Aging Alters cAMP Signaling and Membrane-Delimited Regulation of if in Sinoatrial Myocytes

Abstract

We recently demonstrated that age-dependent declines in intrinsic and maximum heart rates are associated with slower action potential (AP) firing rate and a hyperpolarizing shift in the midpoint activation voltage (V1/2) of the funny current (If) in isolated sinoatrial myocytes (SAMs). Given that cAMP is an important intracellular modulator in SAMs, we hypothesized that reduced cAMP concentration may contribute to the slower APs and shifted If in aged SAMs. To test this hypothesis, we applied a saturating concentration of cAMP via the patch pipette in whole cell voltage-clamp and current clamp experiments in acutely isolated SAMs from young and old mice. Here, we show that this exogenous cAMP completely reversed the age-dependent changes in both AP firing rate and If in SAMs. In contrast, maximal stimulation of endogenous cAMP production, via co-application of the PDE inhibitor, IBMX and the adenylyl cyclase activator, forskolin, did not rescue either AP firing rate or If in aged SAMs. These data indicate that a deficit in cAMP production and/or responsiveness may contribute to the age-dependent changes in SAM excitability. To further determine how cAMP regulates If in aged versus young SAMs, we compared If activation in excised inside-out membrane patches from SAMs from young and old mice. Surprisingly, we found that the age-dependent hyperpolarized shift in the V1/2 of If persisted in the cell-free patches, indicating that soluble cAMP alone cannot account for the shifted If in aged SAMs. Rather, aging appears to alter an additional, membrane-associated factor that regulates If in SAMs. Future work will focus on resolving the underlying mechanism(s) responsible for the age-dependent shift in the V1/2 of If, and its relationship to AP generation in SAMs.