In order to study specific anatomical relationships among organelles involved in the movement and secretion of mucin granules by airway epithelium, a three-dimensional replica of a single goblet cell from porcine trachea was constructed by means of transmission electron microscopy of serially prepared ultrathin sections. Many features of the goblet cell that could not be deduced from single planes of section were revealed. The cell is columnar in configuration. The cytoplasm contains not one, but several discrete clusters of mucin granules, each associated with a separate Golgi apparatus. Microtubules and microfilaments often are observed in close association with both mucin granules and coiled filamentous mitochondria, suggesting a role in the intracellular movement of these organelles. These morphological observations are in agreement with previous functional studies of airway mucin secretion, and elucidate further the cellular biology of airway mucosal goblet cells and the secretory process.

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Stimulation of intact platelets by ADP results in a shape change followed by aggregation in the presence of fibrinogen. ADP was found to induce a shape change in chymotrypsin-treated platelets that was similar in extent and initial velocity to that of intact (untreated) platelets. Scanning-electron microscopy verified an ADP-induced shape change in chymotrypsin-treated platelets. This shape change could be completely blocked by stimulators of platelet adenylate cyclase (forskolin, prostaglandin E1, and prostacyclin). On the other hand, the aggregation of chymotrypsin-treated platelets by fibrinogen was not dependent on the presence of ADP and could not be blocked by forskolin, prostaglandin E1, or prostacyclin, even though the levels of cyclic AMP (cAMP) formed in chymotrypsin-treated platelets were comparable to levels that completely inhibited the ADP-induced aggregation of intact platelets. This lack of inhibition of platelet aggregation was not due to degradation of the adenylate cyclase or prostaglandin receptors, since chymotrypsin-treated platelets were found to have a functional adenylate cyclase system that could be stimulated by forskolin, prostaglandin E1, and prostacyclin and inhibited by ADP and epinephrine, similar to that of intact platelets. These results provide direct evidence that cAMP does not interact with fibrinogen binding sites once they have become permanently exposed on the surface of platelets. Pretreatment of platelets with chymotrypsin therefore appears to be a useful tool that allows for the dissociation of platelet shape change from aggregation, without inhibiting either response.

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