ELECTRON MICROSCOPICAL STUDY OF RELAXATION AND CONSTRICTION IN FROG ARTERIOLES

Mesenteric arterioles of frogs were selected under a dissecting microscope and compact vasoconstriction was induced by local application of a microdrop of epinephrine. Selected arterioles were fixed in situ at maximum constriction by flooding with buffered osmium tetroxide or glutaraldehyde. Fixation was faster than any vasoconstriction initiated by the onset of fixation. The arterioles were embedded in epoxy resin and regions of dilation and constriction were identified optically in the resin block. Ultrathin sections were cut at known levels for examination in the electron microscope. The localized constriction permitted the use of the relaxed, dilated region of the arteriole as a structural control for comparison against the fine structure of the constricted segment of the same arteriole. In constriction there was a dramatic change from the relaxed condition. The endothelial cells assumed a teardrop configuration and their nuclei protruded toward the lumen. The muscle cells and their nuclei became short and thick and demonstrated strongly indented borders, and the elastic membrane assumed a deeply pleated appearance. Both endothelium and smooth muscle cells appeared to adhere tightly to the elastic membrane for they faithfully replicated its convolutions. Bundles of thick (120 Å) and thin (70 Å) filaments were seen in the endothelial cytoplasm. The thick filaments were not contractile although the thin variety may shorten during vasoconstriction. Copyright © 1969 Wiley‐Liss, Inc.