Two independently regulated secretory systems within the acini of the submandibular gland of the perinatal rat

Abstract

Rat submandibular glands undergo a transient stage of differentiation in the perinatal acini. Two morphologically distinct cell types (Type I and Type III) appear and secrete several electrophoretically identifiable proteins. To examine the regulation of the secretion of these proteins and their association with secretion by the two cell types, 4-day-old glands were incubated with secretory agonists and antagonists, and the cultures were evaluated for secreted proteins in the medium and morphologically assayed degranulation of the cells. Stimulation of glands with beta-adrenergic agonists resulted in the secretion of three proteins, A, B1, and B2, at Rf 0.26, 0.36, and 0.40 in native, anionic gels at pH 8.3. Cholinergic stimulation caused the appearance of a different protein, C, at Rf 0.12, but only slight secretion of A, B1, and B2. With both beta-adrenergic and cholinergic stimulation, another protein, D, was seen near the origin. Partial purification of the proteins was effected by gel chromatography followed by preparative IEF (Isoelectric point focusing). Proteins A, B1, and B2 were recovered at isoelectric pH 4.5, 5.4, and 5.1 respectively. SDS gels of IEF fractions and secretion medium indicate Mr 25K, 26K, 27K, 97K, and greater than 150K for A, B1, B2, C, and D respectively. beta-Adrenergic stimulation caused degranulation only of Type I cells and cholinergic stimulation only of Type III cells. We propose the existence of two independently regulated secretory systems in the perinatal acini, with Type I cells secreting protein C on stimulation of cholinergic receptors, and Type III cells secreting A, B1, and B2 through beta-adrenergic receptors. With either stimulation, protein D is secreted, and thus it may serve a general function in the secretion process.