Vagal control of mucociliary clearance in murine lungs: a study in a chronic preparation

Abstract

We conducted several experiments that focused on the effect of vagal control on mucociliary clearance (MCC) in murine lungs. We hypothesized that loss of vagal control by chronic denervation (i.e. vagotomy) would reduce both basal MCC and the increase in MCC typically observed upon stimulation of capsaicin sensitive C-fibers. Vagotomy was performed on the right side of C57BL/6 mice and MCC was measured 5 days later. Mucociliary clearance was measured by gamma scintigraphy after oropharyngeal aspiration of the radioisotope (99m)technetium and was expressed as the amount of radioactivity removed from the right lung 6h later. Baseline MCC was unaffected by vagotomy, averaging 6.5+/-4.9% and 6.8+/-5.8%, in 6 vagotomized and 6 non-vagotomized mice (controls), respectively. Mucociliary clearance increased significantly to 12.7+/-5.9% in 9 non-vagotomized mice treated with 1.6 x 10(-9) M capsaicin, a vagally-mediated, nociceptor stimulus (p=0.041). Capsaicin was admixed with (99m)technetium and administered by oropharyngeal aspiration. In contrast, MCC was unchanged from control values in 9 vagotomized, capsaicin-treated animals, averaging 6.0+/-5.5% (p=0.024). These findings suggest that loss of vagal control through denervation does not affect basal MCC in C57BL/6 mice, but does appear to reduce the capacity of mice to respond to nociceptor agents that stimulate MCC. These data could have implications for patients whose lungs are denervated due to lung transplantation, since they may be at risk for an inadequate MCC response to inhaled irritants and inflammatory mediators, which are also nociceptor stimuli.

Figure 1

Line drawing showing the border of the right lung of a representative mouse from our study in dark gray, the central lung region (cross-hatched in light gray) and the lung periphery in dark gray. The number of counts and pixels in the central and peripheral regions were quantified and a count density established (counts divided by pixels). A central to peripheral (C:P) ratio was calculated to quantify the distribution of radioactivity in each mouse lung. The C:P ratio was calculated by dividing the central count density by the peripheral count density.

Figure 2

Perspective view (i.e. rendering view) of one of the C57BL/6 mice in this study, showing the radioisotopic image superimposed on the CT scan. This image was obtained approximately 6 hours after administration of the radioisotope and provides a view looking down at the animal. The two lungs and stomach appear in color. Radioactivity in the lungs resulted from oropharyngeal aspiration of ^99mtechnetium-sulfur colloid. Radioactivity in the stomach resulted from removal of the radioisotope by MCC. The radioactivity was then swallowed, appearing in the stomach.

Figure 3

Representative gamma camera images of lungs in an untreated C57BL/6 mouse at time 0 and at 6–6.5 hours after oropharyngeal aspiration of the radioaerosol. The right lung is shown at the bottom of each panel. Qualitatively, radioactivity retained in the lungs of these animals diminished over time due to a combination of radioactive decay and mucociliary clearance. The “hot spot” below the left lung (found at the top of the panel) at 6 hours post aspiration shows activity that was removed from the lungs by mucociliary clearance, swallowed and accumulated over time in the stomach.

Figure 4

Mean MCC (±SD) from the right lung between 6–6.5 hours in 6 C57BL/6 non-vagotomized control mice (white bars), 9 non-vagotomized C57BL/6 mice treated with capsaicin aerosol (striped bars), 6 vagotomized C57BL/6 mice (black bars) and 9 vagotomized C57BL/6 mice treated with capsaicin aerosol (checked bars). Between 6–6.5 hours, there was no significant difference between MCC in the control animals (white bar), compared to animals that had undergone vagotomy (black bar). However, MCC was statistically significantly greater in intact animals treated with capsaicin (striped bar), compared to controls (white bar) and animals that underwent vagotomy prior to treatment with capsaicin (checked bar).