Characterization of serotonin (5-HT) and glucose patterns and their hepatic receptor profiles during the transition period in dairy cows
The liver is crucial for metabolism and partitioning of nutrients to the mammary gland during lactation. Recent evidence suggests that non-neuronal serotonin (5-HT) participates in glucose metabolism, but little is known regarding 5-HT actions in the liver during the transition period in dairy cattle. Our objective was to explore glucose and 5-HT circulating patterns, and to characterize the 5-HT and glucose receptor profiles in the liver during the transition from pregnancy to lactation in dairy cows. Multiparous pregnant Holstein cows (n=6, avg. lactation=3.5) were utilized to collect daily blood samples from 7d pre-calving (-7d) through 7d post-calving (+7d) and liver biopsies were performed at -7d, +1d and +7d. Total RNA was extracted and cDNA was synthetized to measure mRNA expression of 5-HT receptors (HTR, isoforms 1A, 1B, 1D, 1F, 2A, 2B, 2C, 5A, and 7) and glucose transporters (SGLT-1, Glut-1, -2, -5, -8, -9 and 10) by RT-PCR. Glucose concentrations decreased pre-calving (-7 to -1d, 72 vs. 57 ± 3.5 mg/dl, P=0.045), increased post-calving (+1 and +3d, 74 ± 4.1 mg/dl, P=0.008), decreasing again and reaching lowest concentrations at +7d (50.2 ±1.7 mg/dl, P=0.0012). Serum 5-HT concentrations decreased abruptly pre-calving (-7 to -5d, 916 vs. 90 ± 35 ng/ml, P=0.001), remained low until +3d (160 ± 35 ng/ml, average for -3, -1 and +1d), and increased again at +5 and +7d (355 ± 27, average for +5 and +7d). Hepatic mRNA expression of HTR-1D, -2D and -7 were decreased while HTR-2A was increased at +1d compared to -7d (P<0.045). Only HTR-1F increased 2.5-fold at +7d, compared to both -7 and +1d (P<0.048). HTR-4 mRNA expression was undetectable at +7d, and HTR-2C was undetectable at +1d. HTR-5A was not expressed at all in the liver. Hepatic expression of Glut-2, Glut-5 and SGLT1 were decreased on both +1d and +7 compared to -7d, while Glut-1 was increased 2-fold at +7d compared to -7d (P<0.04). These results indicate that 5-HT can potentially play a role in liver glucose homeostasis during the transition period in dairy cows, possibly through the modulation of specific receptors. Additional research is needed to further explore the functional role of these HTRs in the liver during the transition from pregnancy to lactation.
5-hydroxytryptamine, liver, lactation