Temporal dissociation in the ability of ouabain and furosemide to inhibit K+-transport in rat submandibular salivary gland slices at various postnatal ages

Developmental aspects of fluid and electrolyte secretion in salivary glands

The salivary glands of rodents undergo considerable cytodifferentiation after birth and are useful models for the study of functional development, including the mechanisms of fluid and electrolyte secretion. In the rat submandibular gland, secretion of salivary fluid cannot be elicited until approximately 2 weeks of age. The currently accepted model of salivary fluid secretion indicates that this process depends on the activation, on stimulation of cholinergic receptors, of several ion transport systems, resulting in a net transport of osmotically active ions (primarily Cl- and Na+) across the acinar epithelium. This creates the necessary osmotic gradient for the transacinar movement of water. The process is associated with a signal transduction pathway involving the formation of phosphoinositide products (primarily inositol triphosphate or IP3) and the mobilization of Ca2+. The latter regulates monovalent ion conductances (K+, Cl-), which are critical for the secretory process. Immature submandibular glands and cells of early postnatal rats have a lower density of cholinergic receptors and release less K+ and Cl- than mature cells and gradually develop other ion transport systems (such as a Na, K, 2Cl cotransport system) involved in the secretory process. Surprisingly, they form more IP3 and show a larger increase in cytosolic Ca2+ when stimulated with maximal or supramaximal concentrations of agonist. Therefore, they show some interesting dissociations in the signal transduction mechanism that suggest differences in the coupling between receptors and membrane phosphoinositides, between IP3 and IP3-dependent Ca2+ stores, and between the Ca2+ signal and the monovalent ion transport systems which are critical for secretion.

Intracellular [Ca2+] and K+ and Cl- efflux responses in submandibular cells of neonatal and adult rats

The effects of graded doses (5 x 10(-8) to 10(-5)) acetylcholine on intracellular Ca2+ and on 86Rb and 36Cl efflux were compared in submandibular cell clusters of 1 and 7 day-old and adult rats. Initial Ca2+ peaks were similar at agonists concentrations lower than 10(-7) M but the release of Rb+ and Cl- were smaller in cells of young animals. At higher agonist concentrations, Ca2+ peaks were higher in immature cells; however, initial Cl- (but not Rb+) efflux was similar to that of mature cells. Plateau Ca2+ levels were independent of age and agonist concentrations but the content of Cl- and Rb+ varied greatly and differences between age groups were less evident. These data confirm a dissociation between intracellular Ca2+ levels and Ca(2+)-mediated ion transport in immature salivary cells.

Intracellular pH changes induced by exposure to weak acids and bases in submandibular cells of early postnatal rats

The intracellular pH of submandibular cells of adult, 1-day-old and 1-week-old rats was measured with the pH sensitive fluorimetric indicator SNARF-1, following shifts induced by exposure to a weak acid (sodium butyrate) or a weak base (NH4Cl). The effects of several ion transport inhibitors and agonists on both the pH change and the rate of recovery were examined. The results indicate that, in most respects, the processes involved in pH regulation were similar in the neonatal and mature cells. However, cholinergic and peptidergic stimulation accelerated recovery from sodium butyrate-induced acidification more, and from NH4Cl-induced alkalinization less, in cell preparations from 1-day-old animals.

Na+ influx mechanisms in submandibular salivary cells of newborn rats

Uptake of this isotopic tracer was essentially the same as in cells derived from salivary glands of adult animals, both in the absence and in the presence of 1 microM acetylcholine, 10 microM monensin or 10 microM A23187. Tracer accumulation in resting cells was inhibited in the immature cells by 1 mM amiloride but the inhibition was significantly smaller than in fully mature cells (16 and 33%, respectively). Bumetanide inhibited tracer uptake by 28% in mature cells not exposed to agonist, but had essentially no effect in those of newborn rats. The tracer content of adult cells exposed to acetylcholine in Ca(2+)-free incubation solutions was significantly reduced when compared to that of cells exposed to agonist in solutions containing 1.0 mM CaCl2. A similar but significantly smaller reduction in tracer content was observed in cells of newborn animals incubated in the Ca(2+)-free medium. The inhibition of 22Na uptake observed in the presence of amiloride was the same in both types of cells in the presence or absence of external Ca2+. The findings suggest that Na entry into immature salivary cells of newborn rats is quantitatively similar to that in mature cells, but that the contribution of the loop diuretic-sensitive co-transporter and of the amiloride-sensitive Na/H exchange to Na+ entry is less in the immature cells. Na entry in the immature cells is likely to occur, therefore, primarily by other mechanisms such as Ca(2+)-regulated channels.

Effects of acetylcholine and transport inhibitors on K content in dispersed submandibular salivary cells of newborn and adult rats

These isolated cells were used to measure uptake of the radioactive tracer 86Rb as an index of changes in K+ content. There was time-dependent tracer uptake in the absence of secretagogue in both one-day-old and adult cells. Kinetic analysis of this uptake revealed two components, a fast one during the first 5 min of incubation and a slower one between 5 and 30 min. The early (fast) component was slower in the newborn cells (0.084 nmol mg-1 min-1) than in adult cells (0.116 nmol mg-1 min-1). Values for the second component of uptake were, respectively, 0.015 and 0.009 nmol mg-1 min-1. Basal uptake of K+ was inhibited by both ouabain and furosemide (at 1 mM concentrations) but, after 5 min of incubation, the effect was more pronounced in adult cells. The K+ content (i.e. K+ uptake) was increased after exposure to 1 microM acetylcholine in both cell types, but the effect was larger in those from one-day-old rats after 5 min incubation (16 per cent increase, compared to 9 per cent increase in adult cells). The secretagogue-induced increase in K+ uptake or content was inhibited by ouabain (57 per cent in adult cells, 24 per cent in newborn cells), and by furosemide (25 and 37 per cent, respectively). These findings suggest that K+ uptake in submandibular cells occurs by both an ouabain-sensitive Na, K pump and a diuretic-sensitive transport system. Both appear to have lower resting activities in the cells of newborn rats but can be significantly stimulated cholinergically.(ABSTRACT TRUNCATED AT 250 WORDS)

Na, K ATPase activity during early postnatal development of the rat submandibular gland

The activity of the ouabain-sensitive Na, K ATPase was measured in membrane fractions of the submandibular gland of 1-, 7-, 14- and 21-day-old rats. This activity increased with age and reached adult levels by 21 days.

36Cl fluxes in dispersed-cell aggregates of the submandibular salivary gland of early postnatal rats

Salivary cells were isolated from the gland of rats (1, 7, 14 and 21 days old) by enzymic digestion. They formed small aggregates with typical morphological characteristics and were functional units with characteristic 36Cl fluxes. The rates of isotopic uptake in the absence of secretagogue (basal uptake) were similar at all ages. Uptake was time-dependent, leading to a steady-state isotope content of 6.7-8.9 nmol/mg protein after 3-5 min of incubation. 36Cl accumulation (uptake) in the unstimulated condition was inhibited 33-39 per cent by exposure to 1 mM furosemide at all ages, but was reduced significantly less (50 per cent) by 1 microM acetylcholine in cells of 1-day-old animals than in older ones. In aggregates exposed to 36Cl for 12 min (preloaded with tracer), acetylcholine caused a net efflux of isotope which was significantly less in cells of 1-day-old rats (18 versus 35 per cent in cells of older animals). Thus it was shown (1) that functional cell aggregates can be obtained from early postnatal rats; (2) that when not stimulated, their time-dependent Cl accumulation is partially inhibited by loop diuretics, and so likely to involve a cation/anion cotransport system; and (3) that Cl efflux, which may occur by way of membrane channels, is significantly reduced early in the postnatal period. The putative Cl channels are therefore either not fully developed, or they do not become responsive to stimulation until later. This reduced Cl efflux could be related to differences in Ca2+ mobilization at early and late phases of postnatal glandular development as Cl efflux is a Ca2+-dependent phenomenon in salivary cells.