Autonomic modulation of sinoatrial node: Role of pacemaker current and calcium sensitive adenylyl cyclase isoforms

Abstract

This article reviews work over the past three decades that is related to the contribution of the pacemaker current, I_f, to basal and autonomically regulated spontaneous rate in the sinoatrial node. It also addresses how the actions of the pacemaker current relate to those of Ca homeostasis with respect to basal and autonomically regulated rhythm. In this regard, it explores the relative contributions of Ca-sensitive and Ca-insensitive isoforms of adenylyl cyclase to sinoatrial node automaticity. The latter studies include previously unpublished work making use of mice in which both the type 1 and type 8 Ca-sensitive adenylyl cyclase isoforms were knocked out. These studies indicate that the pacemaker current and the L-type Ca current are distinctly influenced by Ca-sensitive and insensitive adenylyl cyclase isoforms.

Keywords: Adenylyl cyclase; Autonomic responsiveness; L-type Ca current; Pacemaker current; Sinoatrial node; Spontaneous rate.

Fig 1:

ACh dose-response relations for I_f inhibition (mV, ▲), I_K,Ach activation normalized to cell capacity (pA/pF, ●) and the percent slowing of pacemaker rate (beats/min, X), all normalized to the same amplitude. The arrows indicate half-maximal concentrations (0.013 and 0.26 μM), respectively, for the I_f and I_K,Ach curves. Means and SEMs are displayed for the first 2 curves and only mean values for the third. From DiFrancesco et al., 1989, with permission.

Fig 2:

Effect of ACh and ISO on diastolic depolarization rate and currents in SAN cells. A and B, examples of the effect of ACh during recording of spontaneous activity in single cells, and fractional corresponding rate decrease plotted against ACh concentration. The best fit of the experimental data yielded an EC50 value of 0.024 μM. Dose-response curves for I_f and I_Ca,L are shown for comparison. C and D, similar data for ISO, with a yielded EC50 for rate of 0.0268 μM, and the associated dose-response curves of I_f and I_Ca,L. From Zaza et al., 1997, with permission.

Fig 3:

Effect of ISO and cAMP analogues on rate and I_f in the presence and absence of RY. LEFT. Summary of effects on rate of ISO (1 μM) and cAMP analogues (CTP-cAMP, 300 μM; Rp-cAMPs, 50 μM) in control cells (−Ry) and in cells exposed to RY (3 μM, +Ry). RIGHT. The effect of ISO and cAMP analogues on I_f in control cells (left traces) and cells exposed to RY (right traces) in the absence and presence of ISO (a), CPT-cAMP (b) and Rp-cAMPS (c). d: bar graphs of shift of I_f activation curve (in mV) by ISO, CPT-cAMP and Rp-cAMPs in control and after RY. * indicates significant difference between +Ry and corresponding −Ry data. From Bucchi et al., 2003, with permission.

Fig 4:

Summary of changes of action potential (AP) parameters caused by rate-modifying agents. All values are expressed as a fraction of the corresponding control value in the absence of drug. The bar graphs describe the mean steady-state amplitudes (normalized to control) of rate, end diastolic depolarization (EDD) and take-off potential (TOP) following application of IVA 3 μM, RY 3 μM, Rp-cAMPs 50 μM, ACh 0.01 μM and ISO 1 μM. Empty bars, significantly different from 1; grey bars, non-significantly different from 1. From Bucchi et al., 2007, with permission.

Fig 5:

Midpoint of activation of expressed mHCN2 (I_f) I-V relation and its responsiveness to ISO in the absence/presence of BAPTA-AM pre-treatment. Two Way ANOVA was used for group comparison where ISO midpoint values were compared in relation to the presence/absence of BAPTA-AM in AC1 or AC6 groups. *p<0.05 versus corresponding no ISO. From Kryukova et al., 2012, with permission.

Fig 6:

The effect of ISO on SAN spontaneous rate in the absence and presence of RY. TOP: in the absence of RY, both double knockout (DKO) and control (i.e. strain matched wild type; WT) tissues respond comparably to 1 minute exposure to1 μM ISO. ISO was tested following pre-treatment for 20 minutes with 100 nM TTX. Bottom: In contrast, in the presence of RY, WT control no longer responds to ISO, but spontaneous rate of DKO increases with ISO. N=2–6. * p<0.05 relative to WT.

Fig 7:

The pacemaker current (I_f) activation relation in double knockout (DKO) SAN cells and strain matched control cells (WT). TOP: Under basal conditions there is a significant negative shift of the activation relation of pacemaker current in DKO compared to WT n=9–12; p<0.005 BOTTOM: When the midpoint of the activation relations are plotted under control conditions (CTRL) and in the presence of 1 μ,M isoproterenol (ISO), there is a significant shift in both WT and DKO. n=3–4; * p<0.05.

Fig 8:

The L-type Ca current (I_Ca,L) I-V relation in double knockout (DKO) SAN cells and strain matched control cells (WT). LEFT: Under basal conditions (Control), The current density of DKO is significantly greater than that of WT at voltages near the peak of the I-V relation (*). RIGHT: In the presence of 1 μM ISO both I-V relations exhibit an increase in current density and do not differ significantly from each other. n=3–4; * p<0.05.