title: Properties of the pacemaker current (If) in latent pacemaker cells isolated from cat right atrium.
abstract: 1. Single latent pacemaker cells were isolated from the Eustachian ridge of cat right atrium using Langendorff perfusion and enzyme dispersion techniques. Whole-cell patch-clamp techniques were used to study the hyperpolarization-activated inward current (I(f)). 2. All cells studied beat rhythmically. Pacemaker activity was recorded in the voltage range -68 +/- 1 to -54 +/- 2 mV and its cycle length was 901 +/- 67 ms (72 +/- 5 beats min-1) at 34-36 degrees C. Cells were elongated with tapered ends, and appeared bent or crinkled without obvious striations. Mean cell diameter and length were 7.4 +/- 0.5 microns and 93.1 +/- 5.9 microns, respectively (n = 15). Input resistance and total membrane capacitance were 2.2 +/- 0.2 G omega and 27.8 +/- 3.1 pF, respectively. 3. Hyperpolarizing clamp steps more negative than -50 mV elicited a time-dependent increasing inward current that was maximally activated at -120 mV. Activation of I(f) was well within the pacemaker voltage range. Half-maximal activation voltage and slope factor were calculated, using a Boltzmann function, to be -80.5 mV and 8.4, respectively. 4. The fully activated current-voltage (I-V) relationship was approximately linear at voltages more negative than -30 mV and showed outward rectification at more positive voltages. The reversal potential of I(f) was -26 mV and the fully activated conductance was 1.75 +/- 0.14 nS (n = 21). Caesium (2 mM) blocked I(f) at voltages more negative than the reversal potential. Reducing extracellular Na+ or K+ shifted the reversal potential more negative, and increasing extracellular K+ exerted the opposite effect. Reducing extracellular Na+ decreased I(f) amplitude and the slope of the fully activated I-V relationship, and elevated extracellular K+ increased I(f) amplitude and the slope of the fully activated I-V relationship. 5. Some pacemaker cells exhibited a short delay in the onset of I(f) activation whereas other pacemaker cells exhibited little, if any, delay in activation. I(f) currents exhibiting no delay in activation were best fitted by a single exponential function with a mean time constant of 3.20 +/- 1.03 s at -70 mV (n = 4). 6. A nystatin-permeabilized patch recording method was used to record spontaneous pacemaker action potentials and I(f) from the same pacemaker cell. Caesium (2 mM) inhibited I(f) by more than 90% (at -70 mV), and decreased the slope of diastolic depolarization, resulting in a 48 +/- 5% decrease in spontaneous rate.(ABSTRACT TRUNCATED AT 400 WORDS)
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D016275:Atrial Function; D001683:Biological Clocks / Q000502:physiology*; D002415:Cats; D002478:Cells, Cultured; D002586:Cesium / Q000494:pharmacology; D004594:Electrophysiology; D006321:Heart / Q000502:physiology*; D006325:Heart Atria / Q000166:cytology; D015640:Ion Channel Gating / Q000187:drug effects; D007473:Ion Channels / Q000502:physiology; D007700:Kinetics
chemical_list: D007473:Ion Channels; D002586:Cesium