Somatostatin inhibits pepsinogen secretion without influencing cytosolic free Ca2+ increase induced by carbachol and cholecystokinin octapeptide in rat chief cells

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.

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.