To evaluate the importance of volume in the development of hypertension in inbred Dahl salt-sensitive rats (SS/Jr), we measured the changes in blood pressure (BP) that occurred with oral intake of food (salt) and water in rats whose body weight was permitted to increase versus those in which body weight was maintained constant with a servo-control system. We hypothesized that if volume expansion is essential in the development of hypertension, then BP would not increase if body weight was held constant. We found that oral presentation of chow containing 4% salt to SS/Jr rats caused BP to increase 32.2 +/- 2.9 mmHg over 4 days when body weight was controlled at its initial value. Plasma sodium increased from 142.0 to 145.2 meq/l during 4 days of high salt. Neither plasma volume, hematocrit, nor central venous pressure changed significantly on the high-salt diet. In contrast, the inbred Dahl salt-resistant rats (SR/Jr) did not increase their BP during body weight control when given 4% salt. This demonstrates that volume expansion is not an obligatory step in the pressure response to increased salt in SS/Jr rats. Our results obtained with oral presentation of salt, in contrast to intravenous, represent a physiological evaluation of the significance of volume changes in response to dietary salt because no potential regulatory reflexes have been bypassed.

Pressure diuresis refers to the direct effect of arterial pressure (AP) on the rate of urine flow (UF). On the basis of computer modeling, pressure diuresis has been viewed as a long-term mechanism that acts to set the level of the blood volume and, thus, the steady-state AP. There are no systematic studies, however, on the rapidity with which changes in AP induce changes in UF in vivo. Therefore, we measured the delay between induced changes in AP and the subsequent change in UF. Nine anesthetized rats were instrumented with arterial, venous, and ureteral catheters. AP and UF were measured every 2 s, while acute changes in AP were induced by 1) occlusion of the aorta above or below the renal vessels; 2) brief tail pinch; or 3) intravenous administration of acetylcholine (1 microgram), phenylephrine (1 microgram), or angiotensin II (0.1 microgram). The rapidity of the urinary response to induced changes in AP was determined by calculating the delay between a significant change in AP (+/- 2 SD from baseline) and a significant change in UF. The delay averaged 6.0 +/- 0.5 s for all conditions. Also, examining the relationship between the magnitude of the induced changes in AP and the magnitude of the responses in UF revealed an exponential influence of AP on UF. That is, there were proportionately larger changes in UF compared with AP (< or = 10 times greater magnitude) in response to the experimental interventions.(ABSTRACT TRUNCATED AT 250 WORDS)

Currently available methods for the measurement of spontaneous activity of laboratory animals require expensive, specialized equipment and may not be suitable for use in low light conditions with nocturnal species. We developed a gravimetric method that uses common laboratory equipment to quantify the total spontaneous activity of rats and is suitable for use in the dark. The rat in its home cage is placed on a top-loading electronic balance interfaced to a computer. Movements are recorded by the balance as changes in weight and transmitted to the computer at 10 Hz. Data are analyzed on-line to derive the absolute value of the difference in weight between consecutive samples, and the one-second average of the absolute values is calculated. The averages are written to file for off-line analysis and summed over the desired observation period to provide a measure of total spontaneous activity. The results of in vitro experiments demonstrated that: 1) recorded weight changes were not influenced by position of the weight on the bottom of the cage, 2) values recorded from a series of weight changes were not significantly different from the calculated values, 3) the constantly decreasing force exerted by a swinging pendulum placed on the balance was accurately recorded, 4) the measurement of activity was not influenced by the evaporation of a fluid such as urine, and 5) the method can detect differences in the activity of sleeping and waking rats over a 10-min period, as well as during 4-hr intervals recorded during active (night-time) and inactive (daytime) periods. These results demonstrate that this method provides an inexpensive, accurate, and noninvasive method to quantitate the spontaneous activity of small animals.

1. Previous work has demonstrated a positive relationship between experimentally induced changes in arterial pressure (AP) and renal interstitial hydrostatic pressure (RIHP). The purpose of the present study was to test the hypothesis that RIHP is positively correlated with the normal changes in AP that occur spontaneously in conscious rats. 2. Rats were chronically instrumented for the recording of AP (via an aortic catheter) and RIHP. RIHP was measured by implanting a Millar microtransducer, whose tip had been encapsulated in a 35 ,um pore polyethylene matrix (5 mm long, 2 mm o.d.), approximately 5 mm below the renal cortical surface. 3. A total of 56 h of simultaneous analog recording of AP and RIHP was obtained from ten rats. Each 1 h segment was digitized and evaluated at frequencies of 1, 0-1, 0-02 and 0O01 Hz. 4. In forty-nine out of fifty-six of these 1 h recordings taken at 1 Hz, there were significant positive linear correlations between AP and RIHP (mean r = 0 32) with a mean slope of 0-11 mmHg RIHP/1 mmHg AP. Low-pass filtering to 0 01 Hz significantly increased the r value to 0-48. 5. These results demonstrate that spontaneous changes in AP and RIHP are positively correlated. The spontaneous coupling of AP and RIHP may be of importance in the regulation of salt and water excretion by the pressure diuresis mechanism.Both theory and experimental evidence indicate that a close correspondence between arterial pressure (AP) and urinary excretion of salt and water (pressure diuresis) is

The roles of the autonomic nervous system, vasopressin, and angiotensin II in support of blood pressure were evaluated in seven conscious, resting dogs while hydrated or dehydrated. Mean arterial blood pressure (MAP) was monitored, and the dogs were given hexamethonium to block autonomic ganglia. Thirty minutes later, they were given captopril, and after another 30 min, a vasopressin V1 antagonist, d(CH2)5TyrMeAVP, was given. The order okf administration of captopril and d(CH2)5TyrMeAVP was alternated in different experiments. Hexamethonium had no effect on steady-state MAP in either hydrated or dehydrated dogs. In hydrated dogs, the average MAP was 100 mmHg; d(CH2)5TyrMeAVP decreased MAP by approximately 12 mmHg, and captopril decreased MAP by 24 mmHg. The magnitude of the effect of these two inhibitors was independent of the order of their administration. Dehydration doubled the effect of d(CH2)5TyrMeAVP on MAP but had no effect on the response to captopril. The results suggest that 1) autonomic function is not essential for maintenance of arterial blood pressure in resting dogs; 2) during autonomic ganglionic blockade, arterial blood pressure is supported by both angiotensin II and vasopressin; and 3) dehydration increases the role of vasopressin in control of blood pressure.