Secretory vesicles are storage vehicles of neurotransmitters and hormones in secretory cells. Despite their important biological role, their complex composition and release mechanisms are not yet fully understood. To complement amperometry measurements and fluorescence-based methods to characterize neurotransmitter content of secretory vesicles, we here introduce a new method using surface plasmon resonance (SPR) to probe the intravesicular content of isolated intact large dense core vesicles from chromaffin cells and present how using this technique offers real time measurement of vesicle content release when subjected the isolated granules to high osmotic pressure. The observations from these SPR measurements confirm recent amperometric measurements at live chromaffin cells that neurotransmitters are released from vesicles when subjected to high osmotic pressure. Further, a comparison of volume exclusion measurements using SPR to the physical dimensions of the dense core vesicle size as determined by transmission electron microscopy imaging analysis, the molecular dimension of soluble volume versus the solid volume of membrane and dense protein matrix in intact vesicles was determined.
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