Published online before print February 27, 2003, doi: 10.1161/01.RES.0000064175.70693.EC
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Submitted on November 11, 2002
Revised on February 14, 2003
Accepted on February 14, 2003
Ca2+ Sparks in Rabbit Ventricular Myocytes Evoked by Action Potentials. Involvement of Clusters of L-type Ca2+ Channels
Masashi Inoue * and John H.B. Bridge
From the Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah, Salt Lake City, Utah.
* To whom correspondence should be addressed. E-mail: inoue{at}cvrti.utah.edu.
It is not clear how many L-type Ca2+ channels (LCCs) are required to ensure that a Ca2+ spark is triggered during a normal mammalian action potential (AP). We investigated this in rabbit ventricular myocytes by examining both the properties of sparks evoked by APs and the activity of LCCs. We measured Ca2+ sparks evoked by repeated APs with pipettes containing 2 mmol/L EGTA and single LCC activity in cell-attached patches depolarized to +50 mV using pipettes containing 110 mmol/L Ba2+. With 2 mmol/L Ca2+ in the external solution, we observed sparks at the beginning of every evoked AP at numerous locations. Each spark was observed repeatedly at a fixed location and began during a limited interval after the AP peak. These sparks occurred with a probability of approximately unity. However, the chance that an LCC does not open during the interval when a spark is triggered is quite high (
0.13). Therefore, because single channels open with a probability significantly lower than 1, more than one LCC must be available to ensure that sparks are triggered with a probability of approximately unity. We conclude that it is likely that a cluster of LCCs is involved in gating a cluster of ryanodine receptors at the beginning of an AP.
Key words: Ca2+ channels • Ca2+ sparks • excitation-contraction coupling • Ca2+ triggers • trigger clusters
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