on the postnatal development of the lactotrope pop- 
ulation. In addition, studies are in progress to iden- 
tify extracellular factors that regulate ion channel 
expression in pituitary cells. 
Dr. Cota is Professor of Physiology, Biophysics, 
and Neurosciences at the Center for Research and 
Advanced Studies of the National Polytechnic In- 
stitute, Mexico City. 
Article 
HortaJ., Hiriart, M., and Cota, G. 1991- Differential 
expression of Na channels in functional subpopu- 
lations of rat lactotropes. Am f Physiol 261: 
C865-C871. 
IONIC FLUXES IN THE ACROSOME REACTION OF SEA URCHIN SPERM 
Alberto Darszon, Ph.D., International Research Scholar 
Dr. Darszon and his colleagues have continued 
their efforts to understand at the molecular level the 
relationship and regulation of the ionic fluxes that 
are deeply involved in the sea urchin sperm acro- 
some reaction (AR) . 
In sea urchin sperm, the AR is triggered by a 
fucose-sulfate-rich polymer, FSG, contained in the 
outer investment of the egg, the jelly. FSG induces 
the influx of Ca^^ and Na^ and the efflux of and 
H"*^, leading to increases in intracellular concentra- 
tion of Ca^+ ([Ca^^li) and intracellular pH (pH;), a 
K^-dependent transient hyperpolarization that may 
involve channels (in Lytechinus pictus sperm) 
and a Ca^"^-dependent depolarization. 
Desensitization of the Egg Jelly Receptor 
In Strongylocentrotus purpuratus sperm sus- 
pended in 2 mM Ca^^ artificial sea water (ASW) , egg 
jelly and FSG induce a small and transient increase 
in [Ca^^ji without producing the AR. Even after Ca^* 
restoration to 10 mM, its normal concentration in 
ASW, sperm are unable to attain the AR or to respond 
to a new addition of FSG after being washed and 
resuspended in ASW. By varying the time between 
the addition of FSG and the restoration of Ca^^ to 10 
mM, it is possible to determine that the rate of Ca^"^ 
influx inactivation is accelerated as the concentra- 
tion of external Ca^^ increases. The AR after Ca^^ 
restoration also decreases as external Ca^"^ increases. 
However, raising [Ca^*]; with ionomycin does not 
lead to the refractory state in sea urchin spermato- 
zoa. High external pH was used to trigger Ca^^ in- 
flux and the AR without activating the egg jelly re- 
ceptor. External pH was unable to induce a 
refractory state in sea urchin sperm suspended in 2 
mM Ca^+ ASW, even though it increased [Ca^+]i. This 
indicates that FSG or egg jelly is required to achieve 
the refractory state in sea urchin sperm and that de- 
sensitization of the egg jelly receptor is probably 
involved in this process. 
Membrane Potential and Channels 
in the Acrosome Reaction 
To investigate the role of the membrane potential 
changes during the AR, I. pictus sea urchin sperm 
were artificially hyperpolarized with valinomycin 
in K+-free ASW (OKASW). This condition raised pHj, 
caused a small increase in ^^Ca^^, and induced some 
AR. When the cells were depolarized 40-60 s after 
the induced hyperpolarization, the pHj decreased 
and there was a significant increase in ^^Ca^"^ up- 
take, [Ca^^ji, and the AR. The waiting time was re- 
quired to allow the pHj change necessary for the AR 
to occur. 
Planar bilayers into which I. pictus plasma mem- 
branes were incorporated by fusion displayed single 
cation-selective channels of 85 pS. Although the se- 
lectivity sequence of this channel for cations needs 
to be determined, its single-channel conductance is 
close to that found for one of the channels of 5. 
purpuratus plasma membranes. 
Three conclusions can be drawn from these re- 
sults. 1) Pan of the K^-induced depolarization seen 
in I. pictus sea urchin sperm suspended in OKASW is 
due to Ca^"^ influx through a voltage-dependent 
Ca^^ channel. 2) The K^-dependent hyperpolariza- 
tion in sperm of this species is important to trigger 
the AR and may be necessary to a) remove inactiva- 
tion from voltage-dependent Ca^^ channels so that a 
subsequent depolarization could open them and/or 
b) to increase pHj stimulating a Na^-H^ exchange. 
This alkalinization is necessary for the AR and may 
participate in the regulation of a Ca^^ transport sys- 
tem activated in this process. 3) The bilayer experi- 
INTERNATIONAL RESEARCH SCHOLARS 505 
