β-Modulation of Pacemaker Rate: Novel Mechanism or Novel Mechanics of an Old One?
To the Editor:
In a recent article, Vinogradova and coworkers1 show that β-adrenergic receptor (β-AR)–induced rate acceleration of sinoatrial node cells (SANCs) depends on functional subsarcolemmal ryanodine receptors (RyRs). This is supported by evidence that (1) β-AR stimulation increases RyR-associated Ca2+ transients and (2) disruption of RyR function abolishes Ca2+ transients and β-AR–induced increase of diastolic depolarization (DD) and spontaneous rate. The authors conclude that “RyR Ca2+ release flux … links β-AR stimulation to an increase in SANC firing rate” (page 78) and “β-AR modulation of … RyR-generated Ca2+ release … is a novel mechanism to explain β-AR modulation of cardiac chronotropy” (page 78).
This bold conclusion does not consider established evidence concerning mechanisms other than RyRs in β-AR control of chronotropy. In particular, this scenario does not encompass several known facts pointing to direct involvement of adenylate cyclase (AC), cAMP, and pacemaker (If) current, including the following: (1) β-stimulation affects rate by modifying both early and late DD. This appears even at low agonist concentrations2 and implies that the mechanisms involved are not directly correlated to subsarcolemmal Ca2+ release, which occurs in late DD. A steeper slope in early DD is explained by β-AR stimulation of AC, increased cAMP, and enhanced If, whose activation spans the whole DD period; (2) symmetrical to β-AR action, low doses of acetylcholine slow spontaneous rate by modifying early and late DD.2 Neither Ca2+- nor acetylcholine-activated K+ currents are sensitive to acetylcholine concentrations (10 to 30 nmol/L), which are able to inhibit If and slow rate, indicating that If mediates rate slowing by low doses of acetylcholine3,4⇓; and (3) abundant evidence demonstrates involvement of If in generating and modulating rhythmic activity in heart and brain.2,5⇓ Especially compelling is the action of specific rate-reducing agents (UL-FS49, ZD7288, S16257) that are able to induce bradycardia without side effects and are known to act by selective hyperpolarization-activated channel blockade.6–9⇓⇓⇓ Evidence for the relevance of If to pacemaking does not rule out that modification of other components flowing during DD affects diastolic slope; it does however imply that a β-AR–mediated increase of If will accelerate rate.
Vinogradova et al1 rule out the participation of If in the mechanism coupling β-AR stimulation to rate by using Cs+ as an If blocker. The observation that If block by Cs+ does not stop automatic activity has often been used to argue against If being a major contributor to DD.10 However, Cs+ block of If is voltage-dependent, with fractional block increasing at hyperpolarized voltages; at diastolic voltages, If block ranges from 23% (−65 mV) to 38% (−45 mV).2 Thus, Cs+ cannot fully dissect the If contribution; in agreement with a partial If reduction, Cs+ causes only a moderate rate reduction.10
In our view, the results of Vinogradova et al1 cannot rule out the existence of the β-AR–cAMP–If mechanism, although they do indicate that the viability of RyRs is essential for proper β-AR–induced pacemaker rate control. Our interpretation of the requirement for a functional Ca2+-handling system, however, differs from that of the authors. We believe it is compatible with the abovementioned mechanisms known to underlie sympathetic rate modulation.
Ryanodine-treated cells are depleted of Ca2+, which inevitably affects other Ca2+-dependent phenomena, including phosphorylation and dephosphorylation, involved in maintenance of conditions required for proper function of receptors and second-messenger coupling. Clearly, ryanodine treatment affects action potential parameters of SANCs, depressing DD and spontaneous rate, modifications indicating altered ionic channel contributions (Table 1 in Reference 1). These conditions, and, in fact, any condition partially or totally impairing β-AR coupling to ionic channels, would explain a reduced ability of the system to respond to β-stimulation.
The fact that L-type current increased in response to isoproterenol in ryanodine-treated cells1 does not demonstrate that β-AR signaling is normal. This could reflect an action on distal components (eg, phosphatase) or direct G protein coupling. Further, the same authors previously argued for a role of L-type channel in regulating rate.11 Yet, in this study, they show a β-AR increase in ICa,L with no rate effect in ryanodine. Does that imply that their earlier conclusion was incorrect or simply that the present experimental paradigm does not apply to normal SANC activity? We propose the latter and extend that to include all ionic currents involved in normal impulse initiation. Contrary to the authors’ interpretation of their Figure 5, we believe the absence of an isoproterenol-induced increase in inward current during ramp clamp reflects an uncoupling for the β-adrenergic cascade from their normal target channels.
- ↵Vinogradova TM, Bogdanov KY, Lakatta EG. β-Adrenergic stimulation modulates ryanodine receptor Ca2+ release during diastolic depolarization to accelerate pacemaker activity in rabbit sinoatrial nodal cells. Circ Res. 2002; 90: 73–79.
- ↵DiFrancesco D, Ducouret P, Robinson RB. Muscarinic modulation of cardiac rate at low acetylcholine concentrations. Science. 1989; 243: 669–671.
- ↵Shin KS, Rothberg BS, Yellen G. Blocker state dependence and trapping in hyperpolarization-activated cation channels: evidence for an intracellular activation gate. J Gen Physiol. 2001; 117: 91–101.
- ↵Vinogradova TM, Zhou YY, Bogdanov KY, Yang D, Kuschel M, Cheng H, Xiao RP. Sinoatrial node pacemaker activity requires Ca2+/calmodulin-dependent protein kinase II activation. Circ Res. 2000; 87: 760–767.