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Central Sympathetic Control of Uterine Blood Flow during Acute Heat Stress¹

University of Illinois³, Urbana 61801

Abstract

The role of central sympathetic regulation of decreased uterine blood flow (UBF) during acute heat stress was investigated. An electromagnetic blood flow probe was surgically implanted around the left miduterine artery and a catheter inserted into the right carotid artery of ewes between days 120 and 135 of gestation. Ewes were individually housed in controlled temperature metabolic chambers. Four or more days after surgery, ewes were fitted with instruments to measure rectal temperature (Tr), heart rate (HR), respiratory rate (RR), blood pressure (BP) and UBF. One-half hour after calibration of instruments, a 15-min thermoneutral control period was carried out during which carotid artery blood samples were taken at 5-min intervals for PCO₂ and pH determinations. Ewes were then subjected to a heat challenge that reached 40 C after 2 hours. All data were recorded every 5 min as 1-min average values. Arterial blood sampling continued at 5-min intervals. In nine experiments, a sympathetic ganglionic blocking drug, hexamethonium (H), was infused into six ewes at 16 mg/min for 5 min after depression of UBF. In seven experiments, physiological saline (S) was infused into five ewes at 15 ml/min when UBF was depressed. Analysis of variance comparisons within H and S experiments were made between control period values and values obtained at 0, 5 and 10 min after the start of infusions. A second analysis of variance compared S and H treatments at 0, 5 and 10 min from the start of infusions. At the start of H and S infusions, Tr, RR and pH were elevated while PCO₂, BP and UBF were depressed (P<.05) in relation to the thermoneutral period values. Blood pressure and HR were depressed after H but not S infusion-, while UBF remained at the depressed preinfusion rate after infusion of both H and S. Uterine vascular resistance (UVR) remained at the thermoneutral level after H and S infusion despite a 50% reduction in UBF. These data suggest that central sympathetic transmission is not a major component in the control of UBF during acute heat stress.

¹ The authors express appreciation to Ms. Sharon Sittler, Ms. Meredith Magers and Ms. Valarie Hedden for their assistance in the research.

² Present address: Div. of Anim. Sci., Univ. of Nevada, Reno, 89557.

³ Dept. of Anim. Sci.