The beat-to-beat decay of cardiac contractility from highly potentiated levels is bi-exponential.
In order to determine the mode of beat-to-beat decay of contractility from very high levels, we studied the beat-by-beat decay of cardiac contractility following potentiation. Such decay curves are normally analysed using a mono-exponential decay function, which assumes that a fixed fraction of activator calcium ions is recirculated from one beat to the next. We postulated that there might be deviations from such a mono-exponential expression at high levels of contractility. In single sucrose-gap voltage clamp experiments of isolated ferret papillary muscle, we obtained very high contractility by potentiation due to prolonged depolarisations. We found a bi-exponential decay in 9 of 11 muscles studied, in which the initial decay is much faster than the subsequent slower decay, as judged by residual variance of least-squares exponential fitting and by analysis of covariance using a linear equation (force of beat versus force of previous beat), p = 0.0089. In the slower decay period (physiological range), the decay was identical to that following post-extrasystolic potentiation in the same muscles studied with conventional stimulation.
Noble
Mark I M
MI
Department of Cardiology, University of Aberdeen, UK. mimnoble@freenetname.co.uk
Arlock
Per
P
Wohlfart
Bjorn
B
Drake-Holland
Angela J
AJ
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D001696:Biomechanical Phenomena; D020013:Calcium Signaling / Q000502:physiology; D005289:Ferrets; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000502:physiology; D009200:Myocardial Contraction / Q000502:physiology*; D010210:Papillary Muscles / Q000502:physiology*; D012815:Signal Processing, Computer-Assisted; D016278:Ventricular Function, Right / Q000502:physiology*