(S1P) is released at sites of tissue injury and effects cellular responses through activation of G protein-coupled receptors. The role of S1P in regulating cardiomyocyte survival following in vivo myocardial ischemiareperfusion (I/R) injury was examined by using mice in which specific S1P receptor subtypes were deleted. Mice lacking either S1P 2 or S1P3 receptors and subjected to 1-h coronary occlusion followed by 2 h of reperfusion developed infarcts equivalent to those of wild-type (WT) mice. However, in S1P2,3 receptor double-knockout mice, infarct size following I/R was increased by Ͼ50%. I/R leads to activation of ERK, JNK, and p38 MAP kinases; however, these responses were not diminished in S1P2,3 receptor knockout compared with WT mice. In contrast, activation of Akt in response to I/R was markedly attenuated in S1P2,3 receptor knockout mouse hearts. Neither S1P2 nor S1P3 receptor deletion alone impaired I/R-induced Akt activation, which suggests redundant signaling through these receptors and is consistent with the finding that deletion of either receptor alone did not increase I/R injury. The involvement of cardiomyocytes in S1P2 and S1P3 receptor mediated activation of Akt was tested by using cells from WT and S1P receptor knockout hearts. Akt was activated by S1P, and this was modestly diminished in cardiomyocytes from S1P2 or S1P3 receptor knockout mice and completely abolished in the S1P2,3 receptor double-knockout myocytes. Our data demonstrate that activation of S1P2 and S1P3 receptors plays a significant role in protecting cardiomyocytes from I/R damage in vivo and implicate the release of S1P and receptor-mediated Akt activation in this process.cardioprotection; mitogen-activated kinase; G protein-coupled receptors; infarct SPHINGOSINE 1-PHOSPHATE (S1P) is a bioactive lysophospholipid generated through the breakdown of sphingomyelin. A number of regulated enzymes, including sphingomyelinase and sphingosine kinase, control its formation (40). A role for S1P in regulating cellular responses to injury and inflammation has become increasingly well accepted. In the heart, as in other tissues, sphingomyelinase is activated by ischemia-reperfusion (I/R) (anoxia-reoxygenation) and by cytokines such as TNF-␣, suggesting that sphingolipid metabolites (ceramide, sphingosine, and S1P) are generated and may participate in cellular responses to these interventions (5,8,12,23). Sphingosine kinase has also been shown to be activated by I/R in the heart (18). Although intracellular actions of sphingomyelin metabolites had been examined for many years, the cloning of G protein-coupled receptors with specificity for S1P led to recognition that sphingolipid-mediated responses are effected, in large part, through extracellular activation of cell surface receptors (6,16,26).The S1P receptors, originally classified into the edg receptor family, are now referred to as S1P 1 -S1P 5 . The S1P 1 (edg1), S1P 2 (edg5), and S1P 3 (edg3) receptors are ubiquitously expressed, whereas the expression of S1P 4 and S1P 5 receptors...

The Mas receptor is a class I G-protein-coupled receptor that is expressed in brain, testis, heart, and kidney. The intracellular signaling pathways activated downstream of Mas are still largely unknown. In the present study, we examined the expression pattern and signaling of Mas in the heart and assessed the participation of Mas in cardiac ischemia-reperfusion injury. Mas mRNA and protein were present in all chambers of human hearts, with cardiomyocytes and coronary arteries being sites of enriched expression. Expression of Mas in either HEK293 cells or cardiac myocytes resulted in constitutive coupling to the G(q) protein, which in turn activated phospholipase C and caused inositol phosphate accumulation. To generate chemical tools for use in probing the function of Mas, we performed a library screen and chemistry optimization program to identify potent and selective nonpeptide agonists and inverse agonists. Mas agonists activated G(q) signaling in a dose-dependent manner and reduced coronary blood flow in isolated mouse and rat hearts. Conversely, treatment of isolated rat hearts with Mas inverse agonists improved coronary flow, reduced arrhythmias, and provided cardioprotection from ischemia-reperfusion injury, an effect that was due, at least in part, to decreased cardiomyocyte apoptosis. Participation of Mas in ischemia-reperfusion injury was confirmed in Mas knockout mice, which had reduced infarct size relative to mice with normal Mas expression. These results suggest that activation of Mas during myocardial infarction contributes to ischemia-reperfusion injury and further suggest that inhibition of Mas-G(q) signaling may provide a new therapeutic strategy directed at cardioprotection.

There are many papers on tissue remodeling of blood vessels in hypertension, but there are few documents describing the tissue remodeling of the blood vessels following a step lowering of the blood pressure. The present article presents data on the opening angle, the vessel wall thickness, and the thicknesses of the intima-media and adventitia layers of the blood vessels of the lower body (the abdominal aorta, and the common iliac, femoral, saphenous branch, medial plantar, and plantar metatarsal arteries) of the rat after a step lowering of the blood pressure and flow by a controlled constriction of the aorta below both renal arteries. We found a pattern of changes that depend on space (location on the vascular tree), time (after the blood pressure change), and the intensity of disturbance. We model mathematically the dynamics shown by the experimental results by means of the indicial response functions, which are defined as the morphometric changes in response to a step decrease of blood pressure or blood flow. Under the hypothesis that there is a range of linearity between the degree of tissue remodeling and the amplitude of the pressure change, we can use the indicial functions to predict the remodeling of the vessel under an arbitrary history of decreasing blood pressure; and conversely, we can compute the indicial response functions from pertinent results of a single experiment. The totality of all our experiments is consistent with the linearity hypothesis within the range of the experiment. The mathematical analysis and the formulas are presented in the Appendix.

The reversibility of tissue remodeling is of general interest to medicine. Pulmonary arterial tissue remodeling during hypertension induced by hypoxic breathing is well known, but little has been said about the recovery of the arterial wall when the blood pressure is lowered again. We hypothesize that tissue recovery is a function of the oxygen concentration, blood pressure, location on the vascular tree, and time. We measured the changes of blood pressure, vessel lumen, vessel wall thicknesses, and opening angle of each segment of the blood vessel at its zero-stress state after step changes of the oxygen concentration in the breathing gas. The zero-stress state of each vessel is emphasized because it is important to the analysis of stress and strain and in morphometry. Experimental results are presented as histories of tissue parameters after step changes of the oxygen level. Tissue characteristics are examined under the hypothesis that they are linearly related to changes in the local blood pressure. Under this linearity hypothesis, each aspect of the tissue change can be expressed as a convolution integral of the blood pressure history with a kernel called the indicial response function. It is shown the indicial response function for rising blood pressure is different from that for falling blood pressure. This difference represents a major nonlinearity of the tissue remodeling process of the blood vessels.indicial response function ͉ blood vessel opening angle at zero-stress state ͉ nonlinearity of tissue remodeling I t is well known that blood flow in pulmonary arteries becomes hypertensive when an animal breathes a gas the oxygen concentration of which is lower than that of normal sea level air. It is also known that hypertension leads to hypertrophy in blood vessels, and returning to normal blood pressure mitigates the symptom. The question is whether the processes of hypertrophy and recovery are symmetric. Considerable data on the building up of the lung tissue due to pulmonary hypertension have been obtained (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Some data on recovery from hypertension are also given in refs. 1, 4, and 5. These studies considered neither the zero-stress state, which is characterized by an opening angle of the blood vessels, nor the elastic moduli of the blood vessel walls. The significance of the zero-stress state and the change of Young's modulus were recognized later (8,10,11,(15)(16)(17)(18)(19).So far as we are aware, there has been no study on the recovery of the zero-stress state of a vascular tissue that was subjected to hypertension for some time but was returned to the normal condition later. Recovery can be expected to be a complex function of space, time, stress, and strain. The objective of this article is to clarify these points. Materials and MethodsAnimals. Ninety-two male Sprague-Dawley rats (Harlan, San Diego), Ϸ3 months old, body weight 300-350 g, raised in normal air at sea level, were used. These rats were cared for 1 week or more after arrival at the vivari...