VMAT-Mediated changes in quantal size and vesicular volume

It has been well established that the volume of secretory vesicles can be m odulated. However, we present the first data demonstrating that the amount of transmitter in a vesicle can regulate its volume. Amperometry and transm ission electron microscopy have been used to determine that L-3,4-dihydroxy phenylalanine and reserpine increase and decrease, respectively, the volume of single pheochromocytoma cell vesicles as well as their catecholamine co ntent. Because changes in vesicular catecholamine content are tracked by ch anges in vesicle volume, our results indicate that when quantal size is alt ered via the vesicular monoamine transporter the concentration of catechola mines within the vesicles remains relatively constant. This previously unid entified cellular response provides new insight into how catecholamines can be packaged in and released from secretory vesicles.