Patricia J. Metting

Heritability of treadmill running endurance in rats

Treadmill running was evaluated as a phenotype for selective breeding for high- and low-endurance performance from a starting population of 18 male and 24 female outbred Sprague-Dawley rats. Each rat was exercised to exhaustion once per day for 5 consecutive days. The treadmill was set at a constant 15° slope, and the initial velocity of 10 m/min was increased by 1 m/min every 2 min. The total distance run on the single best day out of the five trials was taken as the measure of endurance performance. The original population (males and females combined, n = 42) ran on average for 396 m. The two lowest-performing pairs and two highest-performing pairs were selectively bred through three successive generations. After three generations of selection, performance of the offspring from the high selected line averaged 659 ± 36 m ( n = 20), whereas low-performance offspring ( n = 13) averaged 388 ± 28 m. The narrow-sense heritability, calculated as the regression of individual offspring performance on midparental value for each family, was 0.39 across the three generations. This implies that 39% of the variation in running endurance performance between the low and high selected lines was determined by heritable factors.Treadmill running was evaluated as a phenotype for selective breeding for high- and low-endurance performance from a starting population of 18 male and 24 female outbred Sprague-Dawley rats. Each rat was exercised to exhaustion once per day for 5 consecutive days. The treadmill was set at a constant 15 degrees slope, and the initial velocity of 10 m/min was increased by 1 m/min every 2 min. The total distance run on the single best day out of the five trials was taken as the measure of endurance performance. The original population (males and females combined, n = 42) ran on average for 396 m. The two lowest-performing pairs and two highest-performing pairs were selectively bred through three successive generations. After three generations of selection, performance of the offspring from the high selected line averaged 659 +/- 36 m (n = 20), whereas low-performance offspring (n = 13) averaged 388 +/- 28 m. The narrow-sense heritability, calculated as the regression of individual offspring performance on midparental value for each family, was 0.39 across the three generations. This implies that 39% of the variation in running endurance performance between the low and high selected lines was determined by heritable factors.

Evaluation of spontaneous baroreflex sensitivity in conscious dogs.

1. We evaluated a method of measuring cardiac baroreflex sensitivity (BRS) derived from spontaneous changes in systolic pressure (SP). SP was measured from the ECG signal in seven conscious, resting dogs. 2. Beat‐to‐beat changes in PI (dPI) were positively correlated with beat‐to‐beat changes in SP (dSP) in all dogs tested, suggesting spontaneous baroreflex function. The slope of the regression of dPI on dSP was used as an index of spontaneous BRS. 3. The spontaneous BRS was abolished by hexamethonium, atropine and bilateral carotid sinus denervation. Low dose atropine sulphate produced a paradoxical increase in spontaneous BRS, which has been observed in other studies. The spontaneous BRS was positively correlated with the average pulse interval in resting dogs. 4. Random modulation of heart rate after vagotomy failed to reproduce the strong positive correlation between dSP and dPI; this demonstrated that the correlation was not the result of mechanical coupling between heart rate and arterial blood pressure. 5. The BRS was measured pharmacologically in six dogs using a bolus injection of a vasoconstrictor. The pharmacological BRS was positively correlated with the spontaneous BRS measured after the bolus injection. 6. Finally, the spontaneous BRS was negatively correlated with the average arterial pressure in resting dogs. We conclude that the spontaneous BRS is a useful quantitative indicator of baroreflex function in conscious resting dogs.

Body fluid expansion is not essential for salt-induced hypertension in SS/Jr rats

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.

Adenosine is not essential for exercise hyperaemia in the hindlimb in conscious dogs.

1. The contribution of endogenous adenosine to the increase in hindlimb blood flow that occurs during treadmill exercise was evaluated in conscious dogs. We postulated that if adenosine is essential for the hindlimb hyperaemic response, then pharmacological treatment of the animals with adenosine receptor antagonists should decrease hindlimb blood flow during treadmill exercise. 2. A total of twenty‐three dogs were chronically instrumented for measurement of aortic blood pressure and hindlimb blood flow using electromagnetic or Doppler flow probes on the left external iliac artery. Measurements of arterial blood pressure, hindlimb blood flow and heart rate were made during steady‐state treadmill exercise in both the presence and the absence of adenosine receptor antagonists. Four different protocols were performed using different routes of administration of two adenosine receptor antagonists. Aminophylline was used in most of the experiments, and the effects of the more potent antagonist, 8‐phenyltheophylline, were also evaluated. In addition, the dogs exercised at varying intensities ranging from a low level of 5.5 km h‐1 at 0% gradient to a high intensity of 5.5 km h‐1 at 21% gradient. 3. Aminophylline given as a single intravenous dose, or as a constant infusion either intravenously or directly into the hindlimb artery, did not decrease hindlimb blood flow at low, moderate or high intensities of exercise. Likewise, the blockade of adenosine receptors with 8‐phenyltheophylline, given systemically or as a bolus injection administered directly into the hindlimb circulation during moderate exercise, did not attenuate the hindlimb blood flow response. 4. Our data demonstrate that exercise hyperaemia of the hindlimb is not reduced by antagonism of adenosine receptors. These findings are consistent with the hypothesis that adenosine is not an essential mediator of hindlimb vasodilatation during exercise.

Spontaneous changes in arterial blood pressure and renal interstitial hydrostatic pressure in conscious rats.

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 microns 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 0.01 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.

Phenotypic variation in sensorimotor performance among eleven inbred rat strains

As a first step toward identifying the genes that determine sensorimotor ability (motor coordination) we subjected 11 inbred strains of rats to three different tests for this trait. Rats were tested at 13 wk of age to determine how long they could remain on 1) a rotating cylinder as the velocity of rotation increased every 5 s (1-direction rotation test), 2) a rotating cylinder that reversed direction every 5 s and increased velocity every 10 s (2-direction rotation test), and 3) a platform that was tilted 2 degrees every 5 s from 22 to 47 degrees (tilt test). On all three tests, rats of the PVG strain demonstrated the greatest sensorimotor ability. In contrast, rats of the MNS strain were most often represented among the group of strains that demonstrated the lowest performance on all tests. Considering all three tests, there was a 3- to 13-fold range in sensorimotor performance between the highest and lowest strains. This large divergence between the highest and lowest strains provides a genetic model that can be used to identify intermediate phenotypes and quantitative trait loci that contribute to sensorimotor ability.As a first step toward identifying the genes that determine sensorimotor ability (motor coordination) we subjected 11 inbred strains of rats to three different tests for this trait. Rats were tested at 13 wk of age to determine how long they could remain on 1) a rotating cylinder as the velocity of rotation increased every 5 s (1-direction rotation test), 2) a rotating cylinder that reversed direction every 5 s and increased velocity every 10 s (2-direction rotation test), and 3) a platform that was tilted 2° every 5 s from 22 to 47° (tilt test). On all three tests, rats of the PVG strain demonstrated the greatest sensorimotor ability. In contrast, rats of the MNS strain were most often represented among the group of strains that demonstrated the lowest performance on all tests. Considering all three tests, there was a 3- to 13-fold range in sensorimotor performance between the highest and lowest strains. This large divergence between the highest and lowest strains provides a genetic model that can be used to identify intermediate phenotypes and quantitative trait loci that contribute to sensorimotor ability.

Phenotypic variation in strength among eleven inbred strains of rats

Abstract As a first step toward the long-range goal of identifying the genes that determine strength, we subjected 11 inbred strains of rats to three tests of muscular strength. The tests consisted of measuring (1) the force exerted by the rat as it was pulled by the base of the tail off a grid on the pan of a top-loading electronic balance (scale test); (2) the length of time the rat hung from a 2.5-mm-diameter U-shaped wire (wire-hanging test); and (3) the length of time the rat hung from a vertically oriented grid consisting of 4-mm-diameter rods (grid-hanging test). Six rats of each gender from each strain were tested at 12 weeks of age, once/day for 5 consecutive days. For the two tests that required use of all four limbs (the scale and grid-hanging tests), one strain performed best (DA). In contrast, on the test that required primarily the use of the front limbs (wire-hanging test), the DA was the lowest performing strain and the F344 rats the best. This differential ranking suggests that the tests selected for variance in the morphological distribution of strength among the strains. There was a 1.5- to 5.2-fold divergence observed between the males of the highest and lowest strains on the scale test and grid hanging tests. This large divergence provides the opportunity to search for intermediate phenotypes and quantitative trait loci that contribute to the different performances of the strains on these strength tests.

Quantitative contribution of systemic vascular autoregulation in acute hypertension in conscious dogs.

Experiments were performed in nine conscious dogs to quantitate the contribution of systemic vascular autoregulation to the increases in total peripheral resistance (TPR) and mean arterial pressure (MAP) produced by angiotensin II (ANG II), arginine vasopressin (AVP), and norepinephrine (NE). We hypothesized that if autoregulatory vasoconstriction is significant, then the increase in TPR produced by vasoconstrictor infusion will be greater when MAP is controlled at hypertensive values than when the increase in pressure is prevented by controlling MAP at the animals normotensive value. Each drug was infused at a dose sufficient to increase MAP by 50%. Then, a constant rate of vasoconstrictor infusion was maintained while MAP was controlled at hypertensive or normotensive levels for 15-min periods using a gravity reservoir connected to the left common carotid artery. During AVP infusion, TPR was significantly greater when MAP was controlled at hypertensive than at normotensive values. This autoregulatory-mediated vasoconstriction accounted for approximately three-fourths of the increase in MAP produced by AVP. No significant autoregulatory component was identified for the increases in TPR and MAP produced by ANG II or NE. We conclude that systemic vascular autoregulation is a powerful physiological property that contributes to the hemodynamic response to pressor doses of AVP.

Gravimetric Method for the Dynamic Measurement of Urine Flow

Abstract The rate of urine formation is a primary index of renal function, but no techniques are currently available to accurately measure low rates of urine flow on a continuous basis, such as are normally found in rats. We developed a gravimetric method for the dynamic measurement of urine flow in anesthetized rats. Catheters were inserted directly into the ureters close to the renal pelves, and a siphon was created to collect all of the urine formed as rapidly as it was produced. Urine flow was determined by measuring the weight of the urine using a direct-reading analytical balance interfaced to a computer. Basal urine flow was measured at 2-sec intervals for 30 to 60 min. The dynamic response of urine flow to a rapid decrease in arterial pressure produced by a bolus intravenous injection of acetylcholine (0.5 μg) was also measured. Intrinsic drift, evaporative losses, and the responsiveness of the system to several fixed pump flows in the low physiologic range were evaluated in vitro. The gravimetric method described was able to continuously measure basal urine flows that averaged 37.3 ± 12.4 μl/min. Error due to drift and evaporation was negligible, totaling less than 1% of the measured urine flow. Acetylcholine-induced declines in arterial pressure were followed within 8 sec by a decline in urine flow. These data demonstrate that this new gravimetric method provides a simple, inexpensive, dynamic measurement of urine flow in the μl/min range.

Pulmonary Vascular Efflux of Norepinephrine in Dahl Rats Susceptible or Resistant to Salt-Induced Hypertension

Abstract The purpose of these studies was to determine whether the accumulation of norepinephrine by the pulmonary circulation is altered in the Dahl model of genetic hypertension. Pulmonary norepinephrine accumulation was evaluated by performing a compartmental analysis of the efflux of L-[3H]norepinephrine from perfused lungs after inhibition of the norepinephrine-metabolizing enzymes. The lungs were isolated from Dahl salt-hypertension-susceptible (S) and salt-hypertension-resistant (R) rats that had been on a high sodium diet for 3 weeks. In both S and R rats, norepinephrine was accumulated into a single compartment with an efflux half-time of approximately 23 min, in addition to its distribution in the extracellular space. The size of the extracellular space was significantly increased in the S rats, but there was no difference in the size of the compartment of L-[3H]norepinephrine efflux between S (6.4 ± 1.2 ml/g) and R (3.7 ± 0.7 ml/g) rats. These data indicate that impaired accumulation and efflux of norepinephrine by the lungs does not contribute to the pathogenesis of hypertension in Dahl S rats.