Appearance of venous sphincters in the pulmonary microvascular bed of normotensive and spontaneously hypertensive rats.

The appearance of pulmonary venous sphincters was studied in normotensive and spontaneously hypertensive rats using scanning electron microscopy of microvascular corrosion casts and transmission electron microscopy of tissue sections. Vascular casts were prepared either after lavage with Tyrode solution or after glutaraldehyde prefixation. Pronounced pulmonary venous sphincters were more frequently identified in spontaneously hypertensive rats as compared to corresponding circular indentations in normotensive rats. Tissue sections established venous sphincters in hypertensive animals as consisting of multiple layers of smooth muscle cells in the venous walls. We did not observe any autonomic nerve terminals in close proximity to these bundles of smooth muscle cells. The effect of various casting procedures on the appearance of venous sphincters is discussed. It is concluded that glutaraldehyde prefixation is an appropriate method to demonstrate sphincter functioning, because it causes deepening of sphincter indentations. Pulmonary vascular sphincters are thought to be governed by blood-born substances, vasoactive metabolites, or by tension of oxygen and carbon dioxide. Venous sphincters may influence microvascular flow in general and probably substitute for venous valves in the pulmonary vascular bed where valves are missing.