Conjunctival goblet cells secrete mucus in response to cholinergic (muscari
nic) agonists, but the underlying signaling pathways activated in this tiss
ue are not well understood. Cholinergic agonists usually activate phospholi
pase C to produce inositol 1,4,5 trisphosphate and diacylglycerol. Inositol
1,4,5 trisphosphate increases the intracellular Ca2+ concentration [Ca2+](
i)) while diacylglycerol activates protein kinase C (PKC). PKC and Ca2+, ei
ther by itself or with calmodulin, activate cellular functions. Goblet cell
glycoprotein secretion, our index of mucin secretion, was measured from pi
eces of rat conjunctiva with an enzyme-linked lectin assay using the lectin
Ulex europaeus agglutinin 1 (UEA-I). UEA-I selectively recognizes high mol
ecular weight glycoproteins secreted by the goblet cells. Increasing the [C
a+](i) with the Ca2+ ionophore ionomycin stimulated glycoprotein secretion
from conjunctival goblet cells. Cholinergic agonist-induced secretion was c
ompletely blocked by chelation of extracellular Ca2+ and by the Ca2+/calmod
ulin-dependent protein kinase inhibitors KN93 and W7 as well as their inact
ive analogs KN92 and W5. Activation of classical and novel PKC isozymes by
phorbol 12-myristate 13-acetate anti phorbol 12,13-dibutyrate stimulated go
blet cell glycoprotein secretion. When ionomycin and PMA were added simulta
neously secretion was additive. PKC isozymes were identified by Western blo
tting analyses with antibodies specific to nine of the 11 PKC isozymes (PKC
gamma and zeta were not tested). All nine PKC isozymes were identified in
the conjunctival epithelium. The cellular location of the PKC isozymes was
determined by immunofluorescence microscopy. Goblet cells contained the cla
ssical PKC isozymes PKC alpha, -betaI and -beta II, the novel PKC isozymes
PKC epsilon, -theta, and -mu and the atypical PI(C isozyme PKC zeta. We wer
e unable to determine if PKC activation is required for cholinergic-agonist
induced secretion because the PKC inhibitors chelerythrine and staurospori
ne alone greatly increased secretion. We conclude that Ca2+ plays a major r
ole in cholinergic agonist-induced conjunctival goblet cell secretion, but
this agonist appears not to use Ca2+/calmodulin-dependent protein kinases.
We also conclude that activated PKC can stimulate goblet cell secretion and
that seven different PKC isoforms are present in the goblet cells. (C) 200
0 Academic Press.