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

Abstract Membrane depolarization of airway smooth muscle (ASM) opens L-type voltage dependent Ca 2+ channels (L-VDCC) allowing Ca 2+ entrance to produce contraction. In Ca 2+ free conditions Na + permeates through L-VDCC in excitable and non-excitable cells and this phenomenon is annulled at µM Ca 2+ concentrations. Membrane depolarization also induces activation of Gq proteins and sarcoplasmic reticulum Ca 2+ release. In bovine ASM, KCl induced a transient contraction sensitive to nifedipine in Ca 2+ free medium, indicating an additional mechanism to the SR-Ca 2+ release. It is possible that Na + could permeate through L-VDCC in bovine ASM. KCl induced a transient contraction in Ca 2+ free medium with a fast intracellular Ca 2+ 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 Ca 2+ induced a [Na + ]i increment, enhanced by Bay K8644 and inhibited with D-600. In Ca 2+ free medium, KCl increased [Na + ]i. Ba 2+ currents corresponding to L-VDCC were observed in myocytes and Na + permeated in the presence and absence of Ca 2+ . SBFI-loaded myocytes in Na + and Ca 2+ 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 Ca V 1.2 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.