290 Age and Ca2+ homeostasis in peripheral neurones, J. N. Buchholz et al. cells from young animals with no significant change in old cells (Tsai et al., 1998). In another study using SCG cells, we blocked the contribution of PMCA’s, mitochondrial calcium uptake, and the Na+/Ca2+ exchanger in controlling high K+-evoked [Ca2+]i transients. Under these conditions, the neurones are required + to rely on SERCA to modulate the rate of recovery of high K - evoked transients, and clearly the rate of recovery was slower in old as compared to young SCG neurones (Pottorf et al., 2000c). Overall, one mechanism that may account for an age- related decline in [Ca2+]i homeostasis in peripheral neurones is a decline in SERCA function. A reasonable explanation for a decline in SERCA function is that there is a decline in the genetic expression of SERCA isoforms and this has been shown in cardiac tissue (Maciel et al., 1990). However, in skeletal muscle and myocardium SERCA-mediated 45Ca2+ uptake declines with age while SERCA protein levels remain stable. These data suggest that the change in function of SERCAs is possibly mediated by a decline in their modulation by mechanisms such as phospho- rylation, as opposed to a decline in protein levels per se. (Gafni & Yuh, 1989; Xu & Narayanan, 1998). Further studies are necessary to determine the mechanisms that account for why SERCA function is altered with age. Aging and mitochondrial calcium uptake in peripheral neurones Mitochondrial calcium uptake has been shown to be a robust mechanism in the control and shaping of [Ca2+]i transients in peripheral and central neurones (Thayer & Miller, 1990; Buchholz et al., 1996; Murchinson & Griffith, 1999; Colegrove et al., 2000). In addition, it has been suggested that an age-related decline in mitochondrial function may be a mechanism that promotes cellular aging (Duckles et al., 2006). As mitochondria participate in calcium buffering, if there is a decrease in mitochondrial func- tion with age, or if there is an increased reliance on mitochon- dria to control [Ca2+]i levels, both conditions may possibly result in disruption of mitochondrial function due to calcium overload and apoptosis. There are studies in central neurones supporting an age-related decline in mitochondrial calcium uptake (Vitorica & Satrustegui, 1985; Villalba et al., 1995; Satrustegui et al., 1996). However, our studies in isolated SCG neurones and adrenergic nerve endings suggest that mitochondrial calcium uptake in peripheral neurones may be preserved with age. For example, when SCG neurones are exposed to dinitrophenol to block mitochondrial calcium uptake, peak, and rate of rise of high K+- evoked [Ca2+]i were only increased in neurones from old animals with no significant effect in young neurones (Buchholz et al., 1996). Similarly, in adrenergic nerve endings in tail arteries, dinitrophenol increased stimulation-evoked release in arteries from old animals with no significant effect in young arteries (Tsai et al., 1995). In another study, we blocked PMCA, SERCA, and the Na+/Ca2+ exchanger in SCG neurones to cause the neurones to rely primarily on mitochondrial calcium uptake to regulate [Ca2+]i transients. The results showed that the capacity of mito- chondrial calcium uptake did not change with age (Pottorf et al., 2000a). What is most interesting about these studies is that when multiple calcium modulatory systems are blocked, mito- chondria are still capable of controlling [Ca2+]i transients in SCG neurones and appear to be preserved with age. This increased reliance on mitochondria to control [Ca2+]i may place an added stress on aged neurones. For example, in brain slices from mice, mitochondrial depolarization as an index of mitochondrial cal- cium uptake in response to high K+-evoked [Ca2+]i transients, occurs in both young and old mice. However, in the old animals the rate of mitochondrial repolarization is slower that corre- sponded to slowed recovery of high K+-evoked [Ca2+]i transients (Xiong et al., 2002). Thus, in the case of high neuronal activity in CNS neurones, the ability of the mitochondria to sustain its buffering of repeated [Ca2+]i transients may be compromised. The studies cited above are consistent with other studies in CNS neurones suggesting that mitochondrial calcium uptake is essential in the modulation of [Ca2+]i transients with advancing age (Murchinson & Griffith, 1998, 1999). In a more recent review on CNS neurones, it has been suggested that as more stress is placed on the mitochondria to control stimulation-evoked increases in [Ca2+]i the ‘polarization state’ of the mitochondria may subtly decline over the lifespan (Toescu, 2005). This would in turn lead to a gradual decline in function of the mitochondria in terms of controlling [Ca2+]i and possibly maintaining energy status of the cell. Overall, the cited studies argue that mito- chondrial calcium uptake may become more central to control- ling [Ca2+]i transients in neurones with advancing age. In addition, maintaining mitochondrial function with improved diet and exercise as we age is emerging as an important topic in relation to cellular aging (Duckles et al., 2006). Aging and function of plasma membrane calcium transport systems The PMCA and Na+/Ca2+ exchange systems are two plasma membrane calcium-pumping mechanisms in peripheral and central neurones that play an important role in buffering [Ca2+]i transients via extrusion of calcium from the cytosol (Werth et al., 1996; Blaustein & Lederer, 1999; Pottorf & Thayer, 2002; Pottorf et al., 2006). Blockade of SERCA and prolonged calcium transients have been shown to induce PMCA gene expression and accelerate the function of PMCA via increased interaction of calmodulin with PMCA (Kuo et al., 1997; Pottorf & Thayer, 2002). These data suggest that components of the calcium buff- ering systems can compensate for a reduction in function of one component or in the case of prolonged [Ca2+]i transients. Indeed, in the failing myocardium, the Na+/Ca2+ exchanger levels increase as SERCA levels decline (Hasenfuss et al., 1991). Our studies in isolated SCG neurones support the notion that multiple components of the calcium buffering systems can com- pensate for a loss of SERCA function. Indeed, we have found that SCG neurones become more reliant on mitochondria to control [Ca2+]i transients. When SERCA and mitochondrial calcium uptake were blocked in SCG neurones, the PMCA and Na+/Ca2+ exchanger are required to control high K+-evoked © 2007 Loma Linda University Journal compilation © Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2007