Evidence against the swelling hypothesis for initiation of exocytosis in terminals of chromaffin cell processes.

Exocytosis of transmitter-containing granules was directly visualized in neurite terminals of cultured chromaffin cells under a video-enhanced contrast microscope. The granule diameter did not change immediately before their exocytotic responses. Large granules responded as early as small ones. These findings are inconsistent with the swelling hypothesis for initiation of exocytosis.