Regional distribution of vascular resistance in two models of experimental renovascular hypertension

Brain Metabolism and Arterial Acid-Base Balance Following Bilateral Carotid Occlusion in Normotensive and Experimental Hypertensive Rats

The effects of bilateral common carotid artery occlusion en brain metabolism and arterial acid-base balance were studied in normotensive and experimental renovascular hypertensive rats.

One hour after carotid occlusion in hypertensive rats, supratentorial lactate increased to 383% and lactate-pyruvate ratio to 280% of the controls, while adenosine triphosphate (ATP) decreased to 69%. These metabolic changes were thought to be due to cerebral ischemia. Arterial pC02 was lowered and the pH was raised in the hypertensive animals due to cerebral ischemia induced hyperventilation. In the normotensive rats, carotid occlusion had minimal effects on cerebral metabolism and arterial acid-base balance.

These results suggest that hypertensive rats are more susceptible to cerebral ischemia caused by carotid occlusion than normotensive rats. Increased cerebrovascular resistance in hypertension is discussed as a casual factor in cerebral ischemia.

Disposition of hexobarbitone and antipyrine in DOCA-hypertensive rats

The disposition of antipyrine and hexobarbitone, and their effects on drug metabolizing hepatic enzymes have been investigated in DOCA-hypertensive rats. Antipyrine pharmacokinetic parameters were the same in hypertensive and control animals. Hexobarbitone sleeping time was longer in hypertensive rats compared with controls, while the activity of hepatic hexobarbitone hydroxylase was the same in both groups. Hepatic aminopyrine-N-demethylase activity was elevated in hypertensive rats while aniline hydroxylase and aryl hydrocarbon hydroxylase were lower. Glucuronyl transferase was the same in both groups. The sensitivity of the central nervous system of hypertensive rats to hexobarbitone was not altered, as determined by hexobarbitone concentration in blood and in brain. The total hepatic blood flow (arterial and portal) was significantly increased. Thus it is suggested that the difference in the disposition of the two drugs is probably not due to drug metabolizing enzyme activity. It is likely that the increase in total hepatic blood flow and rapid saturation of hepatic hexobarbitone metabolizing enzymes have significant roles in the slower metabolism and increased activity of hexobarbitone in hypertensive rats as compared with control rats.

Adrenal and vascular tyrosine hydroxylase activity in Goldblatt hypertension

To examine the role of the sympathetic nervous system in hypertension, the in vitro activity of tyrosine hydroxylase was examined in one-kidney, one clip (1K1C) and two-kidney, one clip (2K1C) hypertensive rabbits and their respective controls 2 weeks after surgical procedures. The in vitro activity of tyrosine hydroxylase provides a measure of catecholamine synthesis and serves as a biochemical index of activity of noradrenergic neurons and the adrenal medulla. Mean atrial pressure rose from 91.5 +/- 1.0 to 128.5 +/- 5.6 mm Hg (p less than 0.01) in the 1K1C group and from 91.8 +/- 1.3 to 106.5 +/- 5.0 mm Hg (p less than 0.02) in the 2K1C group, whereas no change in blood pressure was found in their respective controls. Adrenal tyrosine hydroxylase activity was increased 85% in the 1K1C group, as compared with values in one-kidney controls (from 11.8 +/- 1.5 to 21.8 +/- 1.1 pmol CO2/min/mg; p less than 0.0002), and was increased 49% in the 2K1C group, as compared with values in two-kidney controls (from 8.01 +/- 1.2 to 11.9 +/- 1.1 pmol CO2/min/mg; p less than 0.02). In the 1K1C group, proximal mesenteric tyrosine hydroxylase activity was decreased 46% compared with values in one-kidney controls (from 23.5 +/- 5.0 to 12.8 +/- 2.5 pmol CO2/min/mg; p less than 0.03) and distal mesenteric tyrosine hydroxylase activity was decreased 42% (from 7.73 +/- 1.2 to 4.46 +/- 0.8 pmol CO2/min/mg; p less than 0.03). In the 2K1C group, neither proximal nor distal mesenteric tyrosine hydroxylase activity was altered. Tyrosine hydroxylase activity was not detectable in the femoral arteries, or in the thoracic and abdominal aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic characteristics of the intestinal microcirculation in renal hypertension

This study investigated the microvascular changes that affect vascular resistance in the rat small intestine during two-kidney, one clip renal hypertension 4 weeks after renal artery stenosis. To study the intestinal microcirculation, a loop of the small intestine was exteriorized with intact circulation and innervation and a section of the bowel wall was prepared for observation with an intravital video microscopy system. Microvascular diameter, pressure, and flow velocity were measured for first, second, and third branch order arterioles and venules, using an image shearing monitor, servo-null micropipette system, and an optical Doppler velocimeter, respectively. The diameters of the first order arterioles and venules were significantly (p less than 0.05) reduced in hypertensive rats; however, diameters were unaltered in smaller second and third order arterioles and venules as compared with normotensive vessels. In hypertensive rats, mean arterial pressure was significantly (p less than 0.05) elevated (47%) and pressures also were elevated significantly (p less than 0.05) throughout the microcirculation, although by a proportionally smaller amount. Total network flow (i.e., first order arteriole flow) was significantly (p less than 0.05) reduced (40%) in hypertensive rats, but volume flows in individual second and third order arterioles were similar to flows measured in normotensive rats. Calculated total network resistance was increased (124%) in hypertensive rats. Thus, the intestinal microcirculation in rats with two-kidney, one clip renal hypertension is disturbed by elevated pressure and decreased total flow. The presence of normal flows in individual second and third order arterioles without any demonstrable difference in their diameters suggests that the predominant cause of elevated resistance across this segment of the intestinal microcirculation is a reduction in the number of perfused small arterioles.

Central and regional hemodynamics during the acute onset of renal hypertension in rats

The purpose of this study was to quantitate the regional changes in flow and resistance which occur during the acute onset of two-kidney, one clip renal hypertension. Anesthetized rats were implanted with a Doppler flow probe and balloon occluder on one renal artery. To produce acute hypertension, the occluder was inflated to reduce renal flow velocity by 50%. After 90 minutes, mean arterial pressure increased 25% above the prestenosis control period value as a result of a 22% increase in peripheral resistance. Regional flows and resistances were determined prior to and 90 minutes after renal artery stenosis by injection of labelled microspheres into the left ventricle. Our major findings were that: 1) the regional hemodynamic changes in flow and resistance are unevenly distributed among individual organs; and 2) the major site of increased resistance resides in the splanchnic circulation with the largest increases occurring in the small intestine.

Systemic and regional hemodynamic characteristics of bilateral cellophane perinephritis hypertension in conscious rabbits

Using the radioactive microsphere technique, systemic and regional hemodynamic variables were measured in normotensive rabbits and in rabbits with bilateral cellophane perinephritis hypertension. An average decrease in cardiac output of 18 percent was measured in the hypertensive rabbits as a result of a reduction in stroke volume; heart rate remained unchanged. Thus, in the established phase, hypertension was maintained by the elevated total peripheral resistance. A redistribution of cardiac output was observed in the hypertensive rabbits; a significantly higher fraction was received by the brain, small intestine and heart. The weight normalized blood flow to the kidneys, spleen, skeletal muscles, bones and fat was significantly decreased while an increase in vascular resistance was observed in the hypertensive rabbits in all the organs investigated. A negative correlation existed between the weight of the left ventricle and the blood flow to the left ventricle in hypertensive rabbits, suggesting inadequate coronary perfusion as myocardial hypertrophy becomes more pronounced.

Impaired renal blood flow and cortical pressure autoregulation in contralateral kidneys of Goldblatt hypertensive rats

Experiments were conducted on two-kidney, one clip renal vascular hypertensive rats to assess the ability of the kidney contralateral to renal vascular stenosis to autoregulate renal blood flow (RBF), glomerular filtration rate (GFR), and hydrostatic pressures in cortical structures during conditions of acutely reduced renal arterial blood pressure (BP). When observed at their respective, spontaneous BPs, RBF and GFR were not different in the contralateral kidneys of the hypertensive rats (n = 11) compared to normal animals (n = 7). However, the contralateral kidneys exhibited a significantly higher renal vascular resistance (RVR), 28.9 +/- 2.8 mm Hg . min/ml than the control animals, 23.1 +/- 1.5 mm Hg . min/ml. At spontaneous BP (169 +/- 5 mm Hg), urine flow, absolute and fractional sodium excretion, and absolute and fractional potassium excretion were all significantly greater in the contralateral kidneys of hypertensive rats than in kidneys of normal rats. Hydrostatic pressures in cortical structures were similar in the two groups. When BP was reduced acutely, the kidney contralateral to the renal artery stenosis achieved only small decreases in RVR that failed to allow RBF, GFR, or cortical pressures to be maintained. In contrast, normal rats efficiently autoregulated RBF and GFR. In addition, hydrostatic pressures in proximal tubules, distal tubules, and first order peritubular capillaries were maintained during reductions in BP to as low as 100 mm Hg. Urine flow and electrolyte excretion decreased to a greater extent in the hypertensive kidneys, such that at comparable BP these indices of excretory function were not different in the two groups. These observations indicate that the capacity of the contralateral kidney to maintain hemodynamic and glomerular function at reduced BP is compromised severely and suggest the possibility that the impaired autoregulatory capability may contribute to the maintenance of hypertension observed in this model.

Systemic and regional haemodynamics in experimental renal hypertension in conscious rabbits

1. The radioactive microsphere method was used to measure the distribution of cardiac output, regional flows and resistances in conscious normotensive and hypertensive rabbits implanted with an electromagnetic flow probe on the ascending aorta or pulmonary artery. Hypertension was induced by wrapping one kidney with cellophane and removing the other, and studies were performed about 5 weeks later. 2. Heart rate, stroke volume, cardiac output and total renal mass were reduced in the hypertensive animals, while the weight of, and the cardiac output distribution to left ventricle and the remaining kidney were increased. 3. In renal hypertensive rabbits, the weight normalized regional blood flow was diminished in a number of tissues, including the kidney, and, except for some organs in the splanchnic area (stomach, small intestine, mesentery and pancreas) and the fat, there was a rather uniform increase in tissue vascular resistance.

Reactivity of gomerular afferent and efferent arterioles in renal hypertension

Renal tissue from neonatal hamsters was grafted into the cheek pouch of adult hamsters. Renal hypertension was induced in 28 hamsters, and sham operation was performed in 27. When blood pressure became elevated in hypertensive hamsters (10 to 12 days), the renal microcirculation of both groups was evaluated by direct microscopy. Wall and luminal diameter and vascular responses to norepinephrine (NE) and angiotensin II (AII) were determined on glomerular afferent and efferent arterioles and on the corresponding cheek pouch arterioles. In the hypertensive hamsters, the luminal diameter of the afferent arteriole increased, as did the wall/lumen ratio of the efferent arteriole. Although all vessels responded to direct application of NE and AII in both groups, the response of the afferent to AII and the response of the efferent to both NE and AII was greater in the hypertensive hamsters as compared to the sham-operated hamsters. These results indicate that during the development of renal hypertension structural alterations of glomerular vessels and the selective vascular responses to vasoactive agents would lead to an increased glomerular capillary pressure.