Na+ influx is mediated by Na(+)-K(+)-2Cl- cotransport and Na(+)-H+ exchange in sublingual mucous acini

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Abstract
The intracellular free Na+ concentration ([Na+]i) was studied using dual-wavelength microfluorometry of the fluorescent Na+ indicator sodium-binding benzofuran isophthalate (SBFI) to determine the mechanism(s) by which muscarinic stimulation increases the Na+ content in rat sublingual mucous acini. [Na+]i was 15.5 +/- 0.7 mM in acini superfused with a Na(+)-containing medium (135 mM Na+). Application of ouabain, a Na(+)-K(+)-adenosinetriphosphatase inhibitor, resulted in an increase in [Na+]i (approximately 75% in 10 min), whereas replacement of extracellular Na+ with N-methyl-D-glucamine induced a gradual decrease in [Na+]i (approximately 55% decrease in 5 min). The recovery of [Na+]i in Na(+)-depleted acini was K+ and Cl- dependent and was inhibited by bumetanide (Bum), a specific Na(+)-K(+)-2Cl- cotransport inhibitor, and by 5-(N-methyl-N-isobutyl)amiloride (MIBA), an amiloride derivative that specifically blocks Na(+)-H+ exchange. Stimulation with a muscarinic agonist (10 microM carbachol) resulted in a dramatic increase in the [Na+]i [approximately 180%, half time (t1/2) approximately 1 min] and a net increase in Na+ content, as measured with 22Na+ (approximately 110%, t1/2 approximately 1 min). Both the initial rate of the increase in [Na+]i and the magnitude of the net increase in Na+ content were dramatically blunted by Bum and MIBA. Increasing the intracellular free Ca2+ concentration ([Ca2+]i) with ionomycin, a Ca2+ ionophore, resulted in an increase in [Na+]i. Preventing the [Ca2+]i increase by chelating cytosolic Ca2+ with bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid completely abolished the agonist-induced evaluation in [Na+]i.(ABSTRACT TRUNCATED AT 250 WORDS)