Steven L. Britton

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.

Animal genetic models for complex traits of physical capacity.

BRITTON, S.L., and L.G. KOCH. Animal genetic models for complex traits of physical capacity. Exerc. Sport Sci. Rev., Vol. 29, No. 1, pp 7–14, 2001. Understanding the genetic basis for variance in complex physical traits such as aerobic capacity has become an attainable goal. A starting point is the development or identification of animal genetic models that contrast the low and high values for the trait of interest. Genes that cause natural trait variation can ultimately be determined from animal models via genetic linkage.

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.

Hematoporphyrin Derivative Photochemotherapy of Experimental Bladder Tumors

Recent studies have shown that disruption of tumor blood flow is a major consequence of hematoporphyrin derivative photochemotherapy. A series of experiments was undertaken on the transplantable N-(4-(5-nitro-2-furyl)-2-thiazolyl)-formamide induced urothelial tumor in Fischer 344 rats to determine a dose response for both hematoporphyrin derivative and light. Tumor blood flow was used as the biologic criteria of response. Hematoporphyrin derivative doses of 10 micrograms./gm. body weight or above were necessary to cause a significant decrease in tumor blood flow when the tumors were illuminated with 360 joules/cm.2 of noncoherent red light (greater than 590 nm.). With a constant hematoporphyrin derivative dose of 20 micrograms./gm. body weight, significantly lower tumor blood flows were observed with fluences of 240 joules/cm.2 and above. In order to correlate dose response to tumor regression, experiments were done in which tumor dry weights were determined 3 weeks after completion of photochemotherapy (360 joules/cm.2). Hematoporphyrin derivative doses of 10 micrograms./gm. body weight or above were necessary to induce tumor regression. These studies support the hypothesis that disruption of tumor blood flow is a tumoricidal mechanism of hematoporphyrin derivative photochemotherapy.

Cardiac performance in inbred rat genetic models of low and high running capacity

1 Previous work demonstrating that DA inbred rats are superior to COP inbred rats in aerobic treadmill running capacity has indicated their utility as genetic models to explore this trait. We tested the general hypothesis that intermediate phenotypes of cardiac function and calcium metabolism are responsible for the difference in capacity between these strains. 2 Logical cardiac trait differences were estimated at a tissue (isolated papillary muscle), cellular (isolated left ventricular cells), and biochemical level of organization. 3 DA hearts were found to give significantly higher values than COP hearts for: (1) maximal developed tension (38.3 % greater), and rates of tension change in contraction (61 %) or relaxation (59 %) of isolated papillary muscle, (2) fractional shortening (50 %), amplitude of the Ca2+ transient (78.6 %), and caffeine‐induced release of Ca2+ from the sarcoplasmic reticulum (SR; 260 %) in isolated ventricular myocytes, and (3) Na+,K+‐ATPase activity of isolated myocytes (17.3 %). 4 Our results suggest that these trait differences may prove useful for further studies into the genes responsible for natural variations in both ventricular function and aerobic endurance capacity. Understanding the genetic basis of aerobic capacity will help define the continuum between health and disease.

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.

Autoregulation of hind-limb blood flow in conscious dogs.

We evaluated the efficiency of blood flow autoregulation of the hind‐limb vascular bed of eleven conscious dogs during: resting conditions; graded levels of treadmill exercise; and increases in oxygen consumption produced by the administration of 2,4‐dinitrophenol (DNP). Blood flow to the left hind limb was measured with an electromagnetic flow probe on the left external iliac artery. Hind‐limb perfusion pressure was measured from a catheter in the deep femoral artery and was controlled via an externally inflatable occlusion cuff positioned just distal to the flow probe. Arterial pressure was measured in the abdominal aorta. Experiments were performed 5‐16 days after instrumentation. Hind‐limb pressure‐flow (P‐F) relationships were evaluated by decreasing hind‐limb perfusion pressure in 4‐5 small sequential square‐wave steps of 10‐15 mmHg each while measuring flow. Each step decrease in perfusion pressure was maintained for 2 min. The efficiency of autoregulation was quantified by calculating the closed‐loop gain of flow regulation (Gc) at each decrement in perfusion pressure utilizing the equation: Gc = 1‐[(F0‐Fn/F0)/(P0‐Pn/P0)] where F0 and P0 are the starting (control) flows and pressures prevailing prior to decreasing perfusion pressure, and Fn and Pn are the new flows and pressures at each decrement in perfusion pressure. A Gc value less than 0 indicates a predominantly passive P‐F relationship, while a Gc of 1 is perfect autoregulation of flow. When the dogs were at rest, decrements in hind‐limb perfusion pressure were accompanied by almost equivalent decreases in flow, i.e. no autoregulation occurred, and Gc averaged ‐0.177 +/‐ 0.044 over the pressure range from 100‐40 mmHg. During all levels of treadmill exercise (on gradients of 0, 7, or 21%), however, positive Gc values were found that averaged from 0.258 +/‐ 0.046 at a gradient of 0% to 0.392 +/‐ 0.041 at a gradient of 21% and were significantly different from Gc values found during rest at perfusion pressure ranges from 90‐40 mmHg. The administration of DNP directly into the hind‐limb circulation increased hind‐limb blood flow from 199 to 481 ml/min. In the presence of DNP, Gc values were positive over perfusion pressure ranges from 100‐40 mmHg and averaged 0.473 +/‐ 0.054. These data demonstrate that hind‐limb blood flow is not autoregulated in resting dogs, but that significant autoregulation is manifest during conditions that increase oxygen consumption.(ABSTRACT TRUNCATED AT 400 WORDS)

A GRAVIMETRIC METHOD FOR THE MEASUREMENT OF TOTAL SPONTANEOUS ACTIVITY IN RATS

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.