Cardiac and skeletal muscle enzyme levels in hypertensive and aging rats

Alterations in ventricular K(ATP) channel properties during aging

Coronary heart disease remains the principle cause of mortality in the United States. During aging, the efficiency of the cardiovascular system is decreased and the aged heart is less tolerant to ischemic injury. ATP-sensitive K(+) (K(ATP)) channels protect the myocardium against ischemic damage. We investigated how aging affects cardiac K(ATP) channels in the Fischer 344 rat model. Expression of K(ATP) channel subunit mRNA and protein levels was unchanged in hearts from 26-month-old vs. 4-month-old rats. Interestingly, the mRNA expression of several other ion channels (> 80) was also largely unchanged, suggesting that posttranscriptional regulatory mechanisms occur during aging. The whole-cell K(ATP) channel current density was strongly diminished in ventricular myocytes from aged male rat hearts (also observed in aged C57BL/6 mouse myocytes). Experiments with isolated patches (inside-out configuration) demonstrated that the K(ATP) channel unitary conductance was unchanged, but that the inhibitory effect of cytosolic ATP on channel activity was enhanced in the aged heart. The mean patch current was diminished, consistent with the whole-cell data. We incorporated these findings into an empirical model of the K(ATP) channel and numerically simulated the effects of decreased cytosolic ATP levels on the human action potential. This analysis predicts lesser activation of K(ATP) channels by metabolic impairment in the aged heart and a diminished action potential shortening. This study provides insights into the changes in K(ATP) channels during aging and suggests that the protective role of these channels during ischemia is significantly compromised in the aged individual.

Involvement of the calcium-dependent proteolytic system in skeletal muscle aging

Aging is associated with a progressive and involuntary loss of muscle mass also known as sarcopenia. This condition represents a major public health concern with high socio-economics implications. Although sarcopenia is well documented, the aetiology of this condition still remains poorly understood. Calpains are ubiquitous proteases regulated in part by a specific inhibitor, calpastatin. They are well known to have major implications in muscle growth and differentiation. The aim of the present study was to determine if this proteolytic system could be involved in the phenotype associated with sarcopenia. Calpains and calpastatin levels, subcellular distributions and activities were compared between muscles from 3 and 24 months old rats. Altogether, the results we obtained showed an overall increase in calpain activities associated with muscle aging. These findings suggest that the calcium-dependent proteolytic system is indeed involved in sarcopenia.

Fiber type-related changes in rat skeletal muscle calcium homeostasis during aging and restoration by growth hormone

The mechanisms by which aging induces muscle impairment are not well understood yet. We studied the impact of aging on Ca2+ homeostasis in the slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscles of aged rats by using the fura-2 fluorescent probe. In both muscles aging increases the resting cytosolic calcium concentration ([Ca2+]i). This effect was independent on calcium influx since a reduced resting permeability of sarcolemma to divalent cations was observed in aged muscles likely due to a reduced activity of leak channels. Importantly the effects of aging on resting [Ca2+]i, fiber diameter, mechanical threshold and sarcolemmal resting conductances were less pronounced in the soleus muscle, suggesting that muscle impairment may be less dependent on [Ca2+]i in the slow-twitch muscle. The treatment of aged rats with growth hormone restored the resting [Ca2+]i toward adult values in both muscles. Thus, an increase of resting [Ca2+]i may contribute to muscle weakness associated with aging and may be considered for developing new therapeutic strategies in the elderly.

The biochemistry of aging muscle

Between the ages of 20 and 80, humans lose approximately 20-30% of their skeletal muscle mass. This age-related loss of muscle mass, sometimes described as 'sarcopenia of old age', is the consequence of complicated multifactorial processes and is commonly associated with osteopenia or osteoporosis. Consequences of the aging changes in muscle are declining physiological function and loss of muscle strength, typically associated with reduced physical activities. Consequently, falls and subsequent serious injuries are prevalent in the elderly. Thus, it is imperative to try and understand the processes, leading to age-related muscle loss, in order to develop means to retard this phenomenon leading to improved quality of life in the elderly. It is possible to divide the causes of muscle aging to intrinsic factors, involving changes at the molecular and cellular levels, and to extrinsic or environmental factors. The purpose of this review is to describe some of the biochemical processes and the possible mechanisms of muscle aging and to evaluate the importance of various extrinsic factors such as nutrition, exercise and limb immobilization. Changes in the aging skeletal muscle are reviewed with regard to: (a) enzyme activities, protein turnover and repair capacities (b) mitochondrial functioning and energy reserve systems (c) ion content and regulation (d) oxidative stress and free radicals (e) nutrition and caloric restriction (f) exercise and limb immobilization.

Effects of Aerobic Exercise on Energy Metabolism in the Hypertensive Rat Heart

Background and Purpose. In order to explore the possible effects of physical therapy interventions on patients with hypertension, we evaluated the effects of aerobic exercise training on myocardial energy metabolism in an animal model of hypertension. Subjects. We used 36 female spontaneously hypertensive rats (rats with genetically induced hypertension) and 12 normotensive Wistar-Kyoto rats. Methods. The normotensive rats were sedentary and formed the CONsed group. The spontaneously hypertensive rats were randomly divided into 3 experimental groups (12 rats per group). Hypertensive rats that were sedentary formed the HTNsed group, those that received 8 weeks of exercise training formed the HTN×8 group, and those that received 16 weeks of exercise training formed the HTN×16 group. We measured systolic blood pressure, heart wet weight, maximal activities of cardiac energy metabolism enzymes, glucose transporter content, and total concentrations of protein, glycogen, and triglyceride. Results. Systolic blood pressure was greater than 200 mm Hg in the CONsed group at the time of testing. Exercise training modestly (∼11–18 mm Hg) lowered blood pressure in the HTN×8 and HTN×16 groups. Fatty acid enzyme activity was greater in the CONsed group than in the HTNsed and HTN×8 groups, but activity was roughly equivalent between the CONsed group and the HTN×16 group. Glucose enzyme activity was greater in the HTN×16 group than in the CONsed group and the HTNsed group. Intracellular glycogen concentration was greater in the HTN×8 group than in the HTNsed group. Discussion and Conclusion. Results of this study suggest that aerobic exercise training may help to normalize cardiac energy metabolism in mammals with hypertension.