Effect of isoprenaline on I_f current in latent pacemaker cells isolated from cat right atrium: ruptured vs. perforated patch whole-cell recording methods

Whole-cell recording techniques were used to study the time-dependent inward current activated on hyperpolarization (I_f) and its response to isoprenaline in latent atrial pacemaker cells isolated from cat right atrium. To determine whether the response to isoprenaline depended on the type of recording method, we analysed I_f using either a standard ruptured-patch, or a nystatinperforated-patch, whole-cell recording method. All cells beat rhythmically at 35°C and exhibited normal pacemaker action potentials and I_f current, regardless of the recording method. With the ruptured-patch method, pacemaker action potentials ceased activity within a few minutes and I_f amplitude decreased "ran down" to 74% of control within 10 min of rupturing the patch. Isoprenaline (1 μM) elicited variable changes in I_f amplitude among different latent pacemaker cells resulting in no net change in mean current amplitude (n=6). In addition, isoprenaline failed to change the voltage dependence of the I_f activation curve. On the other hand, using a nystatin-perforated-patch method, pacemaker action potentials and I_f exhibited no significant changes over the same 10 min period. Under these conditions, isoprenaline consistently increased I_f in all cells studied (+90% at -80 mV; n=8), and increased the spontaneous rate of pacemaker action potentials by 58±7% (n=5). Moreover, isoprenaline elicited a significant positive shift (+11 mV) in the half-maximal activation voltage of the I_f activation curve (n=3). We conclude that latent atrial pacemakers consistently exhibit I_f current, and that isoprenaline consistently elicits an increase in I_f amplitude. Variabilities in the response of I_f to isoprenaline may result from the loss of internal constituents when using the ruptured-patch, whole-cell recording method.