Elevation of intracellular cAMP by noradrenaline and vasoactive intestinal peptide in striated ducts isolated from the rabbit mandibular salivary gland.

Salivary gland intralobular ducts are responsible for the modification of the electrolyte composition of the primary fluid secreted by the acini. However, the intracellular messengers that regulate this and other intralobular duct cell processes have not been fully characterized. To investigate the possibility that cAMP-mobilizing agonists may be involved, intralobular (striated) ducts were isolated from the rabbit mandibular salivary gland by tissue dissociation and microdissection and maintained in tissue culture overnight. Individual duct fragments were stimulated with the secretory agonists noradrenaline, vasoactive intestinal peptide (VIP) and substance P and their cAMP content measured by acetylated radioimmunoassay. Both noradrenaline and VIP elevated intracellular cAMP content concentration dependently, but substance P did not. The response to noradrenaline was blocked by the beta-adrenoceptor antagonist propranolol, but not by the alpha-adrenoceptor antagonist prazosin. Application of the VIP analogue [D-p-Cl-Phe6, Leu17]-VIP decreased the VIP-induced cAMP response. These results demonstrate that striated intralobular duct cells possess beta-adrenoceptors and peptidergic receptors that are coupled to adenylate cyclase and activated by noradrenaline and VIP, respectively. By elevating ductal cAMP content, these agonists may regulate both the electrolyte content of the primary saliva and the secretion of protein(s) from the ducts.

Structural and functional characterization of striated ducts isolated from the rabbit mandibular salivary gland.

Ductal elements within salivary glands are responsible for modifying the electrolyte composition of primary saliva secreted by the acini. To study the mechanism and regulation of the transport processes involved requires a suitable preparation of functional ducts. To this end we have isolated intralobular ducts from rabbit mandibular salivary glands using the technique of tissue dissociation and microdissection. Light and electron microscopy demonstrated that the ducts corresponded ultrastructurally to striated intralobular ducts of the intact gland. Ducts could be maintained in tissue culture on polycarbonate filter rafts for up to 36 h, during which time the ends of the ducts did not usually seal. The overall resting content of ductal adenosine 3',5'-cyclic monophosphate (cyclic AMP) was 16.0 +/- 3.0 fmol mm-1 and increased dose dependently in response to stimulation with the beta-adrenoceptor agonist isoprenaline (10(-9)-10(-4) M; concentration required to produce a half-maximal response, K0.5 = 2.1 x 10(-6) M). The response to isoprenaline was blocked by the antagonist propranolol. Intracellular cyclic AMP content was also raised by the adenylate cyclase activator forskolin and by prostaglandin E2. Acetylcholine (3 x 10(-8)-10(-5) M) caused a dose-dependent and maintained rise in [Ca2+]i (K0.5 = 2.5 x 10(-7) M). This increase in [Ca2+]i could be reversed by the muscarinic antagonist atropine and appeared to result from a combination of mobilization of intracellular Ca2+ stores and entry of Ca2+ from the extracellular fluid. Noradrenaline induced only a very small, mainly transient rise in [Ca2+]i while phenylephrine failed to increase [Ca2+]i at all. Vasoactive intestinal peptide (5 x 10(-7) M) also produced a marginal, maintained rise in [Ca2+]i. Substance P, bombesin, isoprenaline, and prostaglandin E2 did not elevate [Ca2+]i. Application of the calcium ionophore ionomycin induced a substantial maintained rise in [Ca2+]i. Taken together, these results indicate that isolated and cultured striated ducts (i) possess intact beta-adrenoceptors coupled to adenylate cyclase, putative receptors for prostaglandin E2 and muscarinic receptors, and (ii) represent a viable preparation for the study of the transport mechanisms involved in the ductal modification of salivary fluid composition.

Seasonal variations in the fine structure of the Necturus maculosus urinary bladder epithelium: low transporters and high transporters.

Although the urinary bladder of Necturus maculosus provides an important model system for studying the mechanisms of active Na absorption, little critical attention has been paid to the fine structure of its epithelium. Moreover, two distinct groups of urinary bladders, low and high Na transporters, have been described based on short-circuit current or transepithelial potential difference. In the present study, over an 11-month period, stable electrical parameters (short-circuit current, transepithelial potential difference, and resistance) were recorded from 63 chamber-mounted bladders. Analysis of these parameters revealed a highly significant difference between two groups (low transporters and high transporters) occurring at different times of the year. Consistent with these data, in urine collected from the bladders, the Na concentration in low transporters was significantly higher than that in high transporters. A subpopulation of these bladders was subsequently fixed and examined at the light and/or electron microscopic level. Low-transporting bladders were characterized unequivocally by a thin, stratified squamous epithelium only 6-15 micron thick. High-transporting bladders were composed predominantly of columnar-shaped granular cells up to 70 micron in height, with ciliated, mitochondria-rich, and basal cells present in small numbers. There is thus a correlation between transport activity, as measured by electrophysiological techniques and urine sodium analysis, and the structure of the tissue. Moreover, these parameters exhibit significant seasonal variation, the underlying mechanisms of which remain obscure.