Inositol phosphates and [Ca2+]i signals in a differentiating exocrine cell.

Generation of inositol phosphates and changes in intracellular Ca2+ concentration [( Ca2+]i) upon muscarinic receptor activation were studied in isolated cells from the nasal salt gland of Anas platyrhynchos, comparing responses in the poorly differentiated cells from ducks drinking only tap water (naive cells) with the more fully differentiated actively secreting cells from ducks drinking 1% NaCl solution for 48 h before the experiment (stressed cells). On stimulation, naive cells showed a rapid five- to sevenfold increase in inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], accompanied by similar changes in Ins(1,3,4,5)P4, whereas both values increased only twofold in stressed cells. [3H]quinuclidinyl benzilate binding experiments revealed that these differences in inositol phosphate production were correlated with differences in the numbers of muscarinic acetylcholine receptors. Continuous recordings of [Ca2+]i revealed that Ca2+ release from intracellular stores upon stimulation was similar in both cell types, but the sustained [Ca2+]i signal (dependent on Ca2+ entry) was three times more pronounced in stressed cells. The results suggest that the adaptive differentiation of salt gland cells is associated with the increased expression of the receptor-mediated Ca2+ entry mechanism. In addition, the high rate of phosphoinositide hydrolysis in naive cells upon receptor activation may have a significance in cell growth, proliferation, and differentiation, which are elements of the development of the salt transport capabilities in these cells.