Different effects of endogenous and exogenous cAMP on the voltage dependence of I_f in young and aged SAMs. (A and B) Normalized mean (±SEM) conductance-voltage relationships for I_f in whole-cell recordings from young (A) and aged (B) SAMs in control conditions (black and red symbols), in the presence of IBMX plus forskolin (white and pink) or with 1 mM cAMP in the patch pipette (gray and dark red). (insets) Representative whole-cell I_f current families. Scale bars: 500 pA, 500 ms (A) and 200 pA, 500 ms (B). (C) Mean (±SEM) V_1/2 values for I_f in whole-cell recordings in control conditions (filled), with IBMX and forskolin in the bath (open), or with 1 mM cAMP in the patch pipette (hatched). *, P < 0.05 versus young SAMs in the same conditions (t tests); ‡, P < 0.05 versus control for the same age (ANOVAs with Holm-Šidák post-tests).