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This study examined 4SCa uptake, 45 Ca efflux, and the distribution of exchangeable 4SCa in confluent, quiescent cultures of aortic smooth muscle cells (VSMCs) from normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). These parameters were investigated under basal conditions and after addition of angiotensin II (Ang II) and low (LDL) (Ang II) 1 is an important regulator / \ not only of vascular smooth muscle contraction A. A . but also of vascular smooth muscle cell (VSMC) growth (see References 1 and 2 for reviews). In hypertension, both the functional and structural integrity of vascular smooth muscle are deranged, 12 thus invoking an essential involvement of Ang II in the pathophysiology of hypertension. This is supported by observations that angiotensin converting enzyme inhibitors effectively lower blood pressure 3 and also prevent myointimal proliferation after vascular injury. 4 Hypertension greatly increases the frequency of atherosclerotic cardiovascular sequelae, and lipoproteins greatly affect the impact of hypertension on the pace of atherogenesis (see References 5 and 6 for reviews). The two diseases are closely linked with respect to predisposing risk factors, clinical manifestations, and vascular/cellular pathobiology; thus, the fundamental role of lipoproteins in the atherosclerotic process suggests that they may also be involved in the pathophysiology of hyper- tension. 5-6 Evidence is accumulating to indicate that low density lipoprotein (LDL) and high density lipoprotein (HDL) can influence smooth muscle cell processes that are distinct from those related to cholesterol homeostasis per se, such as growth-related metabolic events, cell morphology, and proliferation and contractile processes (reviewed in Reference 7).Calcium is a pivotal signaling molecule in the regulation of smooth muscle structure and function, and the hypothesis on the involvement of abnormalities of intracellular calcium handling in the pathophysiology of hypertension is based on the following principal observations.

To identify ion transport systems involved in the maintenance of vascular smooth muscle cell volume the effects of incubation medium osmolality and ion transport inhibitors on the volume and 86Rb and 22Na transport in cultured smooth muscle cells from rat aorta (VSMC) have been studied. A decrease of medium osmolality from 605 to 180 mosm increased intracellular water volume from 0.6 to 1.3 microliters per 10(6) cells. Under isosmotic conditions, cell volume was decreased by ouabain (by 10%, P less than 0.005) but was not influenced by bumetanide, furosemide, EIPA and quinidine. These latter compounds were also ineffective in cell volume regulation under hypotonic buffer conditions. Under hyperosmotic conditions, cell volume was decreased by bumetanide (by approximately 7%, P less than 0.05) and by ethylisopropyl amiloride (by approximately 13%, P less than 0.005). Ouabain-sensitive 86Rb influx was decreased by 30-40% under hypoosmotic conditions. An increase in medium osmolality from 275 to 410 mosm resulted in an approximately eightfold increase in bumetanide-inhibited 86Rb influx and 86Rb efflux. The (ouabain and bumetanide)-insensitive component of 86Rb influx was not dependent on the osmolality of the incubation medium. However (ouabain and bumetanide)-insensitive 86Rb efflux was increased by approximately 1.5-2 fold in VSMC incubated in hypotonic medium. Ethylisopropyl amiloride-inhibited 22Na influx was increased by approximately sixfold following osmotic-shrinkage of VSMC. The data show that both Na+/H+ exchange and Na+/K+/2Cl- cotransport may play a major role in the regulatory volume increase in VSMC. Basal and shrinkage-induced activities of Na+/K+/2Cl- cotransport in VSMC were similarly sensitive to inhibition by either staurosporin, forskolin, R24571 or 2-nitro-4-carboxyphenyl N,N-diphenylcarbomate (NCDC). In contrast basal and shrinkage-induced Na+/K+/2Cl- cotransport were differentially inhibited by NaF (by 30 and 65%, respectively), suggesting an involvement of guanine nucleotide binding proteins in the volume-sensitive activity of this carrier. Neither staurosporin, forskolin, R24571 nor NCDC influenced shrinkage-induced Na+/H+ exchange activity. NaF increased Na+/H+ exchanger activity under both isosmotic and hyperosmotic conditions. These data demonstrate that different intracellular signalling mechanisms are involved in the volume-dependent activation of the Na+/K+/2Cl- cotransporter and the Na+/H+ exchanger.

Atherogenesis is associated with alterations in the properties of different cell types, including monocytes/macrophages (foam cell formation), platelets (increased aggregation), endothelial cells (injury), and smooth muscle cells (SMCs) (lipid accumulation or foam cell formation). Oxidized low density lipoproteins (ox-LDL) play a key role in this vascular pathology. This study investigated the ability of ox-LDL to elicit chemical signaling events in cultured human vascular smooth muscle cells (VSMCs). Ox-LDL was found to stimulate phospholipase C-mediated phosphoinositide turnover in human VSMCs. This response occurred rapidly (within 1 minute) and at low concentrations of ox-LDL (half-maximal effective concentration, approximately 5 micrograms/ml). Ox-LDL-stimulated inositol phosphate accumulation in human VSMCs was inhibited by pretreatment of cells with phorbol 12-myristate 13-acetate and with compounds that elevate cyclic AMP or cyclic GMP. Ca2+ antagonists also blocked the effects of ox-LDL on phosphoinositide turnover. Inhibitors of receptor-endocytotic processes (including receptor clustering, cross-linking, and cytoskeleton-dependent internalization) effectively prevented ox-LDL-induced inositol phosphate generation. The data suggest that ox-LDL promotes phospholipase C-mediated phosphoinositide turnover in a manner analogous to that for other Ca(2+)-mobilizing hormones. The results also support an association between phosphoinositide turnover and receptor-mediated endocytosis. Prevention of the direct effects of ox-LDL on SMCs could prove an interesting therapeutic avenue for the prevention of atherosclerosis.

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