Modulation of chloride secretion across the pigmented rabbit conjunctiva

The purpose of the present study was to investigate whether active Cl- secretion in the pigmented rabbit conjunctiva was subject to cAMP, Ca2+ and protein kinase C (PKC) modulation. The excised pigmented rabbit conjunctivas were mounted in the modified Ussing-type chambers for measurement of unidirectional Cl-36 fluxes under the open-circuit condition and of the short-circuit current (I-sc), potential difference, and transconjunctival electrical resistance. The results indicate that Cl- secretion across the conjunctiva was abolished by mucosal application of 1 mM N-phenylanthranilic acid and was reduced by 40% by serosal application of 10 mu M bumetanide. Net Cl- flux was stimulated by 133% by 1 mM 8-Br cAMP, 107% by 10 mu M A23187, and 87% by 1 mu M phorbol 12-myristate-13-acetate (PMA), suggesting that cAMP, Ca2+, and PKC all modulated active Cl- secretion, respectively. There existed a linear correlation between measured changes in net Cl- flux and observed changes in I-sc (r(2) = 0.99). The serial treatment of the conjunctiva with (a) 1 mM 8-Br cAMP and 10 mu M A23187 and (b) 10 mu M A23187 and 1 mu M PMA resulted in sequence-independent, additive stimulation of I-sc. In the case of 1 mM 8-Br cAMP and 1 mu M PMA, additive stimulation of I-sc was observed only when 1 mM 8-Br cAMP was added prior to 1 mu M PMA. These results suggest that a given pharmacological agent may affect more than one channel type and that there might be a possible connection among the channels at the signal transduction level. In summary, Cl- appears to enter the pigmented rabbit conjunctiva from the serosal fluid via Na+-(K+)-2Cl(-) cotransport process and exit to the mucosal fluid via channels, resulting in active Cl- secretion. Active Cl- secretion in the pigmented rabbit conjunctiva appears to be modulated by cAMP, Ca2+, and PKC.