releases, which, in turn, activate ACs. This feed forward "fail safe" system, kept in check by a high basal phosphodiesterase activity, is central to the generation of normal rhythmic, spontaneous action potentials by pacemaker cells.Numerous studies over the past decade have indicated that intracellular Ca 2ϩ release is a key feature of normal cardiac pacemaker cell automaticity (1). More recently it has been demonstrated that the basal level of global cAMP in rabbit sinoatrial nodal cells (SANC) 3 exceeds that in ventricular myocytes (2).The high basal cAMP in SANC mediates robust basal protein kinase A (PKA)-dependent phosphorylation of specific surface membrane ion channels and Ca 2ϩ cycling proteins, which regulates the periodicity and amplitude of spontaneous, sarcoplasmic reticulum generated, local Ca 2ϩ releases in the absence of cell Ca 2ϩ overload (2). Local Ca 2ϩ releases emanate from ryanodine receptors of sarcoplasmic reticulum that lies beneath the sarcolemma (10 -15 nm), near the Na/Ca exchanger (NCX) proteins (3). Local Ca 2ϩ releases occur mainly during the late part of the spontaneous diastolic depolarization and activate an inward NCX current (4 -7). This imparts an exponential character to the late diastolic depolarization (5,8,9), facilitating the achievement of the threshold for opening of L-type Ca 2ϩ channels, which generate the rapid upstroke of the subsequent action potential (AP). Thus, cAMP-mediated, PKA-dependent phosphorylation of surface membrane ion channels and SR Ca 2ϩ cycling proteins control the SANC basal spontaneous rhythmic firing (2).The mechanisms that underlie a high basal cAMP in SANC are unknown. The failure of ␤ 1 or ␤ 2 adrenergic receptor (␤-AR) inverse agonists to alter the spontaneous, basal SANC firing rate indicates that high levels of cAMP are not due to constitutively active ␤-ARs (2). Although a reduction in phosphodiesterase (PDE) activity could, in part, account for elevated cAMP levels in SANC, recent evidence suggests that basal PDE activity of SANC is not reduced, but rather, appears to be elevated (10). Moreover, inhibition of basal adenylyl cyclase (AC) activity in SANC substantially reduces cAMP and cAMP-mediated, PKA-dependent phosphorylation of phospholamban (2) suggesting a high constitutive (basal) level of AC activity. Whereas there is some evidence to indicate that SANC harbor Ca 2ϩ -activated AC isoforms (11, 12), direct evidence for Ca 2ϩ activation of AC activity, and the specific cell microdomains in which this may occur, are lacking. Using multiple approaches we show that both Ca 2ϩ -regulated ACs reside in lipid microdomains and that Ca 2ϩ activation of AC activity occurs within these domains.

Western societies are rapidly aging, and cardiovascular diseases are the leading cause of death. In fact, age and cardiovascular diseases are positively correlated, and disease syndromes affecting the heart reach epidemic proportions in the very old. Genetic variations and molecular adaptations are the primary contributors to the onset of cardiovascular disease; however, molecular links between age and heart syndromes are complex and involve much more than the passage of time. Changes in CM (cardiomyocyte) structure and function occur with age and precede anatomical and functional changes in the heart. Concomitant with or preceding some of these cellular changes are alterations in gene expression often linked to signalling cascades that may lead to a loss of CMs or reduced function. An understanding of the intrinsic molecular mechanisms underlying these cascading events has been instrumental in forming our current understanding of how CMs adapt with age. In the present review, we describe the molecular mechanisms underlying CM aging and how these changes may contribute to the development of cardiovascular diseases.

Voltage-gated Ca(v)1.2 calcium channels couple membrane depolarization to cAMP response-element-binding protein (CREB)-dependent transcriptional activation. To investigate the spatial and temporal organization of CREB-dependent transcriptional nuclear microdomains, we combined perforated patch-clamp technique and FRET microscopy for monitoring CREB and CREB-binding protein interaction in the nuclei of live cells. The experimental approach to the quantitative assessment of CREB-dependent transcriptional signaling evoked by cAMP- and Ca(v)1.2-dependent mechanisms was devised in COS1 cells expressing recombinant Ca(v)1.2 calcium channels. Using continuous 2-dimensional wavelet transform and time series analyses, we found that nuclear CREB-dependent transcriptional signaling is organized differentially in spatially and temporally separated microdomains of 4 distinct types. In rat neonatal cardiomyocytes, CREB-dependent transcription is mediated by the cAMP-initiated CaMKII-sensitive and Ca(v)1.2-initiated CaMKII-insensitive mechanisms. The latter microdomains show a tendency to exhibit periodic behavior correlated with spontaneous contraction of myocytes suggestive of frequency-dependent CREB-dependent transcriptional regulation in the heart.