Effects of Fasting,Feeding,and Bethanechol Chloride on Pancreatic Microsomal Protein Synthesis in Vitro 1 2

Abstract

Introduction. It has been shown that feeding, bethanechol chloride, and cholecystokin-in-pancreozymin administration were associated with augmentation of pancreatic protein synthesis in rats and pigeons (1, 2). Increases in protein synthesis following bethanechol chloride and cholecystokinin-pancreozymin administration were of 1-4 hr duration, were not completely blocked by actinomycin-D pretreatment, and were not associated with parallel increases in incorporation of [ 3 H] uridine into nuclear-RNA (3-5). These characteristics suggested that “short-term” increases in pancreatic protein synthesis might be mediated at the translational level of control. On the other hand, augmentation of protein synthesis following refeeding was associated with increases in both [¹⁴C]-l-phenylalanine incorporation into protein and [ 3 H] uridine incorporation into nuclear-RNA (3, 6). These characteristics suggested that “long-term” increases might be mediated by changes occurring at both translational and transcriptional levels of control.

Protein synthesis at the translational level is influenced by amounts and activities of a number of constituents, such as amino acids, activating enzymes, t-RNA, initiating and terminating factors, monosomes, and polysomes. These cytoplasmic constituents concerned with protein synthesis at the translational level may be separated by use of high speed centrifugation into soluble and particulate fractions. The soluble or supernatant fraction contains factors such as amino acids, activating enzymes, t-RNA, etc.; the particulate or ribosomal fraction contains monosomes, polysomes, and constituents precipitated therewith.

The experiments presented here were designed to determine if increases in pancreatic protein synthesis following feeding or bethanechol chloride administration resulted from changes in factors isolated in the supernatant or particulate fraction of cytoplasm. It is anticipated that these simple experiments employing cell fractionation by high speed centrifugation and ordered in vitro incubation of cell sap and particulate fractions will provide preliminary information which will enable additional investigations directed at identification and isolation of selected constituents, i.e., changes in t-RNA, changes in activating enzymes or changes in ratios of monosomes–polysomes.