Comparison in vitro of the incorporation of [3H]-leucine and N-acetyl-[14C]-mannosamine into proteins and glycoproteins of the parotid, submandibular and sublingual glands of the mouse

Morphological and biochemical changes in mucous cells of the murine sublingual salivary gland during the carbamylcholine-induced secretory cycle

Changes in these cells have been evaluated over 6 h following cholinergic stimulation. Carbamylcholine administration resulted in the release of almost 50 per cent of secretory material within 15 min, which caused a reduction of 33 per cent in cell size. After 2 h the cells were depleted of secretory material. However, in the second hour the release of secretory material was accompanied by an enlargement of the nucleus, Golgi complexes and rough endoplasmic reticulum (RER), which suggests an elevation of biosynthetic activity. The enlargement of the RER was not the result of an increase in RNA, i.e. in the number of ribosomes, but of dilatation of its cisternal spaces. Before release took place, there was a continuous coalescence of secretory granules. After this extensive fusion, which is probably the result of an altered physiological state of the granule membrane and subsequent water uptake caused by cholinergic stimulation, the viscous mucins could be squeezed out, water transport is likely to assist in this ejection. Refilling of the mucous cells was almost complete within 6 h after stimulation.

Metabolism of sialic acids from exogenously administered sialyllactose and mucin in mouse and rat

A mixture of N-acetyl-[4,5,6,7,8,9-14C]neuraminosyl-alpha (2-3(6]-galactosyl-beta (1-4-glucose[( 14C]sialyl-lactose) and N-acetylneuraminosyl-alpha (2-3(6]-galactosyl-beta(1-4)-glucit-1-[3H]ol(sialyl-[3H]lactitol) as well as porcine submandibular gland mucin labeled with N-acetyl- and N-glycoloyl-[9-(3)H]neuraminic acid were administered orally to mice. The distribution of the different isotopes was followed in blood, tissues and excretion products of the animals. One half of the [14C]sialyl-lactose/sialyl-[3H]lactitol mixture given orally was excreted unchanged in the urine. The other half was hydrolysed by sialidase and partly metabolized further, followed by the excretion of 30% of the 14C-radioactivity as free N-acetyl-[4,5,6,7,8,9-14C]neuraminic acid and 60% of this radioactivity in the form of non-anionic compounds including expired 14CO2 within 24 h. The 14C-radioactivity derived from the [14C]sialyl-lactose/sialyl-[3H]lactitol mixture which remained in the bodies of fasted mice after 24 h was less than 1%. In the case of well-fed mice, a higher amount of the sialic acid residues was metabolized. The bulk of radioactivity of the mucin was resorbed within 24 h. About 40% of the radioactivity administered was excreted by the urine within 48 h; 30% of this radioactivity represented sialic acid and 70% other anionic and non-anionic metabolic products. 60% of the radioactivity administered remained in the body, and bound 3H-labeled sialic acids were isolated from liver. Sialyl-alpha (2-3)-[3H]lactitol was injected intravenously into rats; the substance was rapidly excreted in the urine without decomposition. These studies show that part of the sialic acids bound to oligosaccharides and glycoproteins can be hydrolysed in intestine by sialidase and be resorbed. This is followed either by excretion as free sialic acid or by metabolization at variable degrees, which apparently depends on the compound fed and on the retention time in the digestive tract.

Influence of autonomic agonists on the in vitro incorporation of [3H]leucine and N-acetyl[14C]mannosamine into submandibular action of the mouse

The influence of isoproterenol and pilocarpine on the in vitro incorporation of [3H]leucine and N-acetyl[14C]mannosamine into the proteins of the submandibular glands of the mouse has been investigated during a 10 h period. The total uptake of both labelled precursors into the glands was hardly affected by isoproterenol and pilocarpine during the first 2 h of incubation, thereafter both agonists decreased the uptake slightly. The incorporation of [3H]leucine into secreted proteins was largely similar for the control, isoproterenol and pilocarpine during an incubation of 10 h. [14C]ManNAc incorporation showed a lag period of about 2 h and could be observed in the secreted proteins after 2 h. Particularly after 6 h a strong increase was observed for the control and isoproterenol, whereas pilocarpine showed a much lower increase. The secreted protein components were separated by electrophoresis to study the incorporation of the labelled precursors in separate secretory proteins such as submandibular mucin. Apparently, both agonists increased the incorporation of [14C]ManNAc relative to [3H]leucine into submandibular mucin of the mouse. During a period of 10 h the [14C]ManNAc incorporation into the mucin was enhanced 2-3-fold by isoproterenol and 3-4-fold by pilocarpine. A non-radioactive experiment in vitro showed that the molar ratio of the sugar residues did not change. However, the total amount of sugars relative to the amino acids increased by 50%, pointing to an increase in the degree of glycosylation. This suggests that both adrenergic and cholinergic agonists regulate the total number of carbohydrate chains attached to one and the same polypeptide core of the submandibular mucin of the mouse.

Diurnal variation of sialomucin concentration in female mouse submandibular glands measured by radioimmunoassay

The validity of a radioimmunoassay (RIA) for the predominant sialomucin from female mouse submandibular glands was confirmed by immunological and non-immunological approaches. The maximum sensitivity was in the low nanogramme range. The RIA did not cross-react with parotid, sublingual, trachea, stomach or colon tissues, but showed incomplete cross-reactivity with duodenal tissue. An analysis of protein, sialic acid and mucin contents of submandibular glands obtained during diurnal samplings, indicated that mean sialic acid and mucin concentrations, relative to protein in sample groups, decreased overnight approx. 30 per cent and showed a net increase during the day. Sialic acid per gland showed a 3-fold variation among individuals in the population; mucin showed greater than 20-fold variation. An analysis of variation indicated that influence by time of day was the greatest factor affecting sialic-acid concentrations and was statistically significant but, variation in mucin concentration was not. Variation between individuals accounted for more than 90 per cent of the total population variation in mucin concentrations and in sialic acid to mucin ratios. Regression analyses of the sialic acid to mucin ratios at different times of day suggested that the rates of replenishment of sialic acid and mucin may be co-ordinated.

Comparison in vitro of the incorporation of D-[2-3H(N)]-mannose and D-[1-14C]-glucosamine into glycoproteins of dispersed rat submandibular salivary gland cells

The incorporation of two different radiolabelled sugars, D-[14C]-glucosamine and D-[2-3H(N)]-mannose, into cellular and secretory glycoproteins was compared using dispersed rat submandibular cells. Most of the de-novo biosynthesis appeared to be directed toward the synthesis of secretory material as the molecular profile of the 3H-labelled material released following sympathomimetic stimulation and the percentage of total 3H-labelled acid--precipitable material secreted following cholinergic- or adrenergic-receptor stimulation coincided with the data obtained from similar studies using [14C]-glucosamine. The [3H]-mannose label was found in the neutral sugars mannose, galactose, glucose and fucose, with trace amounts of radiolabel in the amino sugars, whereas the [14C]-glucosamine label was present in three different amino sugars; glucosamine, galactosamine and sialic acid.

Influence of isoproterenol on the incorporation in vitro of [3H]-leucine and n-acetyl-[14C]-mannosamine into proteins and glycoproteins of the parotid glands of the mouse

The in-vitro incorporation of [3H]-Leu in murine parotid glands started rapidly and continued during the whole incubation period of 10 h and was not stimulated significantly by isoproterenol, except during the first half hour. Secretion of [3H]-Leu incorporated was already observed after 30 min and continued up to 10 h. Isoproterenol stimulated the secretion of 3H-labelled protein during the whole period, but maximally after 4 h. Incorporation of [14C]-ManNAc proceeded at a much slower rate than that of [3H]-Leu. It was at least 1 h before incorporated [14C]-ManNAc appeared in the incubation medium. The secretion of [14C]-ManNAc incorporated was increased by isoproterenol between 2 and 6 h. A major part of [3H]-Leu was incorporated in amylase. The electrophoretically slowest moving isoenzyme of amylase incorporated substantial amounts of [14C]-ManNAc, in agreement with the presence of sialic acid in the isoenzyme. The onset of the [14C]-ManNAc uptake in amylase was slow compared to [3H]-Leu uptake. The [14C]-label could be detected in the secreted amylase after at least 4 h of incubation. It is suggested that isoproterenol decreased the incorporation of [14C]-ManNAc relative to that of [3H]-Leu into the secreted amylase.