G. Forstner et al., REGULATION OF MUCIN SECRETION IN T84 ADENOCARCINOMA CELLS BY FORSKOLIN - RELATIONSHIP TO CA2+ AND PKC, The American journal of physiology, 266(4), 1994, pp. 70000606-70000612
The relationship between the adenosine 3',5'-cyclic monophosphate-medi
ated protein kinase A (PKA)-dependent stimulatory pathway for mucin se
cretion and Ca2+-mediated and protein kinase C (PKC)-mediated secretio
n was studied in T84 cells, using the postreceptor secretagogues forsk
olin, A-23187, and phorbol 12-myristate 1S-acetate (PMA), the protein
kinase inhibitors staurosporine and 1-(5-isoquinolinylsulfonyl)-2-meth
ylpiperazine (H-7), high- and low-Ca2+ media, and the Ca2+ chelator ,2
-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Stauros
porine (10(-5) M) inhibited both PMA and forskolin at their maximally
effective concentrations, whereas H-7 (5 x 10(-5) M) inhibited only PM
A. Stimulation of mucin secretion by forskolin (5 x 10(-5) M) was not
significantly affected by the reduction of medium Ca2+ to 47 and 129 n
M, equivalent to published values for intracellular Ca2+ concentration
([Ca2+](i)). Stimulation by forskolin was reduced by preloading cells
with BAPTA, but to a much smaller extent than Ca2+-dependent stimulat
ion by A-23187. A-23187-mediated mucin secretion from BAPTA-loaded cel
ls was augmented by high doses of forskolin. Similar concentrations of
forskolin had no effect on A-23187-stimulated secretion in calcium-re
plete cells. Our results indicate that forskolin does not stimulate mu
cin secretion by increasing Ca2+ entry or releasing Ca2+ from intracel
lular stores. Forskolin can stimulate mucin secretion in a Ca2+-indepe
ndent manner but is apparently inhibited by high levels of intracellul
ar Ca2+ induced by Ca2+ ionophores in 1.0 mM Ca2+ media. PKA and PKC p
athways stimulate mucin secretion independently and supportively in th
e presence and absence of Ca2+. The Ca2+-independent protein kinase-de
pendent response to calcium ionophores is more susceptible to H-7 than
secretion mediated by forskolin, making it unlikely that calcium iono
phores stimulate secretion by activating PKA.