Influence of the renal nerves on sodium excretion during progressive reductions in cardiac output

The purpose of this study was to elucidate the role of the renal nerves in promoting sodium retention during chronic reductions in cardiac output. In five dogs, the left kidney was denervated and the urinary bladder was surgically divided to allow separate 24-h urine collection from the innervated and denervated kidneys. Additionally, progressive reductions in cardiac output were achieved by employing an externally adjustable occluder around the pulmonary artery and by servo-controlling right atrial pressure (control = 0.9 +/- 0.2 mmHg) at 4.7 +/- 0.1, 7.5 +/- 0.1, and 9.8 +/- 0.2 mmHg for 3 days at each level. At the highest level of right atrial pressure, the 24-h values for mean arterial pressure (control = 97 +/- 3 mmHg) and cardiac output (control = 2,434 +/- 177 ml/min) were reduced approximately 25 and 55%, respectively; glomerular filtration rate fell by approximately 35% and renal plasma flow by approximately 65%. However, despite the sodium retention induced by these hemodynamic changes, there were no significant differences in renal hemodynamics or sodium excretion between the two kidneys during pulmonary artery constriction. In contrast, after release of the pulmonary artery occluder on day 9, sodium excretion increased more (approximately 28% during the initial 24 h) in innervated than in denervated kidneys. These results suggest that the renal nerves are relatively unimportant in promoting sodium retention in this model of low cardiac output but contribute significantly to the short-term elimination of sodium after partial restoration of cardiac output and mean arterial pressure.

Advantages of continuous measurement of cardiac output 24 h a day

To test the hypothesis that continuous measurement of cardiac output 24 h a day would provide a better day-by-day reproducibility of the daily average cardiac output than acute measurements, we developed a computer-assisted method to monitor cardiac output continuously using an electromagnetic flow transducer. Because the diastolic aortic flow, which is used as a zero-flow reference, can drift significantly with electromagnetic flow probes, automatic tracking of the diastolic flow baseline was considered essential for long-term measurements. To accomplish this, the analog pulsatile flow signal was digitally converted and processed by an IBM PC to correct for signal drift on a beat-per-beat basis. Using this computerized system in 19 chronically instrumented dogs, we compared the values of cardiac output during 5 consecutive control days, measured either for 20 h each day (allowed 4 h for special care) or for 30 min in the morning when the trained dogs were required to lie quietly in their cages. The results show that the coefficient of variation of the five daily averages in cardiac output for each individual dog was three times smaller when cardiac output was measured 20 h each day (2.9 +/- 0.3 vs. 9.7 +/- 1.0%). Whole-day coefficients of variation were also smaller for mean arterial pressure, heart rate, stroke volume, and total peripheral resistance. Because of this greater day-by-day reproducibility, continuous monitoring of cardiac output is likely to be more sensitive to small changes in cardiac output induced by experimental protocols.

Reduced parasympathetic control of heart rate in obese dogs

We investigated why resting heart rate is elevated in dogs fed a high saturated fat diet for 12.7 +/- 1.8 wk. Obese dogs exhibited elevated body weight (59%), blood pressure (14%), and heart rate (25%). Differences in resting heart rate (control, 58 +/- 5 beats/min; obese, 83 +/- 7 beats/min) were abolished after hexamethonium, indicating an autonomic mechanism. Hexamethonium also reduced blood pressure in obese (20 +/- 4 mmHg) but not control (9 +/- 6 mmHg) animals. Propranolol did not affect heart rate in either group, excluding a beta-adrenergic mechanism. Subsequent administration of atropine increased heart rate more in control than in obese dogs (110 +/- 9 vs. 57 +/- 11 beats/min). The sensitivity of the cardiac limb of the baroreflex (Oxford method) was reduced by 46% in the obese group, confirming impairment of the parasympathetic control of heart rate. The standard deviation of blood pressure measurements was normal when expressed as a percentage of the mean arterial blood pressure (control, 11.2 +/- 0.4%; obese, 11.2 +/- 0.5%). Our results indicate that the development of obesity in dogs fed a high saturated fat diet is accompanied by an attenuated resting and reflex parasympathetic control of heart rate.

Long-term electrical stimulation of rabbit skeletal muscle increases growth of paired arteries and veins

We tested whether chronic stimulation of skeletal muscle can increase the growth of paired arteries and veins in rabbit extensor digitorum longus muscle (EDL). The right EDL of female New Zealand White rabbits was stimulated via the common peroneal nerve at 10 Hz using 300 microseconds square waves at 3-4 V. Two-hour periods of stimulation was alternated with 4-h periods of rest, 7 days/wk for approximately 60 days. The left EDL served as control. The hindlimb vascular system was maximally dilated and perfuse-fixed with 3% glutaraldehyde and 2% paraformaldehyde at arterial and venous pressures of 80-100 and 15-20 mmHg, respectively. Muscles were postfixed in OsO4 and embedded in EPOX 812 resin. One millimeter-thick transverse sections were cut at uniform locations through the entire breadth of the muscle and analyzed using videomicroscopy along with computerized morphometric and stereological techniques. All paired arteries and veins on each full muscle section were analyzed. Chronic muscle stimulation caused the wall volume of paired arteries and veins to increase by an average of approximately twofold and the lumen volume to increase by an average of approximately threefold compared with the contralateral muscles. The wall-to-lumen area ratio of the arteries and veins was not affected. Muscle stimulation also caused the numerical density of arteries having a diameter > 100 microns to increase by approximately fourfold and the density of veins having a perimeter > 500 microns to increase by approximately 10-fold.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal cardiovascular responses to exercise during the development of obesity in dogs

This study examined the control of cardiovascular dynamics in response to exercise during the development of obesity in dogs. Left atrial pressure (LAP), mean arterial pressure, and cardiac output (CO) were determined both at rest and in response to treadmill exercise (5.6 km/h, 10% grade) first during a control, lean state and then, once a week while the dogs were maintained on a high fat diet (HFD) for 4 weeks. Body weight increased from 22.8 +/- 1.1 to 33.1 +/- 2.0 kg after 4 weeks of HFD. The dogs developed mild hypertension with increases in resting CO, heart rate, and LAP. The response to exercise was characterized by a decrease in LAP when the dogs were lean and, in contrast, by a dramatic increase in LAP during the development of obesity. In addition, after the third week, the dogs could no longer maintain exercise at the original level. These results suggest that abnormal left ventricular function may occur very early during the development of canine obesity.

A stereological method for estimating length density of the arterial vascular system

We developed a stereological method for quantitating length density (Lv; vessel length per unit reference volume) of the arterial system. Accurate estimation of Lv for a sparse system of blood vessels in a three-dimensional specimen requires information on individual vessel orientation. The method we present extracts the necessary information on vessel orientation from profile geometry. Major and minor diameters of elliptical profiles of sectioned tubular structures are used to calculate Lv. The method does not require special sectioning alignment and does not assume a prior distribution of blood vessels; however, the method does assume that arteries are cylindrical. A physical model consisting of boiled spaghetti mixed with agar in a cylinder was used to test the stereological method. Measurements of over 1,000 elliptical profiles in 5 separate trials have demonstrated that the method can accurately estimate Lv with < 5% error even when tortuosity is high, i.e., when anisotrophy coefficient is 1.55. This method may facilitate a better understanding of the mechanisms of artery growth by making it possible to quantify linear growth of the arterial system.

Interface for the documentation and compilation of a library of computer models in physiology

A software interface for the documentation and compilation of a library of computer models in physiology was developed. The interface is an interactive program built within a word processing template in order to provide ease and flexibility of documentation. A model editor within the interface directs the model builder as to standardized requirements for incorporating models into the library and provides the user with an index to the levels of documentation. The interface and accompanying library are intended to facilitate model development, preservation and distribution and will be available for public use.

Cardiovascular responses to long-term blockade of nitric oxide synthesis

The goal of this study was to determine if there is a basal release of nitric oxide that affects long-term arterial pressure regulation in dogs. Studies were conducted over a 23-day period in eight conscious dogs with indwelling catheters. Nitric oxide synthesis was blocked by continuous intravenous infusion of nitro-L-arginine-methyl ester at 37.1 nmol/kg per minute for 11 days. Arterial pressure increased to 120 +/- 4% of control on the first day, decreased for a few days, and then increased to a maximum value of 122 +/- 6% of control on day 7. Bradycardia was sustained throughout the entire nitro-arginine period. Blockade of nitric oxide synthesis was evidenced by attenuated pressure and flow responses to systemic acetylcholine infusion. The pressor response to phenylephrine was increased for only 1 day, and the hypotensive effects of nitroprusside were enhanced. Also, the variability of arterial pressure was significantly increased during nitro-arginine. Sodium and water balances were positive the first day of nitro-arginine infusion but were unchanged for the entire nitro-arginine period. In conclusion, the data suggest that blockade of the basal release of nitric oxide in dogs causes an increase in the long-term level of arterial pressure without any sustained sodium or water retention.

Role of pressure natriuresis in long-term control of renal electrolyte excretion

If pressure natriuresis is to play an important role in arterial pressure control, renal perfusion pressure must have a long-term effect on urinary sodium excretion. The aim of this study was to quantitate the importance of renal perfusion pressure per se in controlling renal hemodynamics and electrolyte excretion chronically. Female mongrel dogs (n = 6) were instrumented with bilateral renal artery catheters for measurement of renal perfusion pressure and occluders on both renal arteries for servo-control of renal perfusion pressure at different levels; the urinary bladder was split for determination of renal clearances and electrolyte excretion from each kidney separately. Because both kidneys were exposed to the same neurohumoral influences, any changes in renal function could be attributed to differences in renal perfusion pressure between the two kidneys. After 5 days of control, renal perfusion pressure to one kidney was reduced from 86.7 +/- 0.2 to 74.2 +/- 0.6 mm Hg for 12 days, and pressure in the contralateral kidney increased to 91.5 +/- 0.4 mm Hg. Sodium excretion decreased from 41 +/- 2 to 25 +/- 1 mmol/d in the servo-controlled kidney and increased from 41 +/- 1 to 55 +/- 1 mmol/d in the contralateral kidney during 12 days of servo-control. Urine volume, chloride excretion, and potassium excretion exhibited similar patterns during servo-control. In addition, autoregulation of effective renal plasma flow and glomerular filtration rate was relatively well maintained; however, in the low-pressure kidney, glomerular filtration rate was slightly but significantly lower (approximately 8%) than in the contralateral kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic angiotensin-converting-enzyme inhibition improves cardiac output and fluid balance during heart failure

The purpose of this study was to examine the sequential changes in renal and cardiovascular function produced by chronic Benazepril administration at different stages of heart failure in dogs. Heart failure was produced by rapid ventricular pacing in five dogs with a normally functioning renin-angiotensin system (angiotensin normal, AN) and six dogs chronically administered the angiotensin-converting-enzyme inhibitor (ACEI) Benazepril. After 7 days of pacing, cardiac output was significantly higher and total peripheral resistance (TPR) lower in the ACEI compared with the AN dogs. Cumulative sodium and water balance increased significantly in both groups, but after 7 days of pacing there were no significant differences between groups. However, the rate of increase in sodium and water balance was significantly less in the ACEI group. Effective renal plasma flow decreased in the AN and ACEI groups during pacing, but there were no between-group differences, and no significant changes in glomerular filtration rate (GFR) occurred in either group. In the AN dogs, pacing was continued for 7-21 days until the onset of decompensated heart failure. Urinary sodium excretion increased on the first day of ACEI infusion during this stage but returned to pre-ACEI levels during the next 2-3 days. No significant improvement in cardiac output was measured during ACEI in decompensated heart failure. These data suggest that chronic ACEI administration can improve renal and cardiac function in early heart failure without impairing GFR but is less chronic ACEI administration can improve renal and cardiac function in early heart failure without impairing GFR but is less effective in later, decompensated stages.

Effect of skin concavity on subcutaneous tissue fluid pressure

We tested the hypothesis that mechanical factors associated, with a skin concavity can cause the local tissue fluid pressure to become more negative. Perforated Teflon collars, 26 mm in diameter and having various heights (5, 10, 13, and 16 mm), were implanted into the fascial plane of the inguinal and abdominal areas of six sheep. After several weeks, visible signs of edema were no longer apparent, and the skin formed a concavity within the center of each collar. The depth of each concavity was measured using an electronic micrometer, and the tissue fluid pressure beneath the concavity was measured using a needle method. Over the entire range of collar heights, the average depth of the concavities ranged from 1.1 to 4.7 mm in the abdominal tissues and from 1.8 to 5.5 mm in the inguinal tissues. The respective values of tissue fluid pressure averaged -4.6 to -13.0 and -5.7 to -12.8 mmHg. The results therefore indicate that implanting deeper collars leads to the formation of deeper concavities in the skin and also to greater negativity in the free tissue fluid pressure beneath the skin. Linear regression extrapolation to a collar height of 0 mm corresponded to a tissue fluid pressure of -1.0 mmHg in the abdominal tissue and -2.4 mmHg in the inguinal tissues. A model based on excessive pumping of the lymphatic system in the vicinity of a concavity is provided to explain this newly described phenomenon. We conclude that mechanical factors associated with the formation of a skin concavity cause or permit the tissue fluid pressure to reach levels of negativity far greater than those that exist in the absence of a concavity.

Morphometric measurements of chorioallantoic membrane vascularity: effects of hypoxia and hyperoxia

We studied the effects of hypoxia and hyperoxia on the angiogenesis process in the chick embryo chorioallantoic membrane (CAM) using four different morphometric measurements of vascularity. Chick eggs were incubated in various oxygen atmospheres (12, 16, 21, 45, or 70% oxygen) beginning on the 7th day of development, and vascularity was measured on the 14th day. Measurements of vascularity included vessel endpoint density (VED), length density, fractional image area, and a vascular density index. All measurements were made on blood vessels in randomly selected areas of CAM using a computerized image analysis system. An opaque colloidal carbon-albumin perfusate was used as a vascular marker. All four measurements showed that vascularity of CAM was inversely related to the oxygen tension to which the embryos were subjected. The VED, an estimate of total number of pre- and postcapillary vessels, exhibited the greatest degree of change, but overall changes in vascularity were modest. Prolonged exposure to a 12% oxygen atmosphere increased VED by approximately 16%, whereas 70% oxygen decreased VED by approximately 19% compared with room air control groups. We also studied the normal growth of CAM vasculature from days 8 to 18 of development. In these studies, the values of VED increased progressively throughout the entire period of development, whereas the other measurements of vascularity reached maximum values by the 14th day. We conclude that hypoxia stimulates angiogenesis in the CAM in a dose-related manner, hyperoxia inhibits CAM angiogenesis in a dose-related manner, and VED provides a sensitive estimate of vascularity in chick CAM throughout its development.

Atrial natriuretic factor and blood pressure control: role of sodium and aldosterone

This study examined the long-term actions of atrial natriuretic factor (ANF), at physiological levels, on renal function and mean arterial pressure (MAP) and the importance of Na intake and the renin-angiotensin-aldosterone system in modulating those effects. After a control period, ANF was infused intravenously at a rate of 10 ng.kg-1.min-1 for 7 days, followed by 7 days of 20 ng.kg-1.min-1 and 7 days of recovery. After 7 days of ANF at 10 ng.kg-1.min-1, MAP decreased from 87 +/- 3 to 80 +/- 2 mmHg in normal dogs on low sodium intake (LS, 7 meq Na/day) and from 89 +/- 2 to 79 +/- 2 mmHg in adrenalectomized dogs (ADX, 7 meq Na/day) given constant mineralocorticoid replacement. In both groups, no significant change in glomerular filtration rate (GFR) was observed, although sodium excretion increased transiently. ANF failed to cause significant changes in MAP, GFR, or sodium excretion in normal dogs on high sodium intake (HS, 269 meq Na/day). In LS and HS no long-term effects of ANF on plasma renin activity (PRA) and aldosterone were observed. In ADX, as expected, no change in aldosterone was observed. Thus, in normal and adrenalectomized dogs on LS, chronic ANF infusion caused sustained reductions in MAP. HS markedly attenuated the hypotensive effect of ANF. Our data suggest that the long-term effect of ANF is salt sensitive but that decreases in PRA and aldosterone are not essential for the long-term hypotensive effect of ANF.

Growth regulation of the vascular system: evidence for a metabolic hypothesis

Prolonged imbalances between the perfusion capabilities of the blood vessels and the metabolic requirements of the tissue cells often lead to modification of the vasculature to satisfy the tissue needs. This homeostatic response appears to be bidirectional, since the vascularity of a tissue can increase or decrease in parallel with primary changes in metabolic rate. The factors that mediate the responses are not well understood, but oxygen has been implicated as a major control element, since vessel growth increases during hypoxic conditions and decreases during hyperoxic conditions. The following feedback control hypothesis may apply to many different physiological situations. Decreased oxygenation causes the tissues to become hypoxic, and this initiates a variety of signals that lead to the growth of blood vessels. The increase in vascularity promotes oxygen delivery to the tissue cells by decreasing diffusion distances, increasing capillary surface area, and increasing the maximum rate of blood flow. When the tissues receive adequate amounts of oxygen even during periods of peak activity, the intermediate effectors return to normal levels, and this negative signal, in turn, stops the further development of the vasculature. Although the effector mechanisms of the hypoxic stimulus are still being investigated, adenosine, which is produced in hypoxic tissues, appears to mediate hypoxia-induced increases in vascularity in some instances. Roles for fibroblast growth factor as well as mechanical factors associated with vasodilation and increased blood flow are postulated. Although blood vessel growth is a multifactorial process, a major influence in its regulation appears to be metabolic need. If this view is correct, it may be found that many of the quantitatively significant factors that control growth in a given vasculature are themselves modulated or controlled by metabolic signals reflecting the nutritional status of the tissues which that vasculature supplies.

Atrial natriuretic peptide induces sustained natriuresis in conscious dogs

Although acute infusions of atrial natriuretic peptide (ANP) often cause natriuresis, these effects are not sustained, possibly because of reductions in arterial pressure or other compensatory adaptations. The aim of this study was to determine whether physiological increases in intrarenal ANP levels cause sustained natriuresis if changes in arterial pressure and other neurohumoral influences that might obscure the renal responses are controlled. Changes in renal function were quantitated during chronic unilateral renal arterial infusion of ANP at rates of 1, 2, and 4 ng.kg-1.min-1 in conscious dogs (n = 7) with the urinary bladder split to allow continuous measurement of renal excretion in the ANP-infused and contralateral, vehicle-infused kidneys. There was no change in mean arterial pressure at any infusion rate. During 1 ng.kg-1.min-1 infusion of ANP for 5 days, the renal excretory responses were small and variable. However, during 2 and 4 ng.kg-1.min-1 ANP infusion for 7 days, sodium excretion averaged 37.2 +/- 10.0 and 134.8 +/- 19.0% greater, respectively, in the ANP-infused kidneys compared with the vehicle-infused kidneys but there were no changes in glomerular filtration rate or effective renal plasma flow. These results demonstrate that when compensatory changes in arterial pressure and neurohumoral factors are controlled, ANP, at physiological concentrations, causes marked increases in renal excretion. This study supports the concept that ANP's effects to increase renal excretory capability could play a role in long-term control of arterial pressure and body fluid homeostasis.

Regulation of cardiac output during aldosterone-induced hypertension

The classical haemodynamic transients of volume-loading hypertension have been difficult to demonstrate in aldosterone-induced hypertension. Because recent studies have shown that continuous whole-day measurements of cardiac output are superior to short-term recordings, we studied the transient haemodynamic effects of aldosterone-induced hypertension while monitoring arterial pressure and cardiac output (electromagnetic flow probe) continuously for 20 h a day. In six dogs maintained on a fixed sodium intake of 150 mmol/day, we infused aldosterone (12 micrograms/kg per day, intravenously) for 10 days. The aldosterone induced a progressive increase in mean arterial pressure, from a control value of 88 +/- 1 to 107 +/- 2 mmHg. Cardiac output increased progressively, reaching a peak average value on the 4th day of infusion of +14 +/- 5% above control, and remained slightly elevated throughout the infusion period. Total peripheral resistance increased slowly to a value averaging +13 +/- 4% above control. Therefore, our experiments show that aldosterone induces a primary increase in cardiac output followed by a secondary vasoconstriction, which is consistent with the classical transient haemodynamic effects of volume-loading hypertension.

Role of renal nerves in control of sodium excretion in chronic congestive heart failure

The aim of this study was to examine the contribution of the renal nerves to the sodium retention in chronic congestive heart failure produced by rapid ventricular pacing. In 10 female dogs the left kidney was denervated and the urinary bladder was split to allow separate 24-h urine collection from an innervated and a denervated kidney in the same dog. The dogs were placed on an 80-meq/day sodium intake and permitted to recover for at least 2 wk. Control measurements were made for 5 days followed by ventricular pacing at 270-300 beats/min for 6 days. Cardiac output (CO), measured with an electromagnetic flow probe around the ascending aorta, fell from a control of 2.4 +/- 0.3 to 1.4 +/- 0.2 l/min (6 day average) during pacing while mean arterial pressure (MAP) fell from 91 +/- 4 to 71 +/- 3 mmHg. In six dogs, sodium excretion fell to an average of less than 2 meq/day (80 meq/day intake) during the 6-day pacing period in both the innervated and denervated kidneys. In four dogs, sodium excretion returned back toward control on days 3-6 of pacing despite sustained reductions in CO and MAP. However, there were no differences in renal hemodynamics or electrolyte excretion between innervated and denervated kidneys in either the compensated or decompensated dogs. These results suggest that other control mechanisms, besides the renal nerves, are primarily responsible for the sodium retention in this model of chronic congestive heart failure.

A simulation support system for solving large physiological models on microcomputers

Although physiological modeling and computer simulation have become useful research tools to test new scientific theories and to design and analyze laboratory experiments, developing a new model can be a tedious process because the investigator must often write very complex and specific routines for data input and output. To facilitate the design of new models (as well as the use of existing models), we have developed MODSIM, a FORTRAN-based simulation support system for the IBM PC computer than can accommodate very large dynamic models having up to several thousand equations. It provides the investigator with utilities for continuous on-line graphical and/or tabular output, as well as facilities for dynamic interaction with the model. The user must only supply a model as a list of mathematical equations written in FORTRAN, along with the initial values of the model variables and parameters. The model is precompiled, compiled, and then linked to the MODSIM utilities. Without further programming, the user can then solve the model, select variables for graphical output, and stop the model at any time to analyze the data or to change a parameter before resuming the simulation. This simulation system makes it very easy to develop new models that actively interact with the experimental research of the investigator.

Chronic hypotensive effects of verapamil in angiotensin hypertension are steroid independent

This study was designed to examine the mechanisms that contribute to the chronic hypotensive effects of verapamil during angiotensin II-induced hypertension. Hypertension was induced in five dogs by continuous intravenous infusion of angiotensin II (5 ng/kg/min) for 17 days. On the sixth day of angiotensin II infusion when daily sodium balance was achieved, mean arterial pressure (control, 92 +/- 4 mm Hg), plasma aldosterone concentration (control, 5.2 +/- 0.9 ng/dl), and renal resistance (control, 0.28 +/- 0.01 mm Hg/ml/min) were increased 37 +/- 8 mm Hg, 13.6 +/- 5.0 ng/dl, and 0.20 +/- 0.05 mm Hg/ml/min, respectively. At this time there were no significant changes in glomerular filtration rate, effective renal plasma flow, net sodium and water balance, or extracellular fluid volume. Subsequently, when verapamil was infused (at 2 micrograms/kg/min) simultaneously with angiotensin II (days 7-13), there was a net loss of 55 +/- 10 meq sodium, a 7.0 +/- 0.7% fall in extracellular fluid volume, and approximately a 70% reduction in the chronic effects of angiotensin II on mean arterial pressure and renal resistance; in contrast, verapamil failed to attenuate the long-term aldosterone response to angiotensin II. Further, although glomerular filtration rate and effective renal plasma flow tended to increase during verapamil administration, there were no consistent chronic long-term changes in these renal indexes. In comparison with these responses in hypertensive dogs, when verapamil was infused for 7 days before the induction of angiotensin II hypertension, there were no significant changes in any measurements except mean arterial pressure, which fell 11 +/- 1 mm Hg. Thus, these data fail to support the hypothesis that the chronic stimulatory actions of angiotensin II on aldosterone secretion are dependent on a sustained increase in transmembranal calcium influx. Moreover, these data indicate that the pronounced long-term hypotensive effects of verapamil in angiotensin II hypertension are due to impairment of the direct renal actions of angiotensin II rather than the indirect sodium-retaining effects that are mediated via aldosterone secretion.

Vascular development in chick embryos: a possible role for adenosine

We studied the possible role of adenosine in the development of the vasculature using 217 chick embryos. Adenosine (2-32 mumol/day), inosine (16 mumol/day), dipyridamole (0.04-0.4 mumol/day), or aminophylline (400 and 800 micrograms/day) were administered twice each day into the air space on days 11-14. Control embryos received Ringer solution. Whole body vascularity was estimated on day 15 as the whole body structural vascular resistance (SVR), i.e., the hydraulic resistance of the maximally dilated vasculature. Adenosine decreased the SVR in a dose-related manner at the lower dosage amounts but caused a maximum decrease in SVR at the higher dosage amounts averaging 30% below the Ringer control values. Equimolar amounts of adenosine and inosine decreased the SVR by the same extent. Dipyridamole, which potentiates the biological effects of endogenous adenosine, also decreased the SVR in a dose-related manner to values averaging approximately 30% below control. When the effects of endogenous adenosine were blocked by aminophylline, the SVR increased in a dose-related manner to approximately 100% above control at the highest dosage amount. These results suggest that adenosine could have a physiological role in growth regulation of the vascular system in the chick embryo.

Pressure natriuresis and control of arterial pressure during chronic norepinephrine infusion

The goal of this study was to quantitate changes in mean arterial pressure (MAP) and renal function during chronic increases in plasma levels of norepinephrine, and to determine the role of the renal pressure natriuresis mechanism in controlling sodium balance in norepinephrine hypertension. In six conscious dogs in which renal artery pressure (RAP) was allowed to increase during 7 days of norepinephrine infusion (0.2 micrograms/kg per min), sodium excretion (UNaV) rose from 66 +/- 3 to 112 +/- 15 mmol/day and MAP increased from 100 +/- 3 to 109 +/- 3 mmHg on the first day. On days 2-7, UNaV returned toward the control level while MAP averaged 108 +/- 2 mmHg. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) did not change significantly, averaging 85.9 +/- 4.0 and 235 +/- 17 ml/min, respectively, during 7 days of norepinephrine, compared to controls of 84.1 +/- 3.9 and 252 +/- 20 ml/min. When RAP was servo-controlled for 7 days during norepinephrine infusion, the natriuresis was abolished; UNaV averaged 76 +/- 8 during control, 77 +/- 13 during the first day of norepinephrine and 65 +/- 4 mmol/day during 7 days of norepinephrine. GFR and ERPF did not change significantly during norepinephrine infusion with RAP held constant. MAP did not reach a plateau but continued to rise from 102 +/- 3 to 137 +/- 3 mmHg after 7 days of norepinephrine and servo-control of RAP.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term hypotensive effect of beta-agonist in conscious dogs

The purpose of this study was to investigate the arterial pressure response to long-term administration of beta-agonists in the chronically instrumented conscious animal model. Chronically instrumented dogs were given intravenous infusions of ritodrine (2 micrograms.kg-1.min-1) for a period of 2 wk. Several cardiovascular and renal parameters were monitored before, during, and after the ritodrine infusion, and renal function curves were constructed. After the 1st wk of infusion, a new steady state was reestablished, and this was characterized by hypotension, reduced plasma protein concentration, elevated cardiac output, expanded extracellular fluid space, and near normal levels of activity of renin-angiotensin-aldosterone systems. The renal function curve during ritodrine infusion shifted to the left with no change in slope. We propose the following: 1) the persistence of hypotension is most probably related to the resetting of the arterial pressure-kidney blood volume servocontrol mechanisms, and 2) the persistent elevation of cardiac output and reduction in peripheral resistance are most probably related to increased oxygen and nutrient demand during beta-agonist infusions.

Effects of intermittent hypoxia on structural vascular adaptation in chick embryos

We explored whether the blood vascular system of the chick embryo adapts its structure to meet the maximum or average oxygen needs of the tissue cells. Chick embryos were grown in continuous 12% oxygen, continuous 16% oxygen, and intermittent 12% oxygen in which the embryos were exposed to 12% oxygen for 4 h each day. Control groups were grown in room air. Measurements of structural vascular resistance (SVR), i.e., the resistance of the maximally dilated vasculature, were used to estimate the whole body vascularity of the 14- or 15-day-old embryos. Continuous exposure to 12% oxygen decreased SVR by 63.1 +/- 1.2 (SE) %, and intermittent exposure to 12% oxygen decreased SVR by 55.6 +/- 0.5% when compared with a 15-day-old normoxic control group. Based on studies with continuous exposure to different levels of low oxygen, it was predicted that exposure to 19.5% oxygen, the average concentration for the intermittent hypoxia group, would decrease SVR by 15.0 +/- 0.3%. These results indicate that intermittent hypoxia at 12% oxygen was approximately 90% as effective as continuous hypoxia at the same level in decreasing SVR and about four times more effective than 19.5% continuous oxygen. Therefore, the results support the hypothesis that the blood vascular system adapts its structure to meet almost entirely the maximum oxygen needs of the tissue cells.

Computer models for designing hypertension experiments and studying concepts

This paper demonstrates how computer models along with animal experiments have been used to work out the conceptual bases of hypertensive mechanisms, especially the following: (1) The renal-fluid volume pressure control mechanism has a feed-back gain for pressure control of infinity. Therefore, the chronic level to which the arterial pressure is controlled can be changed only by altering this pressure control mechanism. (2) An increase in total peripheral resistance is not sufficient by itself to cause hypertension. The only resistances in the circulatory system that, when increased, will cause hypertension are those along a restricted axis from the root of the aorta to Bowman's capsule in the kidneys. (3) Autoregulation in the peripheral vascular beds does not increase the arterial pressure in hypertension. However, autoregulation can convert high cardiac output hypertension into high peripheral resistance hypertension. (4) In a computer simulation that cannot yet be performed in animals, a simulated hypertension caused by a combination of increased renal afferent and efferent arteriolar resistances has characteristics that match almost exactly those of essential hypertension.

Hypotensive effect of chronic intrarenal infusion of acetylcholine during angiotensin hypertension

We examined the role of the pressure natriuresis phenomenon in long-term arterial pressure control. Uninephrectomized dogs were housed in metabolic cages and made hypertensive with a continuous background intravenous infusion of angiotensin II (AngII, 12 ng/kg/min). To increase the ability of the kidney to excrete salt and water, we infused acetylcholine (ACH, 2.0 micrograms/kg/min), a potent natriuretic agen, directly into the renal artery. In four dogs, ACH decreased mean arterial pressure (MAP) from 144 +/- 5 mm Hg to 113 +/- 3 mm Hg. Sodium excretion increased by about 60% on the first day of infusion and then returned rapidly toward the control value. On cessation of the ACH infusion, there was a transient but marked sodium retention, and the hypertension returned. A control infusion of ACH intravenously rather than into the renal artery in the same four dogs did not affect MAP or sodium excretion during AngII hypertension.