Novel Na+/Ca2+ Exchanger Inhibitor ORM-10962 Supports Coupled Function of Funny-Current and Na+/Ca2+ Exchanger in Pacemaking of Rabbit Sinus Node Tissue

Zsófia Kohajda; Noémi Tóth; Jozefina Szlovák; Axel Loewe; Gergő Bitay; Péter Gazdag; János Prorok; Norbert Jost; Jouko Levijoki; Piero Pollesello; Julius Gy Papp; András Varró; Norbert Nagy

doi:10.3389/fphar.2019.01632

Novel Na⁺/Ca²⁺ Exchanger Inhibitor ORM-10962 Supports Coupled Function of Funny-Current and Na⁺/Ca²⁺ Exchanger in Pacemaking of Rabbit Sinus Node Tissue

Front Pharmacol. 2020 Jan 29:10:1632. doi: 10.3389/fphar.2019.01632. eCollection 2019.

Authors

Zsófia Kohajda^{1

2}, Noémi Tóth², Jozefina Szlovák², Axel Loewe³, Gergő Bitay², Péter Gazdag², János Prorok², Norbert Jost^{1

2}, Jouko Levijoki⁴, Piero Pollesello⁴, Julius Gy Papp^{1

2}, András Varró^{1

2}, Norbert Nagy^{1

2}

Affiliations

¹ MTA-SZTE Research Group of Cardiovascular Pharmacology, Hungarian Academy of Sciences, Szeged, Hungary.
² Department of Pharmacology and Pharmacotherapy, Faculty of Medicine, University of Szeged, Szeged, Hungary.
³ Institute of Biomedical Engineering, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany.
⁴ Orion Pharma, Espoo, Finland.

Abstract

Background and purpose: The exact mechanism of spontaneous pacemaking is not fully understood. Recent results suggest tight cooperation between intracellular Ca²⁺ handling and sarcolemmal ion channels. An important player of this crosstalk is the Na⁺/Ca²⁺ exchanger (NCX), however, direct pharmacological evidence was unavailable so far because of the lack of a selective inhibitor. We investigated the role of the NCX current in pacemaking and analyzed the functional consequences of the I_f-NCX coupling by applying the novel selective NCX inhibitor ORM-10962 on the sinus node (SAN).

Experimental approach: Currents were measured by patch-clamp, Ca²⁺-transients were monitored by fluorescent optical method in rabbit SAN cells. Action potentials (AP) were recorded from rabbit SAN tissue preparations. Mechanistic computational data were obtained using the Yaniv et al. SAN model.

Key results: ORM-10962 (ORM) marginally reduced the SAN pacemaking cycle length with a marked increase in the diastolic Ca²⁺ level as well as the transient amplitude. The bradycardic effect of NCX inhibition was augmented when the funny-current (I_f) was previously inhibited and vice versa, the effect of I_f was augmented when the Ca²⁺ handling was suppressed.

Conclusion and implications: We confirmed the contribution of the NCX current to cardiac pacemaking using a novel NCX inhibitor. Our experimental and modeling data support a close cooperation between I_f and NCX providing an important functional consequence: these currents together establish a strong depolarization capacity providing important safety factor for stable pacemaking. Thus, after individual inhibition of I_f or NCX, excessive bradycardia or instability cannot be expected because each of these currents may compensate for the reduction of the other providing safe and rhythmic SAN pacemaking.

Keywords: Na+/Ca2+ exchanger; ORM-10962; funny-current; pacemaking; sinus-node.