Functional Heterogeneity 
in Prolactin-secreting Cells 
Gabriel Cota, Ph.D. — International Research Scholar 
Dr. Cota is Professor of Physiology, Biophysics, and Neurosciences at the Center for Research and 
Advanced Studies, National Polytechnic Institute, Mexico City. He received his Ph.D. degree in physiology 
and biophysics from the Center and subsequently carried out postdoctoral research with Clay M. 
Armstrong at the University of Pennsylvania, as a Pogarty International Research Fellow. 
OUR laboratory is interested in the cellular 
mechanisms involved in the control of pro- 
lactin secretion. Prolactin is a vertebrate hor- 
mone that participates in the regulation of a di- 
versity of physiological processes, including 
lactation. This versatile chemical messenger is 
produced in the pituitary gland by endocrine 
cells called lactotropes or mammotropes. Basic 
information about lactotrope function and its 
control offers insight into the pathogenesis of hy- 
perprolactinemia, a frequent hypothalamic pitu- 
itary disorder in humans. 
Until recently, lactotropes were commonly 
thought to comprise a homogeneous cell popula- 
tion in the normal pituitary gland. However, stud- 
ies performed on cultured pituitary cells indicate 
the existence of a considerable lactotrope hetero- 
geneity. In these accessible model systems, sub- 
sets of lactotropes that differ in basal secretory 
activity or responsiveness to extracellular regula- 
tory factors have been distinguished. Our work 
over the last three years has focused on the origin 
of such functional differences. 
Lactotrope Subtypes 
We have identified two subpopulations of lac- 
totropes in pituitary cultures derived from adult 
male rats. We used the reverse hemolytic plaque 
assay, an immunological technique that permits 
the microscopic visualization of single-cell se- 
cretions. In this assay, cells releasing the appro- 
priate hormone induce lysis of indicator erythro- 
cytes. The size of the zone of hemolysis, or 
plaque, around an individual secretor provides 
an index of the cumulative amount of hormone 
released by that cell. In keeping with previous 
observations by Jimmy Neill (University of Ala- 
bama) on the secretory behavior of female rat 
lactotropes, we found that under basal con- 
ditions some prolactin-secreting cells form small 
plaques (SP lactotropes) and others induce large 
plaques (LP lactotropes). Thus SP and LP lacto- 
tropes secrete distinct amounts of prolactin per 
unit of time in the basal state. 
Calcium Channel Activity as a Determinant 
of Lactotrope Heterogeneity 
There is much evidence that calcium ion plays 
a major role in pituitary cells as an intracellular 
messenger in hormone secretion. In particular, 
basal prolactin secretion is thought to be sus- 
tained by calcium influx through plasma mem- 
brane calcium channels, which transiently open 
during spontaneous action potentials. This sug- 
gests that differences in basal secretory rate 
among lactotrope subtypes might arise, at least in 
part, from a differential expression of calcium 
channels. To test this possibility, we investigated 
the calcium channel activity of lactotrope sub- 
types, using electrophysiological techniques. 
Our analysis indicates that both SP and LP 
lactotropes express two classes of voltage-gated 
calcium channels in the plasma membrane: 
low-voltage-activated (LVA) and high-voltage- 
activated (HVA) channels. The activity of LVA 
channels does not significantly differ between 
the two lactotrope subtypes. By contrast, the sur- 
face density of HVA channels is markedly higher 
in LP cells than in SP cells. 
In addition, we tested the effect of nifedipine 
on prolactin secretion. Nifedipine is a dihydro- 
pyridine drug that selectively blocks HVA cal- 
cium channels in many types of excitable cells, 
including prolactin secretors. We found that ni- 
fedipine inhibits prolactin secretion by preferen- 
tially suppressing the LP lactotropes. In fact, a 
large proportion (around 50 percent) of LP lacto- 
tropes behave functionally as SP lactotropes in 
the presence of nifedipine. Our results, taken 
together, indicate that calcium entry through 
dihydropyridine-sensitive HVA calcium channels 
contributes to the high basal rate of prolactin se- 
cretion in LP lactotropes. 
Sodium Channels in Lactotropes 
We have characterized an additional difference 
in ion channel activity between the two lacto- 
trope subtypes: membrane depolarization in- 
duces larger whole-cell sodium currents in LP 
lactotropes than in SP lactotropes. Such differ- 
ences are not related to cell-to-cell variations in 
the kinetic properties of the sodium currents and 
persist after current amplitude is normalized by 
cell capacitance, which eliminates membrane 
489 
