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1 Physiology & Biophysics, University of Washington, Seattle, Washington, United States
2 Pharmacology, University of Washington, Seattle, Washington, United States
3 Pharmacology & Toxicology, University of Innsbruck, Innsbruck, Austria
* To whom correspondence should be addressed. E-mail: santana{at}u.washington.edu.
Ca2+ sparklets are local elevations in intracellular Ca2+ produced by the opening of a single or a cluster of L-type Ca2+ channels. In arterial myocytes, Ca2+ sparklets regulate local and global intracellular Ca2+. At present, the molecular identity of the L-type Ca2+ channels underlying Ca2+ sparklets in these cells is undetermined. Here, we tested the hypotheses that Cav1.3 channels can produce Ca2+ sparklets and that Cav1.2 and/or Cav1.3 channels are responsible for Ca2+ sparklets in mouse arterial myocytes. First, we investigated the functional properties of single Cav1.3 channels in tsA-201 cells. With 110 mM Ba2+ as the charge carrier, Cav1.3 channels had a conductance of 20 pS. This value is similar to that of Cav1.2 and native L-type Ca2+ channels. As previously shown for Cav1.2 channels, Cav1.3 channels can operate in two gating modes characterized by short and long open times. Expressed Cav1.3 channels also produced Ca2+ sparklets. Cav1.3 sparklets had properties similar to those produced by Cav1.2 and native L-type channels including quantal amplitude, dihydropyridine sensitivity, bimodal gating, and dual event duration times. However, the voltage dependencies of conductance and steady-state inactivation of the Ca2+ current (ICa) in arterial myocytes were similar to those recorded from cells expressing Cav1.2, but not Cav1.3 channels. Furthermore, nifedipine (10 µM) eliminated Ca2+ sparklets in wild type myocytes, but not in myocytes expressing dihydropyridine-insensitive Cav1.2 channels. Accordingly, Cav1.3 transcript and protein were not detected in isolated arterial myocytes. We conclude that although Cav1.3 channels can produce Ca2+ sparklets, Cav1.2 channels underlie ICa, Ca2+ sparklets, and hence dihydropyridine-sensitive Ca2+ influx in mouse arterial myocytes.
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