A system comprising an implantable electrical cardiac signal sensing circuit, an implantable sinoatrial cardiac action potential detector circuit, and an implantable electrical stimulation circuit in electrical communication with the electrical cardiac signal sensing circuit and the sinoatrial cardiac action potential detector circuit. The electrical cardiac signal sensing circuit is configured to receive one or more intrinsic heart signals from one or more respective electrodes configured for placement in a vicinity of a sinoatrial node of a subject. The implantable electrical stimulation circuit is configured to initiate delivery of at least one inhibitory electrical stimulation pulse in a vicinity of the sinoatrial node in a timed relationship to a sensed sinoatrial cardiac action potential. Other systems and methods are disclosed.