A novel membrane form of guanylyl cyclase (GC-G) has been identified through the isolation of a full-length cDNA clone; it is predicted to contain an extracellular ligand binding domain, a single transmembrane segment, and intracellular protein kinase-like and cyclase catalytic domains. That GC-G represents a guanylyl cyclase was confirmed by both transient expression in COS-7 cells and stable expression in H293 cells. Endogenous cyclic GMP concentrations of transfected or stable cells, however, were much higher than control cells, suggesting an inability of the cells to effectively regulate GC-G cyclase activity. Of six Cys residues found within the extracellular domain of guanylyl cyclase-A (GC-A), the receptor for atrial natriuretic peptide, five are conserved within GC-G. Ligands for the other cyclase receptors, nevertheless, failed to stimulate GC-G expressed in transient or stable cells, suggesting that the unknown ligands possess a structure different from the natriuretic peptides or heat-stable enterotoxins. 125

In ruminants, pancreatic alpha-amylase is the primary enzyme responsible for the initial hydrolysis of alpha-linked glucose in the small intestinal lumen. The objective of this experiment was to examine the effects of altered dietary starch and energy supply on the expression of pancreatic alpha-amylase mRNA, protein and activity in lambs. Wether lambs (n = 24; 28 +/- 0.5 kg body weight) were fed low or high starch diets at 1.2 or 1.8 x net energy of maintenance for at least 28 d before tissue collection. Lambs fed the high energy/high starch diet tended to have more pancreatic alpha-amylase protein (54.5 kDa; P: = 0.08) and had greater activity (P: = 0.03), but alpha-amylase mRNA (1.6 kb) tended to be lower (P: = 0.17). Additionally, rumen fluid total short-chain fatty acid concentration was greater (P: = 0.04) and plasma glucose concentration tended to be greater (P: = 0.07) in lambs fed the high energy/high starch diet. However, pancreatic trypsinogen protein (25. 5 kDa) and jejunal maltase activity were not influenced by dietary treatment, suggesting that different regulatory systems are involved in regulating the tissue protein or activity levels of these two enzymes compared with alpha-amylase. These data suggest that dietary regulation of pancreatic alpha-amylase expression in ruminants is complex and probably regulated by transcriptional and post-transcriptional events.

Glutamate metabolism is essential to support many facets of metabolism. The objective of this study was to determine the tissue distribution of glutamate transporters known to support the tissue metabolism of glutamate. The expression of proteins capable of high-affinity glutamate transport (system X-(AG)) by epithelia isolated from the rumen, omasum, duodenum, jejunum, ileum, cecum, and colon and homogenates of liver, kidney, and pancreatic tissues from wethers (n = 4; BW = 28.4 +/- 8.4 kg) and steers (n = 3; BW = 426 +/- 32.3 kg) fed forage-based diets was evaluated by immunoblot analysis. Proteins EAAC1 (62, 93 kDa) and GLT-1 (142, 188, >202 kDa) were expressed by every tissue examined. In contrast, GLAST1 (140 kDa) was expressed only by the pancreas, and EAAT4 (67 kDa) was detected only in sheep brain. To corroborate protein expression data, the presence and size of transporter mRNA in ileal, liver, and pancreatic homogenates were evaluated by Northern analysis. GLAST1 mRNA (2.4, 4.3 kb) was detected only in the pancreas, whereas EAAC1 (2.2, 2.8 kb) and GLT-1 (12.1 kb) mRNA transcripts were detected in all three tissues. The expression of EAAT4 and GLT-1 mRNA was confirmed by reverse transcriptase-polymerase chain reaction analyses. Sequencing of the resulting partial-length ovine GLT-1 cDNA revealed 100% identity with the rat homolog. Overall, these data demonstrate that sheep and cattle share the same pattern of system X-(AG) transporter expression, which differed among tissues and transporter isoforms. Accordingly, these data provide the fundamental knowledge to initiate research that determines whether the expression of high-affinity glutamate transporters by ruminants is sensitive to ontogenic and(or) dietary regulation.

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