Functional Heterogeneity in Prolactin-secreting Cells 
area as a variable. Thus sodium channels, like 
HVA calcium channels, are not uniformly ex- 
pressed among SP and LP lactotropes. 
Sodium channels should be functionally im- 
portant for the secretory activity of lactotropes, as 
they favor the triggering of action potentials and 
thereby promote the opening of HVA calcium 
channels. Indeed, we have found that blocking 
the sodium channels of cultured cells with exter- 
nal tetrodotoxin drastically decreases the total 
amount of prolactin secreted. Furthermore, popu- 
lation analysis of prolactin plaque sizes suggests 
that tetrodotoxin, like nifedipine, preferentially 
inhibits prolactin secretion from LP cells. 
These findings raise the question of what fac- 
tors regulate ion channel expression in pituitary 
cells. They also renew our interest in the differ- 
entiation of lactotrope subtypes in the develop- 
ing pituitary gland. Our current research is fo- 
cused on these topics. 
490 
