Quantitative analysis of lactase-phlorizin 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) patterns of protein distribution. Lactase-phlorizin hydrolase (LPH), an enterocyte-specific disaccharidase crucial for the digestion of lactose in milk, reveals a characteristic horizontal pattern of expression at birth. However, it is not certain whether this topographic pattern is due 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 regionalization of LPH at birth, we characterized the epithelial structure along the horizontal axis using stereologic techniques and correlated these data with the patterns of lactase activity and LPH mRNA abundance in the small intestine of unsuckled, newborn rats. Epithelial volume and microvillar surface area per unit of intestinal length decreased three-to fourfold from duodenum to distal ileum. In contrast, lactase activity and LPH mRNA abundance were highest in proximal jejunum and lowest in the most proximal and distal ends of the small intestine. Mean lactase activity per cell in proximal duodenum, proximal jejunum, and distal ileum was estimated at 12.0, 26.7, and 5.6 nU/absorptive enterocyte, respectively, and paralleled the concentration of LPH mRNA in the same segments: 20, 45, and 15 molecules of LPH mRNA/absorptive enterocyte. 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 before suckling. Each absorptive enterocyte along the small intestine expresses lactase activity in a position-dependent manner which is controlled at the level of mRNA abundance.