Species‐ and Preparation‐Dependence of Stretch Effects on Sino‐Atrial Node Pacemaking

Abstract

Acute dilation of the right atrium (e.g., via increased venous return) raises spontaneous beating rate (BR) of the heart in many species. Neural mechanisms contribute to this behavior in vivo, but a positive chronotropic response to stretch can also be observed in isolated right atrial tissue preparations and even at the level of single sino-atrial node (SAN) cells. The underlying mechanism has previously been reported to be compatible with stretch-activation of cation nonselective ion channels (SAC). This review reports species peculiarities in the chronotropic response of isolated SAN tissue strips to stretch: in contrast to guinea pig, murine SAN preparations respond to distension with a reduction in spontaneous BR. This differential response need not necessarily involve disparate (sub-)cellular mechanisms, as SAC activation would occur against the background of very different SAN electrophysiology in the two species. On the basis of single SAN cell action potential recordings, this review illustrates how this may give rise to potentially opposing effects on spontaneous BR. Interestingly, streptomycin (a useful SAC blocker in isolated cells) has no effect on stretch-induced chronotropy in situ, and this is interpreted as an indication of protection of SAC, in native tissue, from interaction with the drug.

Postganglionic nerve stimulation induces temporal inhibition of excitability in rabbit sinoatrial node

“…Suppression of mechanical contractions by BDM prevented activation of stretch-activated channels, which can play an important role in pacemaker activity of SAN (11,24), as well as in initiation of spontaneous vagally induced tachyarrhythmias (20). In this study we did not observe spontaneous PNS-induced tachyarrhythmias.…”

Cardiorespiratory interactions in patients with atrial flutter

“…Specifically, cyclical changes in intrathoracic pressure associated with respiratory phases alter systemic venous return to the heart, and thus right atrial preload, in synchrony with respiratory cycle. Mechanical respiratory effects on cardiac volume are present during normal sinus rhythm and might affect heart rate by stretch of the sinus node pacemaker (12,27), thus potentially contributing to RSA. Indeed, a mechanical modulation of heart rate has been suggested to determine RSA in heart transplant recipients (6,7) and to contribute significantly to RSA in conditions associated with reduced vagal tone, as observed in patients with mild heart failure (17) and in healthy subjects during exercise (7,9).…”

Effects of extracellular changes on spontaneous heart rate of normoxia-and anoxia-acclimated turtles (Trachemys scripta)

“…atria from normoxia-acclimated turtles were placed in simulated normoxic saline whereas atria from anoxiaacclimated turtles were placed in simulated anoxic saline; Table·1). The length of the mounted atrial preparation was adjusted with a micrometer screw to produce ~90% of maximal contraction force to limit inter-preparation variation due to the chronotropic effects of cardiac stretch (Cooper and Kohl, 2005). The preparation was allowed 20-25·min to stabilize to allow for washout of any inherent adrenergic agents.…”