Figure 6. Effect of a selective peptide inhibitor of cAMP-dependent protein kinase A (PKI, 20 μM) on the ISO-stimulated I_f and L-type Ca²⁺ current (I_Ca,L) in LDS cells.

A, ISO neither stimulated I_f nor I_Ca,L in the presence of PKI (1 PKI, 2 PKI and ISO). The inset in A displays I_Ca,L at a more extended time scale. Simultaneous recording of I_Ca,L and I_f consisted of a depolarizing step from a holding potential of −50 to 0 mV for 200 ms and a subsequent hyperpolarization to −110 mV for 2 s. B, time course of the experiment displayed in A. The I_f (•) amplitude remained unaltered upon addition of ISO whereas the pronounced decrease in I_Ca,L (^) was due to run down. A statistical summary of these data demonstrates that neither I_f (C; n= 6) nor I_Ca,L (D; n= 4) was stimulated by ISO in the presence of PKI.