Effect of exteriorization of the sheep fetus on its cardiovascular function

Exteriorized sheep or goat fetuses, with placental circulation continuing, have been extensively used for measurements of fetal cardiovascular function. However, the effects of this procedure on fetal cardiovascular function have not been assessed. We inserted vinyl catheters into an umbilical vein, an umbilical or fetal hindlimb artery, and a hindlimb vein. We measured umbilical arterial and venous blood gas tensions, umbilical blood flow using the steadystate diffusion antipyrine method, and fetal arterial and umbilical venous pressures before and after delivery by cesarean section. Following exteriorization, placental vascular resistance consistently increased and umbilical blood flow decreased. Umbilical venous Po 2 increased, but umbilical arterial and venous pH and Pco 2 and arterial Po 2 did not change significantly. Thus exteriorization of the fetus produces major changes in the fetal circulation, but these changes are not indicated by any major alterations in fetal blood gases and pH.

Response of human umbilical artery to changes in transmural pressure

Human umbilical artery was perfused with Krebs-Ringer bicarbonate-glucose medium at 37 C and pH 7.4. An increase in transmural pressure elicited a vasoconstriction. The artery wall contained no significant amount of norepinephrine, which supports the anatomic evidence of absence of nerve supply. Thus, the response to stretch is due to a smooth muscle cell-borne positive feedback mechanism. The myogenic reflex could still be elicited at 44 C but was reversibly abolished by cooling to 4 C. It was reversibly abolished in the absence of calcium. K2SO4-Ringer solution (50 mm in K) markedly increased the tone of the artery after an initial vasodilatation and induced rhythmicity when the perfusion pressure was increased. Bradykinin, 5-hydroxytryptamine, and angiotensin caused vasoconstriction when the artery no longer responded to stretch. Stretch could induce vasoconstriction when angiotensin no longer elicited a response because of tachyphylaxis. The stretch reflex was not blocked by phenoxybenzamine which suppressed the response to 5-hydroxytryptamine. Tachyphylaxis was not observed with bradykinin; the effect of bradykinin was not blocked by phenoxybenzamine.