Isolated precontracted arteries of various vascular beds relax in response to acetylcholine only if the endothelium is present. One explanation for this is that this drug stimulates the endothelial cells to release a vasodilator substance that in turn relaxes the underlying smooth muscle. To determine whether this mechanism is concerned also in the acetylcholine contraction of isolated porcine coronary arteries transverse strips of the extramural part of the left circumflex artery were used for recording isometric tension in the muscle bath. Dose-response curves for acetylcholine showed no significant difference before and after removal of endothelium. As a functional check on the removal of endothelium, the responsiveness of each preparation to a known endothelium dependent dilator (substance P) was tested before and after removal. These findings suggest that endothelial cells are not concerned in the acetylcholine induced contraction of porcine coronary arteries. Acetylcholine appears to act directly on smooth muscle of the porcine artery.

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Based on results from measurements of arterial blood velocity, arterial and venous diameters of major segmental retinal vessels in normal persons and in patients with venous occlusive diseases and in continuation of the two preceding parts of this series of articles, further possibilities for the differential diagnosis of measurements of retinal microcirculation magnitudes are discussed. Whereas the measurement of blood velocity is an important criterion for the assessment of the stasis conditions and the arterial involvement in an occlusive disease, the diameters of the vessels offer essential suggestions to local regulative processes. In this connection, a dependence on pH of the contraction state of the smooth vascular musculature detected in porcine coronary arteries is presented. By its transmission to the arterial retinal vessels, it is possible to unequivocally clarify the local regulative and pathological behavior of arterial retinal vessels in terms of flow physiology.

Some analogues of bradykinin, especially with replacements by other amino acids of phenylalanine in position 8, have been investigated for enzymatic stability against kininase II from rat duodenum microsomes and rat uterus plasma membranes, respectively. As compared with bradykinin, two of the analogues, [8-erythro-beta-phenylserine]- and [8-erythro-alpha-Amino-beta-phenylbutyric acid]-Bradykinin were stable to enzymatic degradation. Therefore, the latter may be used for studies in hormone-receptor interaction.