ADAPTIVE DIFFERENTIATION OF AVIAN EXOCRINE CELLS ALTERS THEIR PH(I) RESPONSE TO MACHR ACTIVATION

Muscarinic receptor-mediated changes in intracellular pH (pi-Ii) were measured in isolated bis(2-carboxyethyl)-5(6)-carboxyfluorescein-loade d cells, suspended in bicarbonate-containing media, from the exocrine nasal gland of freshwater-fed ducklings (Anas platyrhynchos). The pH(i ) recovery from an acid load was sensitive to amiloride, required sodi um ions in the external medium, and was independent of added bicarbona te. These findings are consistent with the hypothesis that the pH(i) r ecovery was mediated by a Na+/H+ exchanger. Muscarinic activation of c ells resulted in a sustained cytosolic alkalinization that was sensiti ve to atropine and that was blocked by amiloride. Activation of protei n kinase C (PKC) or inhibition of protein phosphatases mimicked the ef fect of receptor activation on pH(i), whereas inhibitors of PKC blocke d the response, indicating that phosphorylation of a major pH(i) contr ol mechanism results in a shift of pH(i) to more alkaline values. In c ontrast, fully differentiated salt gland cells isolated from nasal gla nds of salt-stressed ducklings responded to muscarinic receptor activa tion with a transient cytosolic acidification. These findings raise th e question whether the cytosolic alkalinization in muscarinic acetylch oline receptor-activated naive cells may serve as a signal or a permis sive factor for the initiation of adaptive growth and/or differentiati on processes observed in the salt glands of salt-stressed birds.