292 Age and Ca2+ homeostasis in peripheral neurones, J. N. Buchholz et al. conclusion as to the functional consequences of this age-related decline in RyR levels. We have shown that caffeine-evoked release of calcium declines with age (Vanterpool et al., 2005) and the capacity to release calcium from the SER depends on SER calcium filling levels, as well as the modulation of the RYRs. Indeed, we have shown that SERCA function declines in the SCG (Tsai et al., 1998; Pottorf et al., 2000c). Thus, filling levels of the SER may also be compromised that may alter the func- tional capacity of the release mechanism. In addition, It has been reported that RyR function can be influenced by several factors, including phosphorylation, binding proteins, calcium levels, and nNOS, which modulates cADPr levels that in turn modulates CICR by changing the sensitivity of RYRs to changes in [Ca2+]i (reviewed in Galione, 1993; Eu et al., 1999; Balshaw et al., 2002; Meissner, 2002; Danila & Hamilton, 2004). In addition to measuring the impact of age on RyR gene expression and protein levels, we extended our studies to include possible age-related changes in selected modulators of the RYRs, which include phosphorylation and nNOS levels within the SCG (Vanterpool et al., 2006). Total phosphorylation of RyR channels was not altered with age suggesting that changes in steady state phosphorylation and hence regulation by this mechanism is not necessarily occurring. However, nNOS protein expression increases from 6 to 12 months and signifi- cantly declines from 12 to 24 months. As nNOS activity modulates cADPr levels, it is reasonable to speculate that these data may possibly suggest that cADPr levels may also decline with age. As there appear to only be two RYRs contributing to calcium release in the SCG, overall, given the age-related decline in RyR3, coupled with a decline in nNOS levels, we hypothesize that an age-related reduction in RyR3 receptor levels and cADPr levels may account in part for a decline in the function of RYRs. We are currently determining if advancing age alters cADPR levels in the SCG, which may shed light on activity of nNOS during the aging process and regulation of RYRs. Summary and conclusions The data suggest that normal aging in peripheral autonomic neurones is a subtle and complex process and does not always result in dramatic deterioration. Moreover, the aging process does not necessarily alter the function of excitable neurones in a uniform manner. In terms of age-related changes in [Ca2+]i regulation, we present the idea that in order to maintain cell viability peripheral neurones are able to compensate for an age- related decline in the function of at least one calcium-buffering system, SERCA, by increased function of other calcium-buffering systems, namely, the mitochondria and plasmalemma calcium extrusion. Increased mitochondrial calcium uptake may represent a ‘weak point’ in cellular compensation as this over time may contribute to cell death (Ichas & Mazat, 1998; Thibault et al., 1998; Begley et al., 1999). This review summarizes our work on the dynamics of intra- cellular calcium regulation and possible consequences for auto- nomic nerve function with advancing age. The major findings of all of our studies are summarized in Fig. 2. Based on results of our most current studies (Vanterpool et al., 2005, 2006) and our previous work and that of others we propose that an age- related alteration in [Ca2+]i signaling and function of peripheral adrenergic neurones represents a complex interplay of mech- anisms, including increased sensitivity of the neurotransmitter release mechanism to calcium, a decline in SERCA function that alters calcium buffering and refilling of SER calcium stores, reduced RyR3, and nNOS levels, which in turn modulate cADPr levels and the CICR process. The consequences of these changes are currently being studied in our laboratory by direct measure- ment of cADPr during aging and the contribution of release of calcium from SER stores to stimulation-evoked increases in [Ca2+]i. Given the advances in molecular techniques future studies may include a comprehensive study on the impact of age on the genetic expression and protein levels of multiple buffering systems, including soluble calcium-buffering proteins, SERCA, mitochondrial calcium pumps, PMCA, and the Na+/Ca2+ exchanger. In addition, another mechanism that modulates calcium influx and SER refilling are store operated calcium channels. Their genetic expression and function with advancing age has not been studied and may provide a fruitful avenue of future research. Finally, SERCA function declines with age thus possibly altering the levels of SER calcium stores and CICR. With calcium indicators available to study SER calcium levels, it Fig. 2 Summary of major findings of age-related alterations in the function of peripheral autonomic neurones and [Ca2+]i regulation. © 2007 Loma Linda University Journal compilation © Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2007