Modulation by somatostatin of rat submandibular salivary secretion.

Although somatostatin (somatotrophin release inhibitory factor; SRIF) is a well-known inhibitory peptide, there are only a few reports of it acting as a positive modulator. In this work, the action of somatostatin upon rat submandibular protein secretion was studied. In vivo somatostatin infusion (35 microg/(kg h)) raised protein secretion stimulated by adrenergic and peptidergic agents. To rule out possible systemic effects of somatostatin, in vitro experiments were performed. Somatostatin (90 nmol/l) augmented protein release stimulated by noradrenaline (19 micromol/l) and substance P (10 micromol/l), but it did not affect isoprenaline (400 micromol/l)-induced protein release. Phenoxybenzamine (20 micromol/l) reduced the effect of somatostatin on noradrenaline-stimulated protein release. Propranolol (20 micromol/l) increased the noradrenaline-stimulated protein release and this effect was synergistic with the action of somatostatin. The absence of extracellular calcium did not significantly reduce somatostatin enhancement of agonist-induced secretion. Fluorescence measurements of the Ca(2+)-sensitive dye fluo3 showed that cytosolic calcium in acinar cells remained elevated during stimuli when somatostatin was present in the medium. It was concluded that somatostatin modulates rat submandibular protein secretion by prolonging the time that the cytosolic calcium signal remains high after stimulus.

Kinetic analysis of a model of the sarcoplasmic reticulum Ca-ATPase, with variable stoichiometry, which enhances the amount and the rate of Ca transport.

The sarcoplasmic reticulum (SR) Ca-dependent adenosinetriphosphatase (Ca-ATPase) actively transports Ca2+ from the myoplasm to the SR lumen. Under optimal conditions a 2:1 stoichiometry of Ca transport/ATP hydrolysis has been observed, but lower stoichiometries have been reported under several circumstances. A lower stoichiometry under conditions of high Ca2+ load, although thermodynamically less efficient, could in theory increase the rate and the maximal amount of Ca uptake. We analysed, by computing simulation, the transient kinetics of a model of the SR Ca-ATPase with variable stoichiometry. The model is based on current experimental reports and includes the most relevant properties of the system. The results show an acceleration in the rate of Ca uptake, an increase in the net Ca transport, and an increase in the rate of [Ca2+] reduction in the medium, which might be physiologically useful to increase the rate of Ca pumping at high Ca load of the sarcoplasmic reticulum.

Modulation by somatostatin of rat parotid salivary secretion stimulated by cholinergic, adrenergic and peptidergic agents.

Although it is well known that somatostatin (SRIF) modulates several digestive functions, there are only a few reports about its effect on the salivary glands. Here, the action of SRIF parotid secretion was studied, in vivo and in vitro, in male Wistar rats. In vivo SRIF infusion (35 microg/kg per hr) inhibited the parotid flow rate stimulated by methacholine, substance P and noradrenaline. The isoprenaline-stimulated flow rate was also decreased by SRIF, but only at highest dose of the secretory agent. Total protein and amylase secretion were studied. SRIF inhibited the total protein secretion stimulated by the above-mentioned agents, except that by isoprenaline. SRIF did not inhibit in vivo amylase secretion. In order to avoid flow-rate interference with total protein and amylase measurements, in vitro experiments were performed. SRIF (25 nM) strongly inhibited the total protein release stimulated by methacholine (5.1 microM), noradrenaline (19 microM), and substance P (10 microM). The inhibitory effect was not raised by the absence of calcium in the incubation medium. However, in vitro amylase release was not affected by SRIF. It was concluded that SRIF modulates rat parotid secretion stimulated by cholinergic, adrenergic and peptidergic agents, acting on any step in the calcium pathway.

Calcium additional to that bound to the transport sites is required for full activation of the sarcoplasmic reticulum Ca-ATPase from skeletal muscle.

The sarcoplasmic reticulum Ca-ATPase is fully activated when approximately 1 microM [Ca2+] saturates the two transport sites; higher [Ca] inhibits the ATPase by competition of Ca-ATP with Mg-ATP as substrates. Here we describe a novel effect of EGTA and other chelators, raising the possibility of an additional activating effect of Ca in the sub- or low microM range. Sarcoplasmic reticulum membranes were isolated from rabbit skeletal muscles. The ATPase activity was measured after incubation at 37 degreesC in 3 mM ATP, 3 mM MgCl2, 50 mM MOPS-Tris (pH 7.2), 100 mM KCl, and variable CaCl2, EGTA and calcimycin. In the absence of added EGTA and Ca the ATPase activity is high due to contaminant Ca. The determination of the ATPase activity in the presence of increasing amounts of EGTA, without added Ca, yields a decreasing sigmoidal function. Ki ranged between 20 and 100 microM, depending on the enzyme concentration. Pi production is linear with time for several [EGTA] yielding suboptimal ATPase activities, which are inhibited by thapsigargin. These suboptimal Ca-ATPase activities are inhibited by preincubation of the enzyme in EGTA, at pH 7.2. This effect increases upon increasing EGTA concentration and preincubation time. The inhibitory effect of the previous exposure of the enzyme to EGTA is partially but significantly reverted by increasing [Ca2+] during incubations. Calcimycin and EDTA have similar effects as EGTA when added in preincubations. The effect of calcimycin is fully reverted by optimal [Ca2+] in incubations. The effects of EGTA, EDTA and calcimycin in preincubation are not additive. The results suggest that an additional calcium, lost during preincubations from a site with affinity near 1 microM, is necessary for full activation of the ATPase.

Thyrotropin receptor antibodies in parotid saliva.

Parotid saliva was collected with a Carlson-Crittenden device, under citric acid stimulation, in 18 patients with autoimmune thyroid disease. Thyrotropin Receptor Antibodies (TRAb) were measured with a radioreceptor assay in parotid saliva and in serum in the same patients, and a statistical analysis of the data was performed. TRAb levels in parotid saliva were higher than in serum in the 3 pathologies studied (Graves' disease, Hashitoxicosis and Hashimoto's thyroiditis). There was good correlation between salivary and serum levels.