Na(+) permeates through L-type Ca(2+) channel in bovine airway smooth muscle.
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). 50mM 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 CaV1.2 is the only L-VDCC subunit present in ASM.


CONCLUSIONS
under physiological conditions, Na(+) permeates through L-VDCC in bovine ASM, probably contributing to sustain membrane depolarization during agonist stimulation.
Departamento de Investigación en Hiperreactividad Bronquial, Instituto Nacional de Enfermedades Respiratorias, Ciudad de México, Mexico.
Bettina
B
Sommer
Departamento de Farmacología, Facultad de Medicina, Universidad Nacional Autónoma de México, Ciudad de México, Mexico.
Edgar
E
Flores-Soto
Departamento de Farmacología, Facultad de Medicina, Universidad Nacional Autónoma de México, Ciudad de México, Mexico.
Jorge
J
Reyes-García
Departamento de Embriología, Facultad de Medicina, Universidad Nacional Autónoma de México, Ciudad de México, Mexico.
Verónica
V
Díaz-Hernández
Departamento de Investigación en Hiperreactividad Bronquial, Instituto Nacional de Enfermedades Respiratorias, Ciudad de México, Mexico.
Verónica
V
Carbajal
Departamento de Farmacología, Facultad de Medicina, Universidad Nacional Autónoma de México, Ciudad de México, Mexico. Electronic address: lmmr@unam.mx.
Luis M
LM
Montaño
D000818:Animals; D020746:Calcium Channels, L-Type / Q000235:genetics / Q000378:metabolism*; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D005786:Gene Expression Regulation; D009130:Muscle, Smooth / Q000166:cytology / Q000378:metabolism*; D010539:Permeability; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D014132:Trachea / Q000378:metabolism*
D020746:Calcium Channels, L-Type; D012964:Sodium
Airway smooth muscle; Contraction; L-type Ca(2+) channels; Na(+) permeability; Sarcoplasmic reticulum Ca(2+) release; Subunit Ca(V)1.2