In vivo regulation of secretion of bronchiolar Clara cells in rats.

We used ultrastructural morphometric methods to study the in vivo regulation of secretion in bronchiolar Clara cells of rats. The Clara cells studied were located in airways with an internal diameter of 0.21 +/- 0.06 mm (mean +/- SD) at a transpulmonary pressure of 20 cm H2O. We found that pilocarpine caused a 50% decrease in the volume density of secretory granules of Clara cells in 60 min and that atropine blocked this effect. Isoproterenol produced a similar fall in volume density and this was blocked by propranolol. Propranolol also blocked the effect of pilocarpine. The fall in volume density of the secretory granules produced by pilocarpine and by isoproterenol occurred without any change in the surface-to-volume ratio of the granules. This indicates the change in volume density reflected a decrease in number rather than in size of the secretory granules. The observation that propranolol blocks the secretory response to pilocarpine as well as the response to isoproterenol suggests a dual in series cholinergic adrenergic regulation of secretion in bronchiolar Clara cells in rats.