Ionic Channels in Sea Urchin Sperm Physiology 
small peptide isolated from the homologous egg. 
This peptide, called resact, is species specific and 
induces in sperm a transient increase in [Ca^^]; 
and cGMP and an increase in pHj. The egg of 
Strongylocentrotus purpuratus contains a pep- 
tide, named speract, that also elevates cGMP and 
[Ca^^ji and induces an alkalinization and a K"^- 
dependent hyperpolarization. 
Sea urchin sperm, however, are tiny cells (head 
diameter ~2 ytvci). This has precluded a careful 
characterization of their electrophysiological 
properties that would shed light on the molecu- 
lar mechanisms determining their fascinating 
egg-induced behavioral changes. Recently, in col- 
laboration with Donner Babcock and Martha 
Bosma from the University of Washington in Seat- 
tle, we found that it is possible to swell sea urchin 
sperm in diluted sea water. The swollen cells are 
spherical (~4 yum in diameter), immotile, and 
metabolically active, and they can regulate their 
[Ca'^^Ji, pHj, and membrane potential. 
The swollen cells respond to pM concentra- 
tions of speract with an increase in K"^-selective 
permeability that lasts for many seconds. We 
found that this permeability change, as well as 
the changes in [Ca^^]; and pHj that are seen at 
higher speract concentrations, also occurs in 
nonswoUen sperm. An advantage of swollen 
sperm is that they can be much more easily patch 
clamped and single-channel-activity recorded. 
We have observed that pM speract activates a 
small K"^ channel. Thus swollen sperm provide 
new avenues to study ionic channels and their 
regulation by egg factors and second messengers. 
This work is supported by grants from the Mex- 
ican Council for Science and Technology, the 
World Health Organization, and the Miguel Ale- 
man Foundation. 
Confocal immunofluorescent micrograph of 
Salmonella typhimurium (yellow) interact- 
ing with a cultured epithelial cell and 
causing rearrangement of epithelial actin 
filaments (blue) around the invading bac- 
terium. 
Research and photograph by B. Brett 
Finlay. 
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