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Cyclopiazonic acid activates a Ca²⁺-permeable, nonselective cation conductance in porcine and bovine tracheal smooth muscle

Firestone Institute for Respiratory Health, St. Joseph's Healthcare, Department of Medicine, McMaster University, Hamilton, Ontario, Canada

Submitted 1 March 2005 ; accepted in final form 7 July 2005

Capacitative Ca²⁺ entry has been examined in several tissues and, in some, appears to be mediated by nonselective cation channels collectively referred to as "store-operated" cation channels; however, relatively little is known about the electrophysiological properties of these channels in airway smooth muscle. Consequently we examined the electrophysiological characteristics and changes in intracellular Ca²⁺ concentration associated with a cyclopiazonic acid (CPA)-evoked current in porcine and bovine airway smooth muscle using patch-clamp and Ca²⁺-fluorescence techniques. In bovine tracheal myocytes, CPA induced an elevation of intracellular Ca²⁺ that was dependent on extracellular Ca²⁺ and was insensitive to nifedipine (an L-type voltage-gated Ca²⁺ channel inhibitor). Using patch-clamp techniques and conditions that block both K⁺ and Cl^– currents, we found that CPA rapidly activated a membrane conductance (I_CPA) in porcine and bovine tracheal myocytes that exhibits a linear current-voltage relationship with a reversal potential around 0 mV. Replacement of extracellular Na⁺ resulted in a marked reduction of I_CPA at physiological membrane potentials (i.e., –60 mV) that was accompanied by a shift in the reversal potential for I_CPA toward more negative membrane potentials. In addition, I_CPA was markedly inhibited by 10 µM Gd³⁺ and La³⁺ but was largely insensitive to 1 µM nifedipine. We conclude that CPA induces capacitative Ca²⁺ entry in porcine and bovine tracheal smooth muscle via a Gd³⁺- and La³⁺-sensitive, nonselective cation conductance.

sarcoplasmic reticulum Ca²⁺-ATPase; nonselective cation channel; voltage-gated Ca²⁺ channels; capacitative Ca²⁺ entry

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