ABSTRACT In response to physiological stimuli, neuroendocrine cells secrete neurotransmitters through a Ca2 -dependent
fusion of secretory granules with the plasma membrane. We studied insertion of granules in bovine chromaffin cells using
capacitance as a measure of plasma membrane area and fluorescence of a membrane marker FM1-43 as a measure of
exocytosis. Intracellular dialysis with [Ca2 ] (1.5–100 M) evoked massive exocytosis that was sufficient to double plasma
membrane area but did not swell cells. In principle, in the absence of endocytosis, the addition of granule membrane would
be anticipated to produce similar increases in the capacitance and FM1-43 fluorescence responses. However, when
endocytosis was minimal, the changes in capacitance were markedly larger than the corresponding changes in FM1-43
fluorescence. Moreover, the apparent differences between capacitance and FM1-43 fluorescence changes increased with
larger exocytic responses, as more granules fused with the plasma membrane. In experiments in which exocytosis was
suppressed, increasing membrane tension by osmotically induced cell swelling increased FM1-43 fluorescence, suggesting
that FM1-43 fluorescence is sensitive to changes in the membrane tension. Thus, increasing membrane area through
exocytosis does not swell chromaffin cells but may decrease membrane tension.
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