Cardiac hypertrophy in response to an isometric training program in the cat

Pressure-overload hypertrophy is unabated in mice devoid of AT1A receptors

Mechanisms controlling cardiac growth are under intense investigation. Among these, the renin-angiotensin system has received great interest. In the current study, we tested the hypothesis that the renin-angiotensin system was not an obligate factor in cardiac hypertrophy. We examined the left ventricular hypertrophic response to a pressure overload in mice devoid of the AT1A receptor, the putative major effector of the growth response of the renin-angiotensin system. Aortic banding produced similar transband gradients in wild-type and AT1A knockout mice. The left ventricular mass-to-body weight ratio increased from 3.44 +/- 0.08 to 5.62 +/- 0.25 in wild-type ascending aortic-banded mice. The response in the knockout mice was not different (from 2.97 +/- 0.13 to 5.24 +/- 0.37). We conclude that the magnitude of cardiac hypertrophy is not affected by the absence of the AT1A receptor and its signaling pathway and that this component of the renin-angiotensin system is not necessary in cardiac hypertrophy.

The morphologic consequences of systemic training

The development of echocardiography in the 1970s led to the flourishing of the study of the athlete's heart. From the earliest studies, it was apparent that athletes develop enlargement of the left ventricular cavity and thickening of myocardium in response to prolonged repetitive training. The changes in echocardiographic measurements are small and often within quoted normal ranges. By comparison to sedentary controls, however, left ventricular end-diastolic dimension is increased by approximately 10%, posterior wall dimension by 15% to 20%, and calculated mass by up to 45%.

The power athlete

A number of normal daily and athletic activities require isometric or static exercise. Sports such as weight lifting and other high-resistance activities are used by power athletes to gain strength and skeletal muscle bulk. Static exercise, the predominant activity used in power training, significantly increases blood pressure, heart rate, myocardial contractility, and cardiac output. These changes occur in response to central neural irradiation, called central command, as well as a reflex originating from statically contracting muscle. Studies have demonstrated that blood pressure appears to be the regulated variable, presumably because the increased pressure provides blood flow into muscles whose arterial inflow is reduced as a result of increases in intramuscular pressure created by contraction. Thus, static exercise is characterized by a pressure load on the heart and can be differentiated from the hemodynamic response to dynamic (isotonic) exercise, which involves a volume load to the heart. Physical training with static exercise (i.e., power training) leads to concentric cardiac (particularly left ventricular) hypertrophy, whereas training with dynamic exercise leads to eccentric hypertrophy. The magnitude of cardiac hypertrophy is much less in athletes training with static than dynamic exercise. Neither systolic nor diastolic function is altered by the hypertrophic process associated with static exercise training. Many of the energy requirements for static exercise, particularly during more severe levels of exercise, are met by anaerobic glycolysis because the contracting muscle becomes comes deprived of blood flow. Power athletes, training with repetitive static exercise, derive little benefit from an increase in oxygen transport capacity, so that maximal oxygen consumption is increased only minimally or not at all. Peripheral cardiovascular adaptations also can occur in response to training with static exercise. Although the studies are controversial, these adaptations include modest decreases in resting blood pressure, reduced increases in blood pressure and sympathetic nerve activity during a given workload, enhanced baroreflex function, increases in muscle capillary-to-fiber ratio, possible improvements in lipid and lipoprotein profiles, and increases in glucose and insulin responsiveness. Some of these adaptations can occur in cardiac or hypertensive patients with no concomitant cardiovascular complications. In both healthy individuals and those with cardiovascular disease, the manner in which resistance training is performed may dictate the extent to which these adjustments take place. Specifically, training that involves frequent repetitions of moderate weight (and hence contains dynamic components) seems to produce the most beneficial results.

Capillaries measured in canine diaphragm by two methods

We have measured capillary distribution in costal and crural canine diaphragm using two methods: histochemical processing and perfusion fixation. Each of 18 dogs was deeply anesthetized, the abdomen opened, and the left inferior phrenic artery cannulated. The animal was heparinized and overdosed with pentobarbital. The right hemidiaphragm was frozen, either postexcision (Protocol 1) or intact with no preload (Protocol 2), for histochemical processing. The left hemidiaphragm was fixed by perfusion in situ using 2% glutaraldehyde, either with preload (Protocol 1) or without (Protocol 2). Costal and crural regions of each hemidiaphragm were sampled for analysis. Frozen samples were sectioned and processed for acid-stable (pH 4.0) ATPase activity; perfusion-fixed samples were postfixed, stained, embedded in Epon, and sectioned. Measurements were made using a digital imaging system. We found that muscle fibers had smaller cross-sectional areas in costal than in crural diaphragm; capillary-to-fiber ratio (C:F) did not differ by region and regional differences in capillary density could be attributed to differences in fiber size. Results depended critically on methodology. In perfusion-fixed muscle, fiber area was less, C:F was greater, and capillary density was greater than in histochemically-processed tissue. We conclude that capillary distribution is similar in costal vs. crural diaphragm and that perfusion fixation identifies capillaries more effectively than histochemistry.

Supernormal ejection performance is isolated to the ipsilateral congenitally pressure-overloaded ventricle

Congenital left ventricular pressure overload is associated with "excessive" hypertrophy that leads to subnormal afterload (wall stress), permitting enhanced ventricular ejection performance. Whether congenital right ventricular pressure overload is associated with a similar phenomenon is uncertain. It is also unknown whether supranormal ejection performance affects only the overloaded ventricle or is a general process affecting both ventricles. Conflicting data exist about whether the hypertrophic process associated with pressure overload is induced primarily by local loading conditions or by neuroendocrine influences. If the former postulate is true, the hypertrophic response should be confined to the overloaded ventricle; if the latter is true, one might predict that both ventricles would be affected by a less specific response to circulating catecholamines. To help resolve these issues, both right and left ventricular performance was examined in seven patients with isolated congenital pulmonary stenosis (average pulmonary pressure gradient 78 +/- 13 mm Hg), six patients with isolated congenital aortic stenosis (average gradient 80 +/- 10 mm Hg) and six normal subjects. Right ventricular ejection fraction was increased in patients with pulmonary stenosis (61 +/- 2%) compared with the value in normal subjects (53 +/- 2%, p less than 0.01) and in patients with aortic stenosis (50 +/- 3%, p = 0.007). Left ventricular ejection fraction was increased in patients with congenital aortic stenosis (84 +/- 4%) compared with the value in normal subjects (70 +/- 4%, p less than 0.01) and in patients with congenital pulmonary stenosis (65 +/- 2%, p less than 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)

Improvement of cardiac performance by carboxylic ionophore monensin in greyhound and mongrel dogs

The effects of the cardiotonic ionophore monensin on coronary blood flow, cardiac output, left ventricular (LV) O2 consumption, LV contractility and arterial blood pressure were measured in greyhounds and compared with those in mongrel dogs under pentobarbital anaesthesia. Following intravenous injection of 100 micrograms/kg monensin, all parameters rose, the greyhounds showing a relatively greater response in peak values of coronary blood flow, LV contractility and cardiac output. The increase in the calculated LV mechanical work was proportionately greater than the corresponding increase in LV O2 consumption, hence the derived index, external mechanical efficiency, rose rapidly from an initial value of 0.165 to 0.285 (73%) in greyhounds; mongrel dogs showed a more modest rise from 0.094 to 0.120 (28%) during the same interval. The difference in the responses in the two breeds of dogs are attributed mainly to larger heart size and exercise training in greyhounds as compared with untrained mongrel dogs.

Resistive training and selected effects

This article has briefly discussed the various modes and methods of resistive training, and their relative values and effectiveness in achieving desired goals. Evidence has been presented indicating that resistive training can enhance both "physical and emotional fitness as well as reduce some cardiovascular disease risk factors." This may allow a greater satisfaction and enjoyment in daily activities as well as improve athletic performance.