QUANTITATIVE-ANALYSIS OF LACTASE-PHLORHIZIN HYDROLASE EXPRESSION IN THE ABSORPTIVE ENTEROCYTES OF NEWBORN RAT SMALL-INTESTINE

At birth, the mammalian small intestine displays regional differences in morphology as well as complex proximal-to-distal (horizontal) patte rns of protein distribution. Lactase-phlorizin hydrolase (LPH), an ent erocyte-specific disaccharidase crucial for the digestion of lactose i n milk, reveals a characteristic horizontal pattern of expression at b irth. However, it is not certain whether this topographic pattern is d ue to variations in epithelial structure along the length of the small intestine or to regional differences in the transcription of the LPH gene. In order to understand the mechanisms that regulate the regional ization of LPH at birth, we characterized the epithelial structure alo ng the horizontal axis using stereologic techniques and correlated the se data with the patterns of lactase activity and LPH mRNA abundance i n the small intestine of unsuckled, newborn rats. Epithelial volume an d microvillar surface area per unit of intestinal length decreased thr ee-to fourfold from duodenum to distal ileum. In contrast, lactase act ivity and LPH mRNA abundance were highest in proximal jejunum and lowe st in the most proximal and distal ends of the small intestine. Mean l actase activity per cell in proximal duodenum, proximal jejunum, and d istal ileum was estimated at 12.0, 26.7, and 5.6 nU/absorptive enteroc yte, respectively, and paralleled the concentration of LPH mRNA in the same segments: 20, 45, and 15 molecules of LPH mRNA/absorptive entero cyte. Our data indicate that horizontal gradients of lactase activity in the newborn rat intestine do not depend on epithelial organization or on enteral factors, since the horizontal gradient is established be fore suckling. Each absorptive enterocyte along the small intestine ex presses lactase activity in a position-dependent manner which is contr olled at the level of mRNA abundance. (C) 1996 Wiley-Liss, Inc.