Cyclic AMP-reactive proteins in human saliva

Abstract

This study was conducted to compare cyclic AMP-reactive proteins (cARP), the secretory form of regulatory (R) subunits of cyclic AMP-dependent protein kinase (PKA), and cyclic nucleotide phosphodiesterase (PDE) activity in human whole saliva with that of parotid fluid. Additionally, experiments were done to determine whether secretory cARP is altered by environmental stimuli. Earlier work showed that R subunits are present in parotid fluid and in salivary glands of rats. No previous information is available about secretory PDE in saliva. Whole and parotid ductal saliva samples were collected by a non-invasive procedure from healthy volunteers. After photoaffinity labelling with [ 32P]-8-N 3cAMP, the R subunits were identified by autoradiography. Cyclic nucleotide PDE activity was measured as a function of the conversion of the cyclic nucleotide to the tritiated 5′-nucleotide. The results showed that R of the type II cAPK, RII ( M r 50–54 kDa) and/or a slower-moving isoform ( M r 54–56 kDa, RIIa) were present in all parotid saliva samples tested. Whole saliva was positive for RII in more than 95% of the samples tested ( n = 62), but with 50–90% reduction in concentration compared to parotid fluid. Both female and male subjects exposed to controlled auditory (60–80 dB) stimuli responded by a two- to five-fold increase in photoaffinity labelling of cARP (salivary RII, RIIa and RIIfr). There was considerable individual variability, but in all cases the differences in the results were significant ( p < 0.05, n = 20). Whole saliva showed measurable PDE activity in fresh or frozen samples, whereas no PDE activity was detected in parotid fluid. These findings indicate that, similar to rat parotid fluid, human salivary cARP are secretory proteins, but that the presence of RII, RIIa or their combination varies individually. In contrast, PDE is either not a secretory protein or is secreted from glands other than the parotid. As no PDE activity was found in the ductal fluid, cAMP is not hydrolysed in the secretory granule compartment, and cARP may serve as its carrier to the mouth.