With the endothelium present, the maximum response of rat isolated aorta to clonidine was much lower than that to noradrenaline. Removal of endothelium enhanced the response to both adrenoceptor agonists and the clonidine-induced maximum contraction became almost equal to that produced by noradrenaline, although it was much more sensitive to inhibition by flunarizine and nifedipine. These results indicate that clonidine and noradrenaline activate receptors present in the endothelial cells and that these receptors are highly sensitive to clonidine.

The influence of endothelium on the response of rat isolated aorta to alpha-adrenergic agonists has been studied by comparing the response of intact and denuded preparations before and after treatment with calcium entry blockers flunarizine and nifedipine. Endothelium removal enhanced the response of the preparations, especially to alpha 2-agonists that had a weak effect in intact preparations. In the absence of endothelium, about 80% of the maximum response to clonidine was blocked by calcium entry blockers, whereas only 25% of the maximum response to noradrenaline was sensitive to them. In contrast about 40% of maximum noradrenaline-evoked contractions was sensitive to calcium entry blockers in the presence of endothelium. This may be attributed to an increase in the intracellular exchangeable calcium fraction, likely to be due to a slight depolarization of smooth muscle membrane of denuded preparations, which are highly sensitive to the calcium agonist BAY K 8644. The results indicate that a factor liberated by endothelial cells controls both calcium entry and calcium release evoked by alpha-adrenoceptor agonists in vascular smooth muscle.

1 Three stable epithelial cell lines (HCA-7, HCA-7-Col 1 and HCA-7-Col 3) all derived from the same human adenocarcinoma have been cultured on collagen-coated Millipore filters. These epithelial monolayers have been used to record short circuit current (SCC) in response to a variety of secretagogues. Similar monolayers, but grown on plastic dishes, were used for measurements of tissue cyclic AMP. 2 Lysylbradykinin, applied to either side of the monolayers, increased SCC in HCA-7 cells but had little effect on the other two lines. The responses showed rapid desensitization, which could be prevented by cooling to 4°C. Responses to kinin were not significantly attenuated by piroxicam, an inhibitor of cyclo-oxygenase. 3 Other secretagogues, vasoactive intestinal polypeptide (VIP) and carbachol also increased SCC in monolayers. The responses to VIP were greatest in HCA-7-Col 1 monolayers while responses were virtually absent in HCA-7-Col 3. A similar profile was seen with carbachol except that responses of HCA-7 and HCA-7-Col I monolayers were more equal. With one exception the responses to VIP and carbachol showed sidedness, acting only from the basolateral side. 4 The effects of the secretagogues were inhibited by piretanide, a loop diuretic, applied basolaterally. It is presumed that SCC responses represent electrogenic chloride secretion. 5 Treatment with forskolin increased SCC in HCA-7 and HCA-7-Col 1 monolayers with little effect in HCA-7-Col 3. Nevertheless cyclic AMP levels were elevated most in HCA-7-Col 3 and least in HCA-7-Col I monolayers, in reciprocal relationship to the functional response. 6 A23 187 increased SCC when applied to HCA-7 and HCA-7-Col 3 monolayers with little effect on HCA-7-Col 1. 7 The differential responses of the three human cell lines provide unique opportunities to discover the functional responsibilities of entities involved in the chloride secretory process. HCA-7-Col 3 cells which generate high levels of cyclic AMP in response to forskolin but which fail to show a substantial chloride secretory response may be a useful model of some disease conditions.

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1 The local formation of angiotensin II (All) from its precursors, angiotensin I (Al) and tetradecapeptide (TDP) renin substrate, was studied in intact (with endothelium) and rubbed (without endothelium) aortic rings of the rat. 2 AI and TDP renin substrate maximally contracted intact tissues in a similar way to All. The same observations were made in rubbed tissues. 3 The maximal response and the sensitivity of the aorta to these agonists were greater in rubbed than in intact tissues. 4 In intact preparations, methylene blue increased the contractile response to All and TDP to the same extent as endothelium removal. 5 In intact preparations, All receptor blockade completely suppressed all contractile responses, converting enzyme inhibition completely blocked the responses to AI and TDP, and renin inhibition partially blocked the responses to TDP. 6 In rubbed preparations, All receptor blockade completely inhibited all contractile responses, converting enzyme inhibition completely suppressed the responses to AI but only partially inhibited those to TDP, and renin inhibition partially blocked the responses to TDP. 7 In conclusion, the formation of All from TDP and its blockade by a converting enzyme inhibitor and a renin inhibitor shows that converting enzyme and a renin-like aspartic proteinase are present in the aortic wall. Furthermore, the results show that the endothelium is not essential for the conversion of the TDP to All, but modulates the responses to locally formed All through the release of endotheliumderived relaxing factor (EDRF).