Effects of secretagogues on cytosolic Ca2+ levels in rat submandibular granular ducts and acini.

Saliva is thought to be formed by a two-stage process, with the secretion of a "primary fluid" by the acinar cells followed by various ionic modifications in the salivary ducts. Both of these processes are under the control of autonomic stimuli. Although the role of the acini in salivary secretion has been studied in some detail, little is known about properties of ducts, particularly the intralobular ducts that make up the bulk of the ductal tissue. In the present study, microfluorometric methods were employed to examine the responses of intracellular Ca2+ concentration ([Ca2+]i) in individual male rat submandibular acini and intralobular (granular) ducts to various fluid secretory stimuli. We show that granular ducts respond to muscarinic (carbachol) and alpha-adrenergic (epinephrine) stimulation by increasing [Ca2+]i in a manner that is qualitatively similar to acini, but that in contrast to acini, these ducts do not respond to substance P. Because the transduction of a substance P peptidergic signal typically occurs via increased [Ca2+]i, this observation suggests that there are no substance P receptors on granular ducts. Ducts were also found to be somewhat more responsive to both carbachol and epinephrine than acini. Although muscarinic, alpha-adrenergic, and vasoactive intestinal peptide (VIP) stimulation are known to induce the secretion of epidermal growth factor from granular ducts, no significant increase in ductal [Ca2+]i in response to VIP (10(-9) to 10(-6) M) was observed.