Role of the 293b-sensitive, slowly activating delayed rectifier potassium current, i(Ks), in pacemaker activity of rabbit isolated sino-atrial node cells.
OBJECTIVE
(i) to characterize the electrophysiological properties of the slowly activating delayed rectifier potassium current, i(Ks), defined as the 293b-sensitive current, during the action potential (AP) of rabbit sino-atrial node (SAN) pacemaker cells; (ii) to evaluate the contribution of i(Ks) to the pacemaker AP under physiological conditions and during beta-adrenergic stimulation.


METHODS
Rabbit SAN pacemaker cells were studied using the perforated patch clamp technique in voltage-, AP- and current-clamp modes.


RESULTS
Voltage-clamp findings. Block of i(Ks) by 293b is dose-dependent, with an IC(50) (half block) in rabbit SAN cells of 1.35 microM and an IC(80) (sub-maximal block) of 5 microM. Sub-maximal concentrations of 293b have no significant effects on long-lasting and transient inward calcium currents, i(Ca,L) and i(Ca,T), inward hyperpolarization activated current, i(f), and transient outward current, i(to). AP-clamp experiments. The 293b-sensitive current activates near the peak of the SAN pacemaker action potential, reaches a mean maximal current density of 1.0+/-0.3 pA/pF (n=8, cell capacitances 27 to 62 pF, mean 35+/-4.0 pF) during late repolarization, and inactivates towards the end of repolarization. Additionally, in two smaller cells (cell capacitances 15 and 23 pF), no discernible 293b-sensitive current component was detected. Current-clamp data. In spontaneously beating SAN cells under control conditions, sub-maximal block of i(Ks) by 5 microM 293b has negligible effects on action potential characteristics and does not change average cycle length (n=11). In contrast, after pre-treatment with 10 nM isoprenaline to mimic beta-adrenergic stimulation, cells showed a 293b-induced depolarization of maximum diastolic potential by 2.2+/-1%, a decrease in diastolic depolarization rate by 9.9+/-4%, and a slowing of late action potential repolarization by 28.7+/-10.2%, resulting in a prolongation of spontaneous cycle length by 9.8+/-3.0% (P<0.05, n=10; for all parameters).


CONCLUSIONS
Our findings suggest that in rabbit SAN: (i) i(Ks) is activated during the normal pacemaker AP; (ii) the contribution of i(Ks) to beating rate is small under control conditions; and (iii) i(Ks) contributes significantly to spontaneous pacemaker rate during beta-adrenergic stimulation.
Lei
Ming
M
Department of Physiology, University of Oxford, Oxford OX1 3PT, UK. ming.lei@physiol.ox.ac.uk
Cooper
Patricia J
PJ
Camelliti
Patrizia
P
Kohl
Peter
P
D000200:Action Potentials / Q000187:drug effects*; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D002839:Chromans / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D007545:Isoproterenol / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers / Q000494:pharmacology*; D024661:Potassium Channels, Inwardly Rectifying / Q000187:drug effects*; D011817:Rabbits; D012849:Sinoatrial Node / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D013449:Sulfonamides / Q000494:pharmacology*
D000318:Adrenergic beta-Agonists; D002839:Chromans; D026902:Potassium Channel Blockers; D024661:Potassium Channels, Inwardly Rectifying; D013449:Sulfonamides; C103474:6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethylchromane; D007545:Isoproterenol