine weight. Furthermore, the brain was not significantly heavier in the TGHM, and their carotid and cerebral vessels also were not larger. Wall-to-lumen ratios were similar in the aorta, carotid, and middle cerebral arteries suggesting that wall stress was the controlling factor in wall thickness. Surprisingly, the mesenteric vessels had increased wall-to-lumen ratio, which was similar to that seen in hypertensive vascular remodeling but in a normotensive animal. In an attempt to explain this finding it was noted that the pattern of mesenteric vascular networks and even organized structure within the vessel wall itself appeared to be fixed, perhaps by genetic mechanisms. Thus, vascular network structure may be a potentially limiting factor in the ability of the vessel wall to remodel and may have been responsible for the greater wall-to-lumen ratio in TGHM mesenteric vessels. A similar situation in human acromegaly or gigantism could result in a circulation marginally able to correct for other demands on blood flow resulting in about one third of cases being hypertensive.

The humoral renal antihypertensive system: nervous and hemodynamic effects in normotensive and unclipped renal hypertensive rats

A series of studies of the humoral renal antihypertensive system in normotensive and 2K 1C-renal antihypertensive rats is outlined. The rapid structural upward resetting of the cardiovascular system in renal hypertensive rats was associated with a structural downward resetting in the vasculature of the hypotensive clipped kidney. Unclipping of this kidney caused a pronounced release of renomedullary depressor agents, explaining the rapid normalization of pressure seen after unclipping. This normalization of pressure masks a state of pronounced functional hypotension in a structurally still hypertensive cardiovascular system, characterized by marked splanchnic vasodilatation and a lack of neurogenic counter-regulation. Only when this state has lasted long enough to normalize the structural upward resettings, characteristic of hypertension does the cardiovascular system return to normal. Further, cross-circulation techniques have shown that the humoral antihypertensive agents suppress tonic sympathetic activity, thereby inhibiting normal reflex counter-regulation of their vasodilator effects. Presumably this occurs via both vagal cardiac afferents and central actions. Further, behavior and awareness become depressed during intense and prolonged renomedullary release. Finally, experiments for which a normotensive kidney is cross-circulated from a normotensive rat suggest that the humoral renomedullary antihypertensive system has its threshold of release set so low as to contribute to normal blood pressure regulation, presumably in reciprocal balance with the renocortical renin-angiotensin system. Stepwise pressure elevations increasingly enhance release of the depressor agents from the cross-perfused kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal polyamine metabolism in rats with renovascular hypertension

A standardized stenosis was induced by applying a silver clip around the left renal artery in male rats. This resulted in arterial hypertension within 10 days (as determined by increase in heart weight). Ornithine decarboxylase (ODC) activity was determined in the right (untouched) kidney, the left kidney, and the adrenal glands 1 day, 10 days, and 3 months after the operation. There was no difference in ODC activity in the right kidney of the operated animals when compared with matched controls. In the left kidney (with artery stenosis), ODC activity decreased to 40% after 1 day. A partial recovery was seen after 10 days (ODC activity 70% of normal), and after 3 months ODC activity had normalized. Removal of the clip 1 day prior to killing induced in the 3-month group a more than two-fold increase in ODC activity in the previously clipped kidney; ODC activity in the contralateral kidney was not affected. Only minor changes in ODC activity occurred in the adrenal glands following the operation. Contents of putrescine and spermidine were increased in the left (stenotic) kidney, and after clip removal, also in the right (untouched) kidney. Our observations thus indicate that alterations in renal blood flow are rapidly followed by changes in ODC activity. Contents of putrescine, spermidine and spermine seemed to a great extent to be independent of the ODC activity.

'Structural autoregulation'--the local adaptation of vascular beds to chronic changes in pressure

Blood vessels readily adapt their design in response to sustained functional changes. If pressure (P) increases, the resulting thickening of the walls (w) of the resistance vessels, associated with a reduction in average inner radius (ri), keeps wall tension per unit wall layer (T) constant, because the increased w/ri ratio largely balances the raised pressure (Laplace's law: T = P X ri/w). The opposite occurs when there are sustained reductions in pressure. This locally elicited, mainly precapillary structural adaptation is a long-range equivalent to precapillary functional autoregulation and deserves to be called structural autoregulation. In primary hypertension there is an early 'structural resetting' of the systemic precapillary resistance, due to narrowing of ri and to vascular hyperreactivity ensuing from the increase in w/ri. These structural changes imply an increased resistance to flow at normal levels of vascular smooth muscle activity. Furthermore, even mild functional pressor influences will, if sustained, by a positive feedback interaction with the initially mild vascular hyperreactivity gradually accentuate the structural increase in w/ri. Marked rises in pressure may ensue from this interaction, implying that it is a major causative element in primary hypertension. As the renal preglomerular resistance vessels are similarly structurally autoregulated, this implies an early largely parallel resetting of the important renal 'long-term barostat function'. Further, as the walls of large arteries get thicker and stiffer, this helps to reset the baroreceptors. Finally, as the venous capacitance vessels adapt in a similar way the slight rise in average venous pressure in primary hypertension will reduce venous compliance, which helps to 'centralize' the usually slightly reduced blood volume.

Cardiovascular studies in rats with respect to some functional and structural relationships of relevance in hypertension and ordinary aging

Five current lines of cardiovascular studies in rats are outlined, mainly dealing with some functional and structural relationships of particular relevance for hypertension and ordinary aging: 1. Characteristics of the smooth muscles and their neurogenic control in 'Windkessel' arteries, conduit arteries, precapillary resistance vessels and venous capacitance vessels from normotensive rats (WKY) with comparisons to rats with primary hypertension (SHR). 2. Different types of structural renovascular adaptation, comparing aging with advancing SHR hypertension, with 'high-pressure' and 'low-pressure' kidneys in one-clip, two-kidney renal hypertension, and with hypertrophied kidneys in uni-nephrectomized normotensive rats. 3. Relationships between 'structural autoregulation', wall distensibility, vascular reactivity and smooth muscle sensitivity in SHR and WKY hindquarter resistance vessels along with aging. 4. Relationships between wall thickness, luminal dimension and contractility in left ventricles from SHR and WKY during aging, and when one-clip, two-kidney hypertension is superimposed. 5. Interference with the capacity of the neurohormonal mechanisms counteracting blood loss in rats when on chronic low-salt diet.

Structural factors: the vascular wall. Consequences of treatment

Like all tissues, blood vessels readily adapt their structure upon sustained functional changes. For example, pressure increases soon induce a largely proportional wall (w) thickening, where arteries, veins, and true resistance vessels respond alike. The latter also show a modest structural narrowing of average inner radius (ri), where the increased w/ri ratio results in a geometrically based vascular hyperreactivity. In primary hypertension, this type of precapillary resistance vessel adaptation implies an important "upward structural autoregulation" of systemic resistance, which is thereby kept increased also when vascular smooth muscle activity is entirely normal. Further, the systemic vascular hyperreactivity introduces a positive feedback interaction with functional pressor influences. By this mechanism, even marginal increases in "functional drive" may, if only sustained, steadily enhance the structural w/ri increase until it dominates hemodynamics. Similar alterations of the renal preglomerular vessels cause an early upward resetting of the renal barostat function, while wall thickening of large arteries helps to reset the baroreflexes. Also, the capacitance vessels show some structural adaptation that reduces venous compliance and contributes to blood volume centralization. Due to this overall structural resetting of hypertensive cardiovascular systems, the pharmacological problem of normalizing blood pressure is not one of merely damping an increased vascular smooth muscle activity. The task is more formidable, because effector cell activity must be kept even subnormal, despite counterregulatory mechanisms. These mechanisms almost all have been redesigned to maintain the high pressure state, with the long-range goal that true regression toward normality should ultimately ensue.