Diminished Ca2+-sequestering activity of the sarcoplasmic reticulum (SR) is implicated in the age-associated slowing of cardiac muscle relaxation. In attempting to further define the underlying mechanisms, the present study investigated the impact of aging on the contents of major SR Ca2+-cycling proteins and SR protein phosphorylation by endogenous Ca2+/calmodulin-dependent protein kinase (CaM kinase). The studies were performed using homogenates and SR vesicles derived from the ventricular myocardium of adult (6–8 mo old) and aged (26–28 mo old) Fischer 344 rats. Western immunoblotting analysis showed no significant age-related difference in the relative amounts of ryanodine receptor-Ca2+-release channel (RyR-CRC), the Ca2+-storage protein calsequestrin, Ca2+-pumping ATPase (Ca2+-ATPase), and Ca2+-ATPase-regulatory protein phospholamban (PLB) in SR or homogenate. On the other hand, the relative amount of immunoreactive CaM kinase II (δ-isoform) was ∼50% lower in the aged heart. CaM kinase-mediated phosphorylation of RyR-CRC, Ca2+-ATPase, and PLB was reduced significantly (∼25–40%) in the aged compared with adult rat. ATP-dependent Ca2+-uptake activity of SR and the stimulatory effect of calmodulin on Ca2+ uptake were also reduced significantly with aging. Treatment of SR vesicles with anti-PLB antibody (PLBab) invoked relatively less stimulation of Ca2+ uptake in the aged (≤26%) compared with the adult (≤65%) rat. Ca2+-ATPase but not PLB underwent phosphorylation by CaM kinase in PLBab-treated SR with resultant stimulation of Ca2+ uptake. The rates of Ca2+ uptake by PLBab-treated SR were significantly lower (45–55%) in the aged compared with adult rat in the absence and presence of calmodulin. These findings imply that changes in the intrinsic functional properties of SR Ca2+-cycling proteins and/or their phosphorylation-dependent regulation contribute to impaired SR function in the aging heart.

The impact of aging on the Ca2+ pump function of skeletal muscle sarcoplasmic reticulum (SR) was investigated using SR-enriched membrane vesicles isolated from the slow-twitch soleus muscle (SM) and the relatively fast-twitch gastrocnemius muscle (GM) isolated from adult (6-8 mo old) and aged (26-28 mo old) Fischer 344 rats. In addition, isometric twitch characteristics of SM and GM were determined in situ in adult and aged rats under anesthesia. The rates of ATP-supported Ca2+ uptake by SM SR was markedly lower ( approximately 50%) in the aged compared with adult at varying Ca2+ (0.11-8.24 microM) concentrations. Kinetic analysis of the data revealed age-associated decrease in maximum activity reached (Vmax) and increase in the concentration of Ca2+ giving half of Vmax. In contrast, no significant age-related difference was observed in ATP-supported Ca2+ uptake activity of GM SR. The Ca(2+)-stimulated adenosinetriphosphatase (ATPase) activities and the amount of Ca(2+)-ATPase protein did not vary significantly with aging in SM or GM SR. Also, no significant age-related difference was observed in the content of the ryanodine receptor (Ca(2+)-release channel) or the Ca2+ binding protein, calsequestrin in SM and GM SR. In isometrically contracting SM, the time to peak force, half-relaxation time, and contraction duration were significantly prolonged in the aged compared with adult, whereas there was no age-related difference in maximum developed force. None of these isometric twitch parameters differed significantly with age in the GM. These results demonstrate that the effects of aging on skeletal muscle contractile properties and SR function are muscle specific. Furthermore, the data strongly suggest that impairment in SR Ca2+ pump function, apparently due to uncoupling of ATP hydrolysis from Ca2+ transport, contributes to the age-associated slowing of relaxation in the soleus muscle.

Multiple calmodulin (CaM) isoforms are expressed in plants, but their biochemical characteristics are not well resolved. Here we show the differential regulation exhibited by two soya bean CaM isoforms (SCaM-1 and SCaM-4) for the activation of five CaM-dependent enzymes, and the Ca(2+) dependence of their target enzyme activation. SCaM-1 activated myosin light-chain kinase as effectively as brain CaM (K(act) 1.8 and 1.7 nM respectively), but SCaM-4 produced no activation of this enzyme. Both CaM isoforms supported near maximal activation of CaM-dependent protein kinase II (CaM KII), but SCaM-4 exhibited approx.12-fold higher K(act) than SCaM-1 for CaM KII phosphorylation of caldesmon. The SCaM isoforms showed differential activation of plant and animal Ca(2+)-ATPases. The plant Ca(2+)-ATPase was activated maximally by both isoforms, while the erythrocyte Ca(2+)-ATPase was activated only by SCaM-1. Plant glutamate decarboxylase was activated fully by SCaM-1, but SCaM-4 exhibited an approx. 4-fold increase in K(act) and an approx. 25% reduction in V(max). Importantly, SCaM isoforms showed a distinct Ca(2+) concentration requirement for target enzyme activation. SCaM-4 required 4-fold higher [Ca(2+)] for half-maximal activation of CaM KII, and 1.5-fold higher [Ca(2+)] for activation of cyclic nucleotide phosphodiesterase than SCaM-1. Thus these plant CaM isoforms provide a mechanism by which a different subset of target enzymes could be activated or inhibited by the differential expression of these CaM isoforms or by differences in Ca(2+) transients.

Previous studies have shown that the inotropic response of the heart to beta-adrenergic stimulation declines with aging. This alteration has been attributed partly to an age-related impairment in the activation of the beta-adrenoceptor-G protein-adenylate cyclase complex. To further understand the mechanisms underlying the age-related deficit, the present study compared beta-adrenergic-mediated contractile response, cAMP accumulation, and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins in isolated perfused hearts from adult (6-8 months) and aged (28-30 months) Fischer 344 rats. In isometrically contracting, electrically paced (240 beats per minute) hearts perfused at constant flow rate (9 ml/min per gram ventricle), the baseline contractile performance differed significantly between adult and aged hearts. Thus, contraction duration was prolonged (approximately 15%, p < 0.001) in the aged relative to the adult heart, and this was due to increases in time to peak tension and relaxation time. Further, developed peak tension, normalized per gram ventricular wet weight, was significantly lower (approximately 20%, p < 0.05) in the aged compared with the adult heart. In these isolated perfused heart preparations, beta-adrenergic stimulation with isoproterenol (ISO, 0.001-1 microM) evoked concentration-dependent positive inotropic and lusitropic responses, both of which were significantly lower (15-20%, p < 0.05-0.001) in the aged compared with the adult heart. These age-related differences were manifested as relatively smaller ISO-induced increases in 1) developed peak tension, 2) maximum rate of tension development (+dT/dt), and 3) maximum rate of relaxation (-dT/dt) in the aged compared with the adult heart. The ISO-induced abbreviation of time to half relaxation was also less marked in the aged heart. Under similar experimental conditions, ISO (0.1 microM)-induced increase in tissue cAMP content was also lower (approximately 18%, p < 0.05) in the aged heart. ISO (0.1 microM)-induced phosphorylation of the sarcoplasmic reticulum protein phospholamban and myofibrillar protein troponin I was significantly diminished (approximately 38% and 25% decline, respectively, for phospholamban and troponin I; p < 0.05-0.001) in the aged compared with the adult heart. No significant age-related difference was, however, evident in ISO-induced phosphorylation of C protein of myofibrils. These data suggest that age-related decrements in beta-adrenergic-mediated cAMP accumulation and phosphorylation of phospholamban and troponin I contribute to the diminished contractile responses of the aged heart to beta-adrenergic stimulation.

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