Do caged-Ca2+ compounds mimic the physiological stimulus for secretion?
We measured the exocytotic response induced by a rapid and global increase in the concentration of Ca2+ or GTP gamma S (achieved by flash photolysis of caged compounds) in mast cells and chromaffin cells. Secretion was measured by following both cell membrane capacitance and amperometry. When Ca2+ was used to trigger secretion, we observed an immediate increase in capacitance, however, the first amperometric spike observed was delayed with respect to the change in capacitance (approximately 5 s in mast cells and approximately 0.6 s in chromaffin cells). In contrast, when GTP gamma S was used to trigger secretion, no such discrepancy was seen, both measurements reported a secretory response that was similarly delayed with respect to the stimulus. These data suggest that the capacitance increase, when triggered by a large Ca2+ step, reports events that do not result in the fusion of vesicles containing oxidizable substances. These results contrast with the smaller secretory responses evoked by the transient opening of Ca2+ channels. Under these more physiological conditions, pulsed-laser imaging studies revealed that the Ca2+ influx was localized to discrete areas at the plasma membrane. Thus the Ca2+ stimulus triggered by DM-nitrophen fails to reproduce the small localized changes seen during a physiological stimulation and creates an artifactual non-secretory capacitive response.
Department of Physiology and Biophysics, Mayo Clinic, Rochester, MN 55905, USA.
A F
AF
Oberhauser
I M
IM
Robinson
J M
JM
Fernandez
D000818:Animals; D017610:Calcium Compounds / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D002838:Chromaffin System / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D005089:Exocytosis / Q000187:drug effects*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D019044:Image Cytometry; D066298:In Vitro Techniques; D008407:Mast Cells / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D051379:Mice; D008810:Mice, Inbred C57BL; D018408:Patch-Clamp Techniques; D010782:Photolysis
D017610:Calcium Compounds; D016244:Guanosine 5'-O-(3-Thiotriphosphate)