Inhibition by thiocyanate of muscarinic-induced cytosolic acidification and Ca2+ entry in rat sublingual acini.

Thiocyanate (SCN-) plays a critical part in an oral antimicrobial system by acting as a substrate for peroxidases. Salivary glands concentrate SCN- from blood up to 5 mM in saliva; however, the influence of SCN- on salivary acinar-cell function is unknown. The present study examined the effects of SCN- on the regulation of cytosolic pH (pHi) and free Ca2+ concentration ([Ca2+]i) in rat sublingual mucous acini using the pH- and Ca(2+)-sensitive fluorescent indicators, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein and fura-2, respectively. SCN- induced a concentration-dependent inhibition of the carbachol-stimulated cytosolic acidification (K1/2, approx. 1.4 mM SCN-). Cytosolic pH recovery from an acid load was not changed by substitution of Cl- by SCN-, suggesting that Na+/H+ exchange activity was not affected by SCN-. SCN- did not alter the initial carbachol-stimulated increase in [Ca2+]i; however, the sustained [Ca2+]i increase was inhibited by > 65% (K1/2, approx. 1.0 mM SCN-). Furthermore, SCN- prevented the carbachol-stimulated Mn2+ influx, indicating that it inhibits the divalent-cation entry pathway. Consistent with decreased Ca2+ mobilization being involved in the blockade of the agonist-induced acidification by SCN-, only total replacement of Cl- with SCN- significantly inhibited the acidification induced by the Ca2+ ionophore ionomycin. The permeability to SCN- through the Ca(2+)-dependent Cl- channels was 5.2-fold higher than the permeability to Cl-. These results suggest that inhibition of the agonist-induced cytosolic acidification by high-concentration SCN- may be mediated by both competitive inhibition of HCO3- efflux and by blockade of Ca2+ influx.