tachment of a sacrificial reductant may be useful, 
especially if the reductant can be regenerated by the 
reducing intracellular milieu. 
The recent upsurge of interest in the labile and 
reactive molecule nitric oxide (NO) as a putative 
paracrine neurotransmitter and mediator of excito- 
toxicity provided encouragement to construct a 
"caged" form of NO, i.e., a complex form that can 
be suddenly released by photolysis. Such a molecule 
would permit NO to be delivered with precise local- 
ization and timing, which is not possible now. Pre- 
liminary results of Dr. Makings suggest that hemoglo- 
bin (Hb) in which Fe(II) has been replaced by 
Mn(II) may cage NO. Hb'MnNO is isoelectronic 
with Hb-FeCO, which (unlike Hb-FeNO) photo- 
dissociates efficiently. Further efforts are aimed at 
optimizing the preparation and stability of 
Hb-MnNO, finding lower-molecular-weight ana- 
logues, and applying them to relevant biology. 
Chelators that cage Ca^^ have been of consider- 
able value in quantifying its triggering role in many 
important processes, such as muscle contraction 
and exocytosis. Dr. Stephen Adams serendipitously 
discovered a new photolabile chelator that is several 
hundredfold more sensitive to light and undergoes 
1 0-fold larger changes in Ca^^ affinity than the best 
previous molecules, while retaining the Ca^"^:Mg^"^ 
selectivity that is important for intracellular appli- 
cations. 
Roles of Controversial and Novel 
Second Messengers 
Dr. Carsten Schultz continued exploring 
membrane-permeant acetoxymethyl (AM) esters of 
phosphate-containing messengers. He discovered 
that the dibutyryl AM ester of the controversial mes- 
senger inositol 1 ,3,4,5 -tetrakisphosphate (IP4) 
sometimes causes depression of elevated cytosolic 
Ca^^ levels back toward basal levels in REF-52 fibro- 
blasts. This result seems more in accord with A. L. 
Boynton's proposal that IP4 stimulates reuptake of 
Ca^^ into intracellular stores than with R. F. Irvine's 
hypothesis that IP4 opens Ca^* channels in the 
plasma membrane. 
Dr. Schultz also collaborated with Dr. Steven Pan- 
dol to show that the permeant IP4 derivative neither 
stimulates amylase secretion from pancreatic acinar 
cells nor adds to the substantial secretion due to the 
analogous permeant IP3 derivative. Another collabo- 
ration with Drs. Kim Barrett and Alexis Traynor- 
Kaplan suggested that IP4 may uncouple Cl~ secre- 
tion from Ca^^ elevation in T84 colon carcinoma 
cells. This would be a novel physiological action of 
an inositol polyphosphate downstream from Ca^^. 
Dr. Clotilde Randriamampita sought the mysteri- 
ous messenger that is suspected to be generated by 
emptying of intracellular Ca^^ stores and to signal 
the plasma membrane to open Ca^^ channels to re- 
plenish them. If the message can get from the lumen 
of the endoplasmic reticulum to the plasma mem- 
brane, it crosses at least one membrane, so it might 
be able to cross several and diffuse extracellularly. 
Dr. Randriamampita has found that activated Jurkat 
lymphocytes release a diffusible factor that can 
cause Ca^^ influx into nearby macrophages or astro- 
cytoma cells. Collection and analysis of Jurkat su- 
pernatants suggest that the factor is heat-stable, 
trypsin-insensitive, low molecular weight, anionic, 
and weakly adherent to reverse-phase silica, but 
does not seem to be any of the obvious candidates 
such as ATP, ADP, AMP, adenosine, OTP, cGMP, leu- 
kotrienes C4 or B4, platelet-activating factor, arachi- 
donic acid, or NO. Analysis of this possibly novel 
autocrine/paracrine messenger is continuing. 
The last three projects described above (those of 
Drs. Adams, Schultz, and Randriamampita) were 
supported by a grant from the National Institutes of 
Health. 
Dr. Tsien is also Professor of Pharmacology in 
the School of Medicine of the University of Califor- 
nia, San Diego, and Professor of Chemistry at the 
University of California, San Diego. 
Article 
Sammak, P.J., Adams, S.R., Harootunian, A.T., 
Schliwa, M., and Tsien, R.Y. 1992. Intracellular 
cyclic AMP, not calcium, determines the direc- 
tion of vesicle movement in melanophores: direct 
measurement by fluorescence ratio imaging. / 
Cell Biol 117:57-72. 
NEUROSCIENCE 447 
