10(-6) M n-formyl-methionyl-leucyl-phenylalanine (FMLP) stimulated Ca2
+ flux in human neutrophils is characterized by a profile composed of
two peaks of different amplitude and breadth. beta-Endorphin inhibited
the magnitude and modulated the kinetics of the second peak in a mann
er which was dose-dependent and could reflect either negative cooperat
ivity or heterogeneity of binding sites. The second peak arises from c
alcium channel activity since in the presence of nifedipine or EGTA it
was not evident while the first peak was reduced about 24%. Similarly
, at 15 degrees C, where we were unable to detect any channel activity
, the first peak was diminished by 35% and beta-endorphin had no detec
table effect on this peak. These results led us to conclude that the f
irst peak is chiefly composed of Ca2+ recruited from cytosolic stores
which are relatively insensitive to the above treatments and a smaller
fraction of calcium originating in calcium channel activity. Hence, w
e reason that beta-endorphin modulates only the calcium ion flux arisi
ng from calcium channel function.