The Cardiovascular System

The small arteries play an important functional role in establishing the increased peripheral resistance found in essential hypertension. This paper concerns the direct measurement of the intrinsic mechanical and contractile properites of two categories of small arterial resistance vessels in the mesenteric bed of 5-month-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The vessels had mean internal diameters of 246 fim and 153 /*m when relaxed at 100 mm Hg effective transmural pressure. Segments (1 mm) were mounted in such a way that internal circumference could be controlled and the circumferential wall tension (T) measured. After mounting, each vessel was maximally activated (by K* depolarization at 37°C in the presence of 5 mM Ca) at an internal circumference for which AT was approximately maximal, where AT = Tactlve Trelaxed. From the average values of AT measured we have estimated (on the basis of Laplace's equation) that the SHR vessel would average values of AT measured we have estimated (on the basis of Laplace's equation) that the SHR vessels would have been able to contract against 34% greater pressures than the WKY vessels (P < 0.001). Optical measurements of the dimensions of these vessels showed a 23% greater wall thickness in the SHR vessels (P < 0.02). There were no significant differences in the calculated active wall stresses of the SHR and WKY vessels; this suggests that the greater contractility found in the SHR vessels may be due to their having a greater smooth muscle cell content. These vessel measurements have been examined as well as the rats' blood pressures and heart to body weight ratios. The comparison points to the possibility that the disturbance to the cardiovascular regulatory system which results in hypertension produces similar cellular responses in both the myocardium and the peripheral vasculature.

Contractile Properties of Small Arterial Resistance Vessels in Spontaneously Hypertensive and Normotensive Rats

Artery wall calcification associated with atherosclerosis frequently contains fully formed bone tissue including marrow. The cellular origin is not known. In this study, bone morphogenetic protein-2a, a potent factor for osteoblastic differentiation, was found to be expressed in calcified human atherosclerotic plaque. In addition, cells cultured from the aortic wall formed calcified nodules similar to those found in bone cell cultures and expressed bone morphogenetic protein-2a with prolonged culture. The predominant cells in these nodules had immunocytochemical features characteristic of microvascular pericytes that are capable of osteoblastic differentiation. Pericyte-like cells were also found by immunohistochemistry in the intima of bovine and human aorta. These findings suggest that arterial calcification is a regulated process similar to bone formation, possibly mediated by pericyte-like cells.

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Bone morphogenetic protein expression in human atherosclerotic lesions.

The endothelial lining of blood vessels is subjected to a wide range of haemodynamically-generated shear-stress forces throughout the vascular system1. In vivo and in vitro, endothelial cells change their morphology2,3 and biochemistry4 in response to shear stress in a force- and time-dependent way, or when a critical threshold is exceeded5'6. The initial stimulus–response coupling mechanisms have not been identified, however. Recently, Lansman et al.7 described stretch-activated ion channels in endothelial cells and suggested that they could be involved in the response to mechanical forces generated by blood flow. The channels were relatively non-selective and were opened by membrane stretching induced by suction. Here we report whole-cell patch-clamp recordings of single arterial endothelial cells exposed to controlled levels of laminar shear stress in capillary flow tubes. A K+ selective, shear-stress-activated ionic current (designated IK.s) was identified which is unlike previously described stretch-activated currents. IK.s varies in magnitude and duration as a function of shear stress (half-maximal effect at 0.70 dyn cm−2), desensitizes slowly and recovers rapidly and fully on cessation of flow. IK.s activity represents the earliest and fastest stimulus–response coupling of haemodynamic forces to endothelial cells yet found. We suggest that localized flow-activated hyperpolarization of endothelium involving IK.s may participate in the regulation of vascular tone.

Haemodynamic shear stress activates a K + current in vascular endothelial cells

Hemangiopericytoma. An analysis of 106 cases.

Ultrastructure of mammalian venous capillaries, venules, and small collecting veins.

The ultrastructure of mammalian arterioles and precapillary sphincters.

The ultrastructure of the adrenal cortex of the rat under normal and experimental conditions

The ultrastructure of mouse lymph node venules and the passage of lymphocytes across their walls.

This investigation describes the differentiation of the type I pneumocyte from undifferentiated pulmonary epithelium. Cells lining subpleural alveolar septa were photographed from serial sections with the electron microscope and a three dimensional representation of each cell was obtained by transferring the contours of the cell membranes from micrographs to transparent plastic sheets which were then spaced to scale and stacked. The results of this study indicate that: (1) the only reliable criterion for differentiating between type I and type II cells is the commencement of attenuation of the type I cell; (2) differentiation of the type I cell occurs via the formation of one or more cytoplasmic attenuations that eventually fuse peripherally, thereby surrounding the unattenuated cell soma; (3) with respect to individual cells, blood-air barriers tend to form in distal areas of the attenuating cytoplasm before proximal areas; (4) both type I and type II pneumocytes retain certain characteristics that reveal their common origin.

Johannes A.G. Rhodin

Papers

Ultrastructure of mammalian venous capillaries, venules, and small collecting veins.

The ultrastructure of mammalian arterioles and precapillary sphincters.

The ultrastructure of the adrenal cortex of the rat under normal and experimental conditions

The ultrastructure of mouse lymph node venules and the passage of lymphocytes across their walls.

An electron microscopic study on the type I pneumocyte in the cat: differentiation.