Relaxin increases heart rate by modulating calcium current in cardiac pacemaker cells

Abstract

Relaxin (RLX), a reproductive hormone of the insulin family, increases heart rate in experimental animals. The cellular and ionic mechanisms responsible for this positive chronotropic effect remain unknown. We have investigated the actions of RLX on the action potential and underlying transmembrane ionic currents in single sinoatrial node cells of the rabbit heart under whole-cell voltage-clamp conditions, using both nystatin- perforated-patch and membrane-ruptured techniques. In this preparation RLX (0.8 to 80 nmol/L) caused reversible increases in the rate of spontaneous action potentials and a dose-dependent increase in the L-type calcium current, I(Ca(L)). The best-fit Langmuir relation for the augmentation of I(Ca(L)) yielded a threshold concentration of 1 nmol/L and a K(D) of 14 nmol/L. These effects of RLX appear to be mediated by increases in intracellular cyclic AMP (cAMP), since RLX was without effect after application of (1) the β-adrenergic agonist isoprenaline (1 μmol/L) or (2) superfusion of the intracellular second messenger cAMP (100 μmol/L) or 8- Br-cAMP (100 to 200 μmol/L). Internal dialysis with an inhibitor of cAMP- dependent protein kinase (PKI, 7 μmol/L) abolished the effects of RLX. These results provide the first electrophysiological evidence that RLX modulates heart rate and contractility by increasing I(Ca(L)) and suggest that the biochemical mechanism involves the formation of cAMP and activation of cAMP- dependent protein kinase.