Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, PC257

Poster Communications

Can stimulation frequency modulate Ca2+ handling of sheep atrial myocytes?

D. K. Wrigley1, J. D. Clarke1, M. A. Richards1, A. W. Trafford1, K. M. Dibb1

1. Unit of Cardiac Physiology, University of Manchester, Manchester, United Kingdom.

The human atrium is the site of the most common cardiac arrhythmia observed in clinical practice, atrial fibrillation (AF). It is hypothesised that abnormal Calcium (Ca2+) handling in the atrial myocardium can lead to the pathogenesis of AF. At present no data exists on the effect of pathological stimulation frequencies on atrial Ca2+ handling. We sought to investigate Ca2+ handling in young adult sheep atrial myocytes, across a range of stimulation frequencies. Atrial myocytes were isolated from the left auricle of healthy young adult sheep. Experimental protocols were carried out under perforated patch, voltage clamp conditions at 37°C. Intracellular Ca2+ ([Ca2+]i) was measured using Fluo-5F at stimulation frequencies of 1,3, and 5Hz. An increase in the rate of stimulation resulted in a significant decrease in Ca2+ transient amplitude (264 ± 74 nmol/L, 194 ± 82 nmol/L, 106 ± 51 nmol/L at 1, 3, and 5Hz respectively; p<0.005). This decrease in Ca2+ transient amplitude was accompanied by a rise in diastolic Ca2+ levels over the same frequency range (115 ± 32 nmol/L, 154 ± 27nmol/L, 198 ± 27nmol/L at 1, 3, and 5Hz, respectively; p<0.001). To elucidate the mechanisms responsible for the observed decrease in Ca2+ transient amplitude, we examined the two main determinants of this, sarcoplasmic reticulum (SR) Ca2+ content, and L-type Ca2+ current (ICa-L). SR Ca2+ content demonstrated a trend to increase with stimulation frequency (61.7 ± 10.8 μmol/L-1, 84.9 ± 20.9 μmol/L-1, 100.0 ± 37.4 μmol/L-1, at 1, 3, and 5Hz respectively; p=0.054). A decrease in ICa-L was observed at 5Hz in comparison to 1Hz (0.58 ± 0.22 vs. 1.33 ± 0.35 pA/pF, respectively; p <0.005). Similar qualitative results for Ca2+ transient amplitude, ICa-L, and SR Ca2+ content were observed in old sheep. At pathological rates that mimic those observed in human AF, significant changes in Ca2+ handling during cardiac excitation contraction coupling (ECC) of young adult sheep are observed. These changes may be the cause of the arrhythmogenesis that is observed in human AF. Further investigation under current clamp conditions is required to determine if similar alterations in atrial Ca2+ handling are observed.

Where applicable, experiments conform with Society ethical requirements