Differential effects of ivabradine and ryanodine on pacemaker activity in canine sinus node and purkinje fibers

Abstract

Introduction: It is generally accepted that at least 2 major mechanisms contribute to sinus node (SN) pacemaking: a membrane voltage (mainly I(f) ) clock and a calcium (Ca) clock (localized submembrane sarcoplasmic reticulum Ca(2+) release during late diastolic depolarization). The aim of this study was to compare the contributions of each mechanism to pacemaker activity in SN and Purkinje fibers (PFs) exhibiting normal or abnormal automaticity.

Methods and results: Conventional microelectrodes were used to record action potentials in isolated spontaneously beating canine SN and free running PF in control and in the presence of 0.1 μM isoproterenol. Ryanodine (0.1-3 μM) and ivabradine (3 μM) were used to inhibit sarcoplasmic reticulum Ca(2+) release or I(f), respectively. To induce automaticity at low membrane potentials, PFs were superfused with BaCl(2). In SN, ivabradine reduced the rate whereas ryanodine had no effect. Isoproterenol significantly accelerated automatic rate, which was decreased by ivabradine and ryanodine. In normally polarized PFs, ryanodine had no effects on the automatic rate in the absence or presence of isoproterenol, whereas ivabradine inhibited both control and isoproterenol-accelerated automaticity. In PF depolarized with BaCl(2), ivabradine decreased BaCl(2) -induced automatic rate while ryanodine had no effect.

Conclusion: In canine SN, I(f) contributes to both basal automaticity and β-adrenergic-induced rate acceleration while the ryanodine-inhibited Ca clock appears more involved in β-adrenergic regulation of pacemaker rate. In PF, normal automaticity depends mainly on I(f). Inhibition of basal potassium conductance results in high automatic rates at depolarized membrane potentials with SN-like responses to inhibition of membrane and Ca clocks.

Figure 1

Effects of ivabradine (Iva) on automaticity in SN and PF. A and B, representative transmembrane potentials recorded from spontaneously beating SN preparation and free running PF in control (upper traces) and in the presence of 3 µmol/L ivabradine (lower traces). C through F, respective summary data showing ivabradine effects on rate and maximum diastolic potential (MDP) in SN (C and E) and in PF (D and F). N=10 for SN, n=6 for PF. *P<0.05 versus respective control.

Figure 2

Effects of ryanodine (Rya) on automaticity in SN and PF. Representative transmembrane potentials recorded from SN preparation (A) and PF (B) in control and in the presence of 0.1 and 3 µmol/L ryanodine. C through F, summary data showing the effects of 0.1 and 3 µmol/L ryanodine on the rate and maximum diastolic potential (MDP) in SN (C and E) and 0.1 µmol/L ryanodine in PF (D and F). N=6 for SN and PF.

Figure 3

Effects of ivabradine (Iva) on automaticity in the presence of isoproterenol (Iso) in SN and PF. A and B, representative transmembrane potentials recorded from spontaneously beating SN preparation and free running PF in control (upper traces), in the presence of 0.1 µmol/L isoproterenol (middle traces) and in the presence of isoproterenol and 3 µmol/L ivabradine (lower traces). C through F, respective summary data showing the rate and maximum diastolic potential (MDP) in SN (C and E) and in PF (D and F). N=6 for SN and for PF. *P<0.05 versus respective control. ⁺P<0.5 versus respective Iso.

Figure 4

Effects of ryanodine (Rya) on automaticity in the presence of isoproterenol (Iso) in SN and PF. A and B, representative transmembrane potentials recorded from spontaneously beating SN preparation and free running PF in control, in the presence of 0.1 µmol/L isoproterenol, in the presence of isoproterenol plus 0.1 µmol/L ryanodine and in the presence of isoproterenol plus 3 µmol/L ryanodine. C through F, respective summary data showing the rate and maximum diastolic potential (MDP) in SN (C and E) and in PF (D and F). N=6 for SN and n=8 for PF. *P<0.05 versus respective control. ⁺P<0.5 versus respective Iso.

Figure 5

Effects of ivabradine (Iva) and ryanodine (Rya) on barium-induced automaticity at low membrane potentials in Purkinje fibers. A and B, representative transmembrane potentials in control, in the presence of 0.4 mmol/L BaCl₂, and after adding Iva (3 µmol/L), or Rya (3 µmol/L). C and D, summary data showing the rate and maximum diastolic potential (MDP). N=5 for Iva and Rya. *P<0.05 versus respective control. ⁺P<0.05 versus respective BaCl₂.