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Ca²⁺ Sparks in Rabbit Ventricular Myocytes Evoked by Action Potentials

Involvement of Clusters of L-Type Ca²⁺ Channels

From the Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah, Salt Lake City, Utah.

Correspondence to Masashi Inoue, CVRTI, University of Utah, 95 South 2000 East Back, Salt Lake City, UT 84112-5000. 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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