Sean W Limesand

PMID: 16636305;Abstract:

Developmental changes in ovine myocardial glucose transporters and insulin signaling following hyperthermia-induced intrauterine fetal growth restriction (IUGR) were the focus of our study. Our objective was to test the hypothesis that the fetal ovine myocardium adapts during an IUGR gestation by increasing glucose transporter protein expression, plasma membranebound glucose transporter protein concentrations, and insulin signal transduction protein concentrations. Growth measurements and whole heart tissue were obtained at 55 days gestational age (dGA), 90 dGA, and 135 dGA (term = 145 dGA) in fetuses from control (C) and hyperthermic (HT) pregnant sheep. Additionally, in 135 dGA animals, arterial blood was obtained and Doppler ultrasound was used to determine umbilical artery systolic (S) and diastolic (D) flow velocity waveform profiles to calculate pulsatility (S - D/mean) and resistance (S - D/S) indices. Myocardial Glut-1, Glut-4, insulin signal transduction proteins involved in Glut-4 translocation, and glycogen content were measured. Compared to age-matched controls, HT 90-dGA fetal body weights and HT 135-dGA fetal weights and gross heart weights were lower. Heart weights as a percent of body weights were similar between C and HT sheep at 135 dGA. HT 135-dGA animals had (i) lower fetal arterial plasma glucose and insulin concentrations, (ii) lower arterial blood oxygen content and higher plasma lactate concentrations, (iii) higher myocardial Glut-4 plasma membrane (PM) protein and insulin receptor β protein (IRβ) concentrations, (iv) higher myocardial glycogen content, and (v) higher umbilical artery Doppler pulsatility and resistance indices. The HT ovine fetal myocardium adapts to reduced circulating glucose and insulin concentrations by increasing plasma membrane Glut-4 and IRβ protein concentrations. The increased myocardial Glut-4 PM and IRβ protein concentrations likely contribute to or increase the intracellular delivery of glucose and, together with the increased lactate concentrations, enhance glycogen synthesis, which allows for maintained myocardial growth commensurate with fetal body growth. Copyright © 2006 by the Society for Experimental Biology and Medicine.

Complications in pregnancy elevate fetal norepinephrine (NE) concentrations. Previous studies in NE-infused sheep fetuses revealed that sustained exposure to high NE resulted in lower expression of α2-adrenergic receptors in islets and increased insulin secretion responsiveness after acutely terminating the NE infusion. In this study, we determined if the compensatory increase in insulin secretion after chronic elevation of NE is independent of hyperglycemia in sheep fetuses and whether it is persistent in conjunction with islet desensitization to NE. After an initial assessment of glucose-stimulated insulin secretion (GSIS) at 129 ± 1 days of gestation, fetuses were continuously infused for seven days with NE and maintained at euglycemia with a maternal insulin infusion. Fetal GSIS studies were performed again on days 8 and 12. Adrenergic sensitivity was determined in pancreatic islets collected at day 12. NE infusion increased (P  0.01) fetal plasma NE concentrations and lowered (P  0.01) basal insulin concentrations compared to vehicle-infused controls. GSIS was 1.8-fold greater (P  0.05) in NE-infused fetuses compared to controls at both one and five days after discontinuing the infusion. Glucose-potentiated arginine-induced insulin secretion was also enhanced (P  0.01) in NE-infused fetuses. Maximum GSIS in islets isolated from NE-infused fetuses was 1.6-fold greater (P  0.05) than controls, but islet insulin content and intracellular calcium signaling were not different between treatments. The half-maximal inhibitory concentration for NE was 2.6-fold greater (P  0.05) in NE-infused islets compared to controls. These findings show that chronic NE exposure and not hyperglycemia produce persistent adaptations in pancreatic islets that augment β-cell responsiveness in part through decreased adrenergic sensitivity.

PMID: 8924500;Abstract:

In their habitat, Fundulus heteroclitus (Cyprinodontidae) spawn against a variety of specific substrates that become available only at spring tides; they thus exhibit a semilunar reproductive cyclicity. In the laboratory at 28 ± 0.6°C and 14L:10D, this cyclicity is free running and can be observed by means of daily egg collection and frequent ovarian sampling. Daily egg collection cycles gathered from three different spawning groups were pooled chronologically; the cycles were determined by nonlinear regression sine- curve matching and so were dated from Days -6 to +6, with Day 0 as the peak egg collection day. Ovaries were sampled on Days -6, -4, -2, -1, +1, +2, and +4. The pooled data indicate that early-maturing follicles (1.3- to 1.6-mm diameter) are recruited from a constant reserve of vitellogenic follicles (0.7- to 1.2-mm diameter) and become abundant early in the cycle but are depleted during Days -6 to +4 by the formation of late-maturing follicles and ovulated eggs. A midcycle peak of ovulated eggs in the ovary corresponds to the egg collection peak. This progression of follicles followed by spawning decreases the total count of the follicles (≥ 0.7-mm diameter) and eggs in the ovary by 52% (from 454 to 219 per 10 g of female weight). The semilunar follicular cycle in this Fundulus model thus includes 1) an early-cycle follicle recruitment, 2) an early- and midcycle follicle maturation, and 3) a midcycle egg ovulation and spawning. This follicular cycle also proceeds in fish deprived of a spawning substrate. A female in the wild would thus execute the follicular cycling in a timely sequence, in anticipation of each recurring spring tide, whether or not a natural substrate becomes available. We conclude that F. heteroclitus provides a useful nonmammalian model for the study of cyclic reproductive activity in the laboratory.

Uterine inflammation results in a poor uterine environment and early embryonic loss in the mare due to an inhibition of maternal recognition of pregnancy caused from increased prostaglandin F2α (PGF2α). Oxytocin binds to endometrial cell receptors to activate prostaglandin synthesis. An oxytocin receptor antagonist (Atosiban) and a cyclooxygenase inhibitor (indomethacin) both decrease PGF2α production. The aim of this study was to evaluate the in vitro effects of Atosiban and indomethacin on equine uterine prostaglandin secretion. Equine endometrial explants were harvested on day two of behavioral estrus. Endometrial explant cultures were challenged with oxytocin (250nM) and PGF2α concentrations were measured over time. Explants were also cultured with Atosiban and indomethacin for 6h to determine the influence on PGF2α secretion. When endometrial explants were challenged with oxytocin, PGF2α concentrations were greater (P0.0001) at each time point over the 24h of culture as compared to controls. Oxytocin failed (P0.001) to elicit PGF2α release in explants cultured with either Atosiban or indomethacin. These findings show equine endometrial explants can be stimulated with oxytocin to increase secretion of PGF2α and this secretion can be inhibited through an oxytocin receptor antagonist and a Cox inhibitor, suggesting that this response to oxytocin involves an oxytocin receptor mediated event that activates the prostaglandin synthesis cascade through cyclooxygenase. Furthermore, this data suggests a role for the use of these inhibitors in vivo to decrease uterine PGF2α secretion and prevent early luteal regression and embryonic loss.