Journal
JOURNAL OF MOLECULAR AND CELLULAR CARDIOLOGY
Volume 43, Issue 1, Pages 39-48Publisher
ELSEVIER SCI LTD
DOI: 10.1016/j.yjmcc.2007.04.017
Keywords
I-f current; pacemaker rate; SR Ca transients; ivabradine; ryanodine; diastolic depolarization; SA node; autonomic modulation
Categories
Funding
- NHLBI NIH HHS [HL-28958] Funding Source: Medline
Ask authors/readers for more resources
Two distinct intracellular mechanisms have been proposed to affect the firing rate of cardiac pacemaker cells: one involves modulation of the I-f current by the second messenger cAMP, and one relies upon disruption or alteration of SR Ca2+ transients during activity. Although both mechanisms are necessary for proper automaticity and autonomic rate control, the specific contribution of each to pacemaking is still debated. We investigated if the two processes can be separated based on potentially different effects on action potential characteristics during rate modulation. To identify specific I-f-mediated effects, we used the selective I-f blocker ivabradine and found that ivabradine (3 mu M) slows rate (- 16.2%) by selectively reducing (-31.9%) the steepness of early diastolic depolarization (EDD). On the other hand ryanodine (3 mu M), used to evaluate the effects of abolishment of SR Ca2+ transients, slowed rate (-31.3%) by depolarizing the take-off potential (TOP, 18. 1 %) without affecting EDD. We therefore used these two parameters to identify I-f-based or SR Ca2+ transients-based processes and analyzed the effects on action potential's characteristics of Rp-cAMPs (50 VM), a membrane permeable cAMP analogue directly activating f-channels; we found that Rp-cAMPs accelerates rate by increasing EDD (+42.3%) without modifying TOP. Finally, rate modulation was achieved by muscarinic (ACh 0.01 PM) or J beta-adrenergic (Iso 1 mu M) stimulation; in both cases, rate changes were associated with modifications of EDD (ACh, - 29.3% and Iso, +47.6%) and not of TOP. We conclude that rate-related changes in the EDD induced by autonomic agonists are mediated by If and not by processes involving SR Ca2+ transients. (C) 2007 Elsevier Inc. All rights reserved.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available