SERCA2a and mitochondrial cytochrome oxidase expression are increased in hearts of exercise-trained old rats.

Aging of rats results in slower activities of calcium transport by cardiac calcium adenosinetriphosphatase (ATPase) of the sarcoplasmic reticulum (SR) and mitochondrial cytochrome oxidase (COX). These enzyme activities are faster after exercise training of previously sedentary old rats. Our purpose was to determine whether the expression of the genes encoding SR calcium ATPase (SERCA2a) or COX is altered by exercise training. Old (24-mo-old) male Fischer 344 rats were assigned to SO (sedentary old) or EO (exercised old) groups and compared with younger (12-mo-old) sedentary rats (SM). EO rats were trained on a treadmill for 8-10 wk. SERCA2a and COX mRNAs were lower (P < 0.05) in SO compared with SM and EO, whereas glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and cardiac alpha-actin mRNAs were similar across groups. The immunoreactive protein contents of cardiac calcium ATPase, cytochrome c, sarcomeric actin, and GAPDH followed the changes, when observed, in mRNA contents. Thus pretranslational mechanisms may be modified in some genes during aging and exercise training of previously sedentary old rats.

A chronically instrumented rat model to assess the altered baroreflex due to exercise.

We developed a conscious, chronically instrumented, exercise-trained rat model and examined the time course of the training-induced alteration of baroreflex function in response to hypertensive conditions. Exercise-trained (ET) animals ran at 18 m.min-1, 15% grade, for 60 min.d-1, 5 d.wk-1 for 5 wk. Baroreflex tests were conducted on day 6 each week. Regression line slopes relating the change in mean arterial pressure (delta MAP) to the change in heart rate (delta HR) were used to assess baroreflex sensitivity. Intravenous injections of phenylephrine were used to create hypertensive conditions. Compared with the C group, slopes of ET animals were reduced (from 2.1 to 1.2 bpm.mm Hg-1, P < 0.05) as early as week 3 of training in response to increasing doses of PE, and reached 0.8 bpm.mm Hg-1 by the end of training. The reflex bradycardiac response (delta HR) to PE was reduced (P < 0.05) depending on the dose of PE and the duration of training: in micrograms PE.kg-1 body weight, 5 (71% +/- 6% of control at week 2), 3 (70% +/- 7% of control at week 3, and 1 (61% +/- 10% of control at week 4). The pressor (delta MAP) to PE remained constant throughout training. Thus, using a chronically instrumented rat model that maintains the ability to run, we observed that the ability of the arterial baroreflex to produce bradycardia during pressor events was substantially reduced following as few as 2 wk of training.

Physiological effects of inverse agonists in transgenic mice with myocardial overexpression of the beta 2-adrenoceptor.

G-protein-coupled receptors are thought to have an inactive conformation (R), requiring an agonist-induced conformational change for receptor/G-protein coupling. But new evidence suggests a two-state model in which receptors are in equilibrium between the inactive conformation (R), and a spontaneously active conformation (R*) that can couple to G protein in the absence of ligand (Fig. 1). Classic agonists have a high affinity for R* and increase the concentration of R*, whereas inverse agonists have a high affinity for R and decrease the concentration of R*. Neutral competitive antagonists have equal affinity for R and R* and do not displace the equilibrium, but can competitively antagonize the effects both of agonists and of inverse agonists. The lack of suitable in vivo model systems has restricted the evidence for the existence of inverse agonists to computer simulations and in vitro systems. We have used a transgenic mouse model in which there is such marked myocardial overexpression of beta 2-adrenoceptors that a significant population of spontaneously activated receptor (R*) is present, inducing a maximal response without agonist. We show that the beta 2-adrenoceptor ligand ICI-118,551 functions as an inverse agonist, providing evidence supporting the existence of inverse agonists and validating the two-state model of G-protein-coupled receptor activation.

Mechanisms for the responses of cardiac muscle to physical activity in old age.

The decline of maximal cardiac output (Qmax) is a major factor responsible for the lower maximal oxygen consumption of elderly mammals. The lower Qmax is associated with aging-related decreases in maximal heart rate (HR-max) and maximal stroke volume (SVmax). The mechanism(s) for the slower HRmax, unchanged by exercise training, is unknown. The decrement in SVmax, however, can be improved, as shown by the enhanced systolic and diastolic properties of the elderly heart after exercise training. One major problem is diastolic dysfunction observed in the absence of disease. Diastolic dysfunction (a decrease in peak ventricular filling after systole or a prolonged relaxation of contracted muscle) results from in part a downregulation of the sarcoplasmic reticulum's (SR) calcium ATPase that sequesters cytosolic calcium via the hydrolysis of ATP. Exercise training of sedentary old mammals produces a faster relaxation and an upregulation of the SR calcium ATPase. Yet the characteristic shift of myosin toward the slower isoform is unaltered by exercise training. The molecular signals and mechanisms underlying these aging-related alterations in sedentary and physically active individuals are unknown. An enhancement of cardiac function by exercise training, though, is preserved in advanced age.

The role of calcium in the energetics of contracting skeletal muscle.

In its second messenger role in skeletal muscle, calcium coordinates the function of muscle (contractile activity) with its overall energetics, thereby controlling the provision of ATP in a time of need. Not only is ATP required for crossbridge turnover in the myofibrils, but it is also needed for the maintenance of ion pumps, nuclear activity, and so forth. When oxygen is limiting, the sustained contractions of both fast and slow muscle (after the immediate burst of activity) is primarily supported by glycogenolysis and the glycolytic pathway (anaerobic). Calcium is important to this process, and the compartmentation of the glycogen particle and some of the enzymes associated with the glycolytic pathway in the terminal cisternae of the sarcoplasmic reticulum ensures that the provision of glucose-6-phosphate to the glycolytic pathway for the generation of the needed ATP proceeds rapidly. The activation of phosphorylase and glycogenolysis by calcium-troponin-C is another example of the tight control of cellular energetics deemed possible by compartmentation within the cell. The regulation by calcium, therefore, is only dependent on the diffusion of calcium rather than diffusion of substrate. When oxygen is not limiting (i.e. when a new steady-state is reached), the aerobic metabolism of pyruvate and fatty acids may be regulated in part by calcium at least in slow skeletal muscle. Oxidative phosphorylation, where ADP is phosphorylated to ATP, is though to be controlled by the concentration of ADP in skeletal muscle. However, because of the obvious compartmentation of the mitochondria within the slow muscle fibre and the higher free calcium required for peak force development (5 mumol/L), the kinetics are theoretically favourable for the calcium cycle in slow muscle mitochondria to play an important role in the regulation of aerobic substrate oxidation, as it does in the heart. Although this hypothesis is attractive based on the available data, the direct demonstration of a major role for calcium as a regulator of substrate oxidation in slow muscle awaits experimentation.