FUNCTIONAL PROPERTIES OF PROLACTIN-SECRETING CELLS 
Gabriel CoTA, Ph.D., International Research Scholar 
Dr. Cota's research is aimed at understanding the 
cellular mechanisms involved in the control of pro- 
lactin secretion in the vertebrate pituitary gland. 
Prolactin is a versatile hormone that participates in 
the regulation of a variety of physiological pro- 
cesses, including lactation. It is produced by pitu- 
itary cells called lactotropes or mammotropes. Until 
recently lactotropes were commonly thought to 
comprise a homogeneous cell population in the 
normal gland. However, studies performed on cul- 
tured pituitary cells indicate the existence of 
distinct subsets of prolactin secretors that differ 
in basal secretory activity or responsiveness to extra- 
cellular regulatory factors. A major goal of Dr. Cota's 
research is to define the intrinsic properties of lac- 
totrope subpopulations that determine their differ- 
ent secretory behavior. 
Lactotrope Subtypes 
Dr. Cota's group has explored the differences in 
the basal rate of prolactin secretion among individ- 
ual lactotropes in primary cell cultures obtained 
from pituitaries of adult male rats. Prolactin release 
was visualized and quantified at the single-cell level 
by using the reverse hemolytic plaque assay. In this 
immunological technique, cells releasing the ap- 
propriate hormone induce lysis of indicator erythro- 
cytes, and the size of the zone of hemolysis, or 
plaque, around an endocrine cell provides an index 
of its secretory activity. Two subtypes of prolactin 
secretors were identified: small-plaque (SP) lacto- 
tropes that released small amounts of prolactin per 
unit time under basal conditions and comprised 
about 6% of all pituitary cells, and large-plaque (LP) 
lactotropes that secreted prolactin at higher basal 
rates and accounted for about 14% of the pituitary 
cell population. 
Ca^^ Channel Activity and Lactotrope 
Heterogeneity 
Like other endocrine cells, lactotropes are electri- 
cally excitable and exhibit spontaneous depolariza- 
tions of membrane potential. Basal prolactin secre- 
tion is thought to be sustained by Ca^^ influx 
through voltage-gated Ca^"^ channels in the plasma 
membrane , which transiently open during spontane- 
ous action potentials. Dr. Cota's laboratory has ana- 
lyzed the Ca^^ channel activity of identified prolac- 
tin secretors using electrophysiological methods. 
Whole-cell recordings of ionic currents with the 
patch-clamp technique revealed the presence of 
two classes of voltage-gated Ca^^ channels in the 
plasma membrane of each lactotrope examined: 
low-threshold and high-threshold channels. The ac- 
tivity of low-threshold channels did not signifi- 
cantly differ between SP and LP lactotropes. By con- 
trast, the surface density of high-threshold channels 
was markedly larger in LP secretors. Plaque assays 
were again used to demonstrate that blocking the 
high-threshold Ca^^ channels with pharmacological 
agents selectively inhibits prolactin secretion from 
LP lactotropes. In fact, many of these cells behaved 
functionally as SP secretors in the presence of the 
channel blockers. These results indicate that high- 
threshold Ca^^ channels are differentially expressed 
in lactotrope subtypes, and strongly suggest that 
Ca^"*^ entry through such ionic channels is a crucial 
determinant of the amount of prolactin released 
from a cell. 
Functional Relevance of Na^ Channels 
in Lactotropes 
During the past year Dr. Cota and his colleagues 
have succeeded in characterizing an additional dif- 
ference in ion channel activity between the two lac- 
totrope subtypes. In patch-clamp experiments, 
membrane depolarization was found to induce 
whole-cell Na^ currents in LP secretors that were 
stronger than in SP cells. Such differences in Na"^ 
current amplitude were not related to cell-to-cell 
variations in the kinetic properties of the ionic 
currents and persisted after normalizing current am- 
plitude by cell capacitance, a procedure that elimi- 
nates membrane area as a variable. Thus, like high- 
threshold Ca^^ channels, Na^ channels are not 
uniformly expressed among SP and LP lactotropes. 
Na^ channels should be functionally important 
for the secretory activity of lactotropes, as they favor 
the triggering of action potentials and thereby pro- 
mote the opening of high-threshold Ca^^ channels. 
Consistent with this view, it was found that tetrodo- 
toxin, a potent blocker of Na^ channels, drastically 
decreases the total amount of prolactin secreted 
from the cultured pituitary cells. Furthermore, pop- 
ulation analysis of prolactin plaque sizes suggested 
that this inhibitory action of tetrodotoxin can be 
explained by the preferential suppression of LP 
lactotropes. 
Research in Dr. Cota's laboratory is now focused 
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