PHORBOL ESTER-STIMULATED EXOCYTOSIS IN LACRIMAL GLAND - PKC MIGHT NOTBE THE SOLE EFFECTOR

In this work we show that, although both phorbol 12-myristate 13-aceta te (PMA) and 4beta-phorbol 12,13-dibutyrate (PdBu) stimulate the prote in discharge in the rat lacrimal gland with the same half-maximal effe ctive concentration (EC50 approximately 2 x 10(-7) M), PdBu is more ef ficient in eliciting this response compared with PMA. We also show tha t sphingosine and chelerythrine have no inhibitory effect on the prote in discharge stimulated by PMA or PdBu at concentrations up to 2 x 10( -4) and 3 x 10(-5) M, respectively. With staurosporine, a complete inh ibition could not be obtained even at 1 muM. However, only with triflu operazine (TFP) we obtained a complete inhibition of the PMA-induced p rotein discharge at 10(-4) M TFP. On the other hand, we show that thre e diacylglycerol-permeant analogues (1-oleoyl-2-acetyl-sn-glycerol, 1, 2-dioctanoyl-sn-glycerol, and 1,2-didecanoyl-sn-glycerol) do not stimu late protein discharge. In a previous report from our laboratory (30), we showed that the rat lacrimal gland expresses the alpha-isoform of protein kinase C (PKC). In this study, using specific antibodies direc ted against the newly identified isoforms of PKC, we show on a diethyl aminoethyl-cellulose fraction that, besides PKC-alpha, the rat lacrima l gland expresses PKC-epsilon, as previously suggested by Dartt et al. (11), and PKC-delta. Our results question the direct implication of P KC activity as a sole effector of the phorbol ester-stimulated protein secretion in the rat lacrimal gland.