Differential control of the hyperpolarization-activated current (i(f)) by cAMP gating and phosphatase inhibition in rabbit sino-atrial node myocytes

J Physiol. 1997 May 1;500 ( Pt 3)(Pt 3):643-51. doi: 10.1113/jphysiol.1997.sp022049.


1. The actions of the phosphatase inhibitor calyculin A on the hyperpolarization-activated cardiac 'pacemaker' current (i(f)) were determined in single cells isolated from the sino-atrial (SA) node of the rabbit. 2. Cells were incubated for 8 min in Tyrode solution containing calyculin A (0.5 microM) and then superfused with normal Tyrode solution. The mean normalized i(f) measured in eight cells at mid-activation voltages during and after exposure to calyculin A increased maximally by 47% with a time constant of 466 s, a time much longer than that required for cAMP-mediated i(f) stimulation (about 8 s). 3. In two-pulse protocols, calyculin A treatment increased i(f) at full as well as at mid-activation voltages, indicating a higher i(f) conductance. 4. Measurement of the conductance-voltage (gf(V)) relation by voltage ramp protocols confirmed a conductance increase by calyculin A, with no significant change in the position of the activation curve on the voltage axis. Data pooled together from ramp and two-pulse protocols yielded a calyculin A-induced increase in fully activated i(f) conductance of 39.6 +/- 6.4% (n = 16 cells). 5. The positive and negative shift of i(f) voltage dependence in response to beta-adrenergic (1 microM isoprenaline) and muscarinic stimulation (1 microM acetylcholine), respectively, was preserved after the calyculin A-induced increase in conductance. The shift of the i(f) activation curve induced by 1 microM isoprenaline was significantly larger in calyculin A-treated cells (8.8 vs. 5.8 mV). 6. These data indicate that phosphatase inhibition increases i(f) in a manner distinct from the direct cAMP pathway and potentiates the beta-adrenergic-mediated i(f) modulation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetylcholine / pharmacology
  • Adrenergic beta-Agonists / pharmacology
  • Animals
  • Biological Clocks / drug effects
  • Biological Clocks / physiology*
  • Cyclic AMP / physiology*
  • Electrophysiology
  • Enzyme Inhibitors / pharmacology*
  • In Vitro Techniques
  • Ion Channel Gating / drug effects
  • Ion Channel Gating / physiology*
  • Isoproterenol / pharmacology
  • Marine Toxins
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Oxazoles / pharmacology
  • Phosphoric Monoester Hydrolases / antagonists & inhibitors*
  • Phosphorylation
  • Rabbits
  • Sinoatrial Node / cytology
  • Sinoatrial Node / drug effects
  • Sinoatrial Node / metabolism*


  • Adrenergic beta-Agonists
  • Enzyme Inhibitors
  • Marine Toxins
  • Oxazoles
  • calyculin A
  • Cyclic AMP
  • Phosphoric Monoester Hydrolases
  • Isoproterenol
  • Acetylcholine