Abstract
1. The hyperpolarization-activated 'pacemaker' current (i(f)) was recorded in inside-out patches excised from rabbit sino-atrial (SA) node cell membranes. 2. Single-channel activity could be resolved in patches containing only a few channels; the voltage dependence of single-channel size and single-channel conductance (0.97 pS) were similar to those measured previously in cell-attached conditions. 3. Perfusion of the intracellular side of the patch membrane with 10 microM cAMP facilitated the opening of single i(f) channels on hyperpolarization. The cAMP-induced i(f) current activation occurred without modification of the single-channel conductance. 4. Modification by cAMP of the probability of channel opening was investigated with respect to the latency to first opening during hyperpolarization and in patches containing a large number of channels (macro-patches). First-latency histograms showed that cAMP shifts the probability curve of first openings to shorter times, in agreement with a cAMP-induced facilitation of channel opening. In macro-patches, measurement of the voltage dependence of the open probability by a slow voltage ramp protocol showed that cAMP shifts the probability curve to more positive voltages without modifying its shape. 5. In cell-free macro-patches the normalized open probability curve in control solutions was centred around -121.9 mV, a voltage some 30 mV more negative than in cell-attached macro-patches. Negative shifting of the curve after patch excision could only partly be explained by the removal of intracellular cAMP, and progressed with time during the ramp protocol, suggesting the presence of a run-down process independent from cAMP.