ably influence the class of neuropeptide ultimately 
formed. 
III. Regulation of Neuroendocrine Secretion. 
The release of a hormone or neurotransmitter 
from the specialized secretory vesicles into which 
they are transported and stored at high concentra- 
tion is dependent on a series of biochemical events, 
which have not been fully elucidated. Initiation of 
secretion in most cells appears to be involved criti- 
cally with an increase in the cytosolic calcium con- 
centration arising from the influx of calcium 
through voltage-sensitive calcium channels and/or 
release of calcium from internal subcellular organ- 
elles. Likewise, the routes of removal of calcium 
have not been fully ascertained but include intra- 
ceUular buffering capacity, the plasma membrane, 
and intracellular organelles. One impediment to 
solving the secretion paradigm is the extreme 
speed under which ion fluxes and fusion events 
proceed under physiological conditions. In many of 
these systems, secretion occurs over a millisecond 
time range, with ion fluxes exquisitely controlled 
by membrane voltage and ion channel conduc- 
tance. To investigate the mechanism of action of 
ion-translocating ATPases, ion channels, ion fluxes, 
and the fusion event. Dr. Johnson and his col- 
leagues have applied and perfected techniques of 
electrophysiology— digital time-resolved microspec- 
trofluorometry of single cells, fast-sensitive detec- 
tors, and rapid perfusion systems. The data indicate 
the extremely complex regulatory control of secre- 
tion. For example, in collaboration with Dr. M. 
Morad, it has been found that the secretion from 
the catecholamine-containing chromaffin cell can 
be modulated by regulation of the acetylcholine re- 
ceptor, by the frequency of stimulation on the 
opening of the calcium channel, and by the feed- 
back inhibition of chemical messengers released 
from the secretory vesicle upon calcium influx. Ex- 
periments have also indicated that release of H"*" 
from the secretory vesicle (where the pH is 5.5) 
into the extracellular space has a marked localized 
inhibitory effect on calcium channel conductance. 
The continuing application of sensitive and rapid 
biophysical techniques to the study of the temporal 
and spatial distribution of ion fluxes and gradients 
should provide insights into the mechanism of se- 
cretion. 
Dr. Johnson is also Associate Professor of Medi- 
cine, Physiology, and Biochemistry and Biophysics 
at the University of Pennsylvania School of Medi- 
cine and Attending Physician at the Hospital of the 
University of Pennsylvania. 
510 
