Calcium dependence of histamine‐induced increases in capillary permeability in isolated perfused rat hindquarters

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Abstract
Experiments were performed on isolated maximally vasodilated perfused rat hindquarters to evaluate the role of calcium and magnesium for the capillary permeability increase(s) elicited by histamine. Changes in capillary permeability were quantified by determinations of capillary filtration coefficient (CFC) with gravimetric technique, and capillary diffusion capacity (PS) for vitamin B12(MW = 1355) with a single injection indicator dilution technique. During control, vascular resistance was 2.2 PRU100at a flow of 9.4 ml min_1per 100 g, and PS for B12was 3.7 ± 0.1 ml min‐1per 100 g, while CFC was 0.0377 ± 0.0004 ml min‐1mmHg‐1per 100 g. Perfusion with ‘Mg‐free’ solution for 1 h caused a 24% increase in CFC, while neither ‘Ca‐free’ perfusion nor perfusion with verapamil (5 x io‐5m) nor felodipine (1 ± 10‐6m) induced any changes in CFC. Histamine (100–200 μM) caused in all preparations a 150–200 % increase in CFC with only small changes in PS for B12. This histamine effect was absent after 1 h of ‘Ca‐free’ perfusion and was partially blocked after 1 h of perfusion with o. 1 mM calcium, while the calcium antagonists verapamil and felodipine had no effects on the histamine‐induced changes. The results imply that histamine exerts its action on the endothelial cells through a calcium‐dependent process, probably involving low affinity calcium sites but this process could not be inhibited by the calcium antagonists used. Thus, endothelial cell contractility, which probably is responsible for the histamine‐induced increase in capillary permeability, exhibits unique characteristics, differing from those of vascular smooth muscle.

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