High Heart Rate in Pregnancy is Modulated by Augmented Expression of an Ion Channel, If, in Pacemaker Tissue
It is established that in healthy human pregnancies there are changes in cardiovascular status, including a significant (approximately 15 - 20 %) increase in resting heart rate.1 Although the underlying mechanism(s) are not well understood, it has been suggested that this pregnancy-related elevation in heart rate may be mediated by increased efferent activity of and increased sensitivity to sympathetic stimulation, concurrent with decreased sensitivity of the heart to parasympathetic activity.2 This increased sympathetic stimulation may be a reflex response to the pregnancy-related drop in total peripheral resistance and systemic vascular tone consistent with the need to maintain arterial blood pressure.3 The increase in resting heart rate is usually benign, although an increase in the incidence of ventricular arrhythmias may result. At present, both intrinsic (pacemaker activity or automaticity) and extrinsic (e.g. adrenergic tone) reflex responses are being considered as pro-arrhythmic factors; however, there is no consensus concerning the underlying ionic mechanism(s).
In this issue of Circulation, the Fiset laboratory at the Montreal Heart Institute report that in an adult mouse model, pregnancy is associated with a significant and selective up-regulation of the so-called funny current, If in the sino-atrial node or primary pacemaker region of the heart.4 The term funny was coined because this current is activated by hyperpolarization, as opposed to depolarization, of membrane potential. If is modulated (increased or decreased) by the autonomic transmitters norepinephrine and acetylcholine respectively.5 It is carried mainly by Na+ under physiological conditions.6
- Received April 12, 2013.
- Accepted April 17, 2013.