Abstract. In the present study, we investigated structural and functional aspects of stress fibers in a cell type in situ, i.e., the sinus endothelium of the human spleen. In this cell type, stress fibers extend underneath the basal plasma membrane and are arranged parallel to the cellular long axis. Ultrastructurally, the stress fibers were found to be composed of thin actinlike filaments (5-8 nm) and thick myosin-like filaments (10-15 nm x 300 nm). Actin filaments displayed changes in polarity (determined by S-l-myosin subfragment decoration), which may allow a sliding filament mechanism. At their plasmalemmal attachment sites, actin filaments exhibited uniform polarity with the S-1-arrowhead complexes pointing away from the plasma membrane. Fluorescence microscopy showed that the stress fibers have a high affinity for phalloidin and antibodies to actin, myosin, tropomyosin, and a-actinin. Vinculin was confined to the cytoplasmic aspect of the plasmalemmal termination sites of stress fibers, while laminin, fibronectin, and collagens were located at the extracellular aspect of these stress fiber-membrane associations. Western blot analysis revealed polypeptide bands that contained actin, myosin, and a-actinin to be major components of isolated cells. Exposure of permeabilized cells to MgATP results in prominent changes in cellular shape caused by stress fiber contraction. It is concluded that the stress fibers in situ anchored to cell-to-extracellular matrix contacts can create tension that might allow the endothelium to resist the fluid shear forces of blood flow.S TRESS fibers represent the most prominent structural component of the actin filament system of most cell types in tissue culture. Their molecular structure and dynamics in various processes of cell biology are subjects of much recent interest (for review, see references 31, 33, and 50). Previous studies have demonstrated stress fibers to occur also in certain cells in situ such as in flattened fibroblasts located underneath scales of fish (10) or in endothelial cells of the vascular system (14,28, 65,67). These observations demanded more detailed studies regarding the degree of molecular relationship between stress fibers in vitro and in situ, including their association with components of the extracellular matrix. Apart from these more general aspects, a major concern of the present study was to obtain more detailed insight into structural and functional aspects of vascular endothelial stress fibers, which obviously play a crucial role in maintaining intimal integrity (24, 26-28, 65, 67).In search of a source of vascular endothelium suitable for these studies, we found that endothelial cells of the sinus vessels in the human spleen have many advantages in comparison to endothelial cells of other vascular segments (14,15). These advantages are the uniform rod-shaped morphology and parallel alignment of cells, the existence of welldefined sites of contacts to the extracellular matrix, and importantly, a highly ordered pattern of stress fibers. Previo...

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In isolated, electrically driven right auricular strips of the human heart the inotropic effect of phenylephrine was studied. 1. First, the influence of the driving rate on the tension developed (i.e., the frequency-force relationship) was determined by stimulation of the preparations at 0.1, 0.5, 1, 2 and 3 HZ. The force of contraction was lowest at a stimulation rate of 0.1 HZ (36.9 g/g dry weight). The maximally developed force of contraction observed at frequencies of 0.5, 1 and 2 HZ amounted to about 200 g/g dry weight. The values did not significantly differ from each other. 2. The negative log of the EC50 (-log EC50) for the positive inotropic effect of phenylephrine determined at a frequency of 0.5 and 1.0 HZ amounted to 5.28 +/- 0.08 and 5.34 +/- 0.11, respectively. The alpha-adrenolytic drug phentolamine (3 x 10(-6) M) diminished significantly the -log EC50 to 5.01 +/- 0.04 and 4.89 +/- 0.10, respectively. 3. At a frequency of 1 HZ a shift of the concentration-response curve to the right was observed after treatment with the beta-adrenolytic drug pindolol (3 x 10(-8) M); the -log EC50 of phenylephrine decreased significantly to 4.08 +/- 0.07. 4. From these results it is concluded that alpha-adrenoceptors are present in human atria; they mediate positive inotropic effects and are stimulated by phenylephrine.

The histamine blocking properties of burimamide and promethazine were determined on heart rate and contractility of the right and left atrium of the guinea-pig.(1) The positive chronotropic effect in spontaneously beating guinea-pig atria was competitively blocked by burimamide, but not by promethazine.(2) On the other hand, promethazine proved to be a potent antagonist against the positive inotropie response elicited by histamine on the electrical stimulated (2 Hz) left guinea-pig atrium, whereas burimamide was ineffective.(3) The positive inotropic response to histamine on the right atrium was affected by both antihistaminic drugs investigated. It should, however, be stressed that the blocking efficacy of both antagonists was less pronounced than that evoked by burimamide on the chronotropic and by promethazine on the inotropic effect on right and left atrium, respectively.(4) These results suggest that on guinea-pig heart the positive chronotropic effect elicited by histamine is mediated by 'burimamide sensitive' H2-receptors, whereas the positive inotropic effect evoked by histamine is mediated by 'promethazine sensitive' Hi-receptors. The observation that the positive inotropic effect of histamine on the spontaneously beating right atrium could be antagonized by both types of antihistamines favours the view that part of the positive inotropic response to histamine on this preparation is due to a positive frequency-force relationship (Bowditeh-Treppe).