Na+ permeates through L-type Ca2+ channel in bovine airway smooth muscle

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Membrane depolarization of airway smooth muscle (ASM) opens L-type voltage dependent Ca2+ channels (L-VDCC) allowing Ca2+ entrance to produce contraction. In Ca2+ free conditions Na+ permeates through L-VDCC in excitable and non-excitable cells and this phenomenon is annulled at μM Ca2+ concentrations. Membrane depolarization also induces activation of Gq proteins and sarcoplasmic reticulum Ca2+ release. In bovine ASM, KCl induced a transient contraction sensitive to nifedipine in Ca2+free medium, indicating an additional mechanism to the SR-Ca2+ release. It is possible that Na+ could permeate through L-VDCC in bovine ASM. KCl induced a transient contraction in Ca2+ free medium with a fast intracellular Ca2+ increment, reduced by TMB-8. This contraction was abolished by caffeine and CPA, diminished with nifedipine and augmented by Bay K8644. Increasing extracellular Na+ concentration in tracheal myocytes, proportionally augmented the SBFI fluorescence ratio, suggesting an increment in the intracellular Na+ concentration ([Na+]i). 50 mM Na+ with and without Ca2+ induced a [Na+]i increment, enhanced by Bay K8644 and inhibited with D-600. In Ca2+ free medium, KCl increased [Na+]i. Ba2+ currents corresponding to L-VDCC were observed in myocytes and Na+ permeated in the presence and absence of Ca2+. SBFI-loaded myocytes in Na+ and Ca2+ containing Krebs stimulated with carbachol showed a Na+ increment with a plateau. D-600 and 2-APB almost abolished the carbachol-induced Na+ increment. RT-PCR demonstrated that CaV1.2 is the only L-VDCC subunit present in ASM. Conclusion: under physiological conditions, Na+ permeates through L-VDCC in bovine ASM, probably contributing to sustain membrane depolarization during agonist stimulation.

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