Slower AP firing rates in aged SAMs persist during maximal stimulation of endogenous cAMP production. (A) Representative spontaneous APs from perforated-patch recordings from SAMs isolated from young (black) or aged (red) mice under control conditions (left) or in 100 µM IBMX plus 10 µM forskolin (Forsk; right). Scale bar: 250 ms, 50 mV. Dashed lines indicate 0 mV. (B) Mean AP firing rates (±SEM) from young and aged SAMs in control or IBMX plus forskolin. Numbers in parentheses indicate numbers of cells. *, P < 0.05 versus corresponding condition in young cells (t tests); ‡, P < 0.05 versus control conditions for the same age before wash on of IBMX/forskolin (paired t tests). (inset) Schematic illustration of perforated-patch recording configuration. Lightning bolts represent amphotericin B, and red asterisks represent endogenous cAMP remaining in the cell during the recordings.