Aortic vascular and atrial responses to (+/-)-1-O-octadecyl-2-acetyl-glyceryl-3-phosphorylcholine.

The effects of (+/-)-1-O-octadecyl-2-acetyl-glyceryl-3-phosphorylcholine (octadecyl-AGPC) were studied in three types of aortic vascular smooth muscle preparations, namely, strips, rubbed and unrubbed rings, and an atrial preparation in normotensive rats. In the resting tension state, octadecyl-AGPC did not elicit significant contractions in either rubbed or unrubbed ring preparations at concentrations lower than 1 X 10(-4) M. However, at a concentration of 3 X 10(-4) M, octadecyl-AGPC markedly contracted both types of ring preparations. This contractile response was partially antagonized by pretreatment with reserpine and completely blocked by phentolamine (1 X 10(-6) M). In preparations contracted with noradrenaline (NA), octadecyl-AGPC elicited biphasic responses in intact ring preparations; an initial relaxation followed by contraction. Octadecyl-AGPC induced only a slight contraction in strips and a slight relaxation in the rubbed ring preparation. Octadecyl-AGPC did not elicit any significant effect on chronotropy or inotropy at concentrations up to 3 X 10(-5) M. When the concentration was 1 X 10(-4) M, octadecyl-AGPC produced significant positive chronotropic and inotropic effects on spontaneously beating right and electrically driven left atrial preparations, respectively. Both effects were blocked by propranolol (5 X 10(-8) M); reserpine pretreatment antagonized only the chronotropic response. In [3H]-dihydroalprenolol [( 3H]-DHA) binding studies, octadecyl-AGPC had a Kd of 427.85 microM and thus was much less potent than isoprenaline (Kd = 465.10 nM) or propranolol (Kd = 4.4 nM) in displacing [3H]-DHA in rat cardiac membrane preparations. 6 In conclusion, relaxation and contraction induced by octadecyl-AGPC in aortic preparations is an indirect rather than a direct effect. An unknown factor released from endothelial cells is responsible for aortic smooth muscle relaxation by octadecyl-AGPC while released NA appears to be responsible for aortic vascular contraction and for the positive chronotropic and inotropic effects in the atrial preparations.

Identification and characterization of angiotensin II receptors in rat epididymal adipocyte membranes.

To better understand the role of the mitogenic vasoactive peptide angiotensin II (AII) in growth and differentiation, we have investigated the existence of membrane receptors for this peptide in rat adipocytes. Following isolation of epididymal fat cells, membrane protein was removed and incubated with varying concentrations of 125I-AII with or without submicromolar concentrations of unlabeled AII. Binding of AII was highly specific, rapid, and reversible. Scatchard analysis indicated that adipocyte membranes contain a high-affinity AII receptor with a Kd of 0.90 nmol/L and a binding site concentration (Bmax) of 53.7 fmol/mg protein. Additional pharmacologic analyses resulted in a rank order potency for peptide agonists and antagonists similar to that reported for the vascular receptor. Determination of subtype specificity with selective organic compounds indicated that the epididymal adipocyte receptor was displaced at low concentrations of DuP753, a selective AT1 subtype antagonist. These studies have successfully identified and characterized a high-affinity membrane receptor for AII in fat cells, further establishing adipose tissue as a peripheral site containing regulatory components of the local renin-angiotensin system.

Distribution of angiotensin II receptors in rat and human adipocytes.

Angiotensin II (AII) receptor binding assays were performed in rat adipocytes from three separate anatomic depots. Fat cells were isolated by collagenase digestion, and plasma membranes were prepared from the epididymal, mesenteric, and retroperitoneal fat depots of male Sprague-Dawley rats at 100 days of age. Binding of 125I-labeled [Sar1,Ile8]AII was rapid, saturable, and specific in membranes from all depots, identifying a receptor with a similar affinity of approximately 1 nM. Site-associated differences in receptor number were observed, with epididymal and mesenteric fat cell membranes exhibiting significantly more receptors than retroperitoneal fat cells when binding was expressed per unit of membrane protein. When corrected for cell volume, the number of receptors per cell ranked epididymal > retroperitoneal > mesenteric. Inhibitory constants for the peptide agonists AII and AIII and the peptide antagonist [Sar1,Ala8]AII indicated similar affinities in all three depots. Because the receptor has been classified pharmacologically into two subtypes, the AT1 selective antagonist losartan, and the AT2 selective antagonist PD 123,319 were used to classify the adipocyte receptor, indicating an AT1 subtype with an affinity for losartan in the mesenteric and retroperitoneal adipocytes that was significantly greater than the epididymal. Similar studies were performed in adipocyte membranes obtained from human omental and subcutaneous adipose tissue, revealing the presence of an AII receptor in both depots with an affinity of approximately 10 nM for losartan. These data indicate site-specific differences in AII receptor number in fat cell membranes from rats and the existence of human adipocyte AII receptors, suggesting that the adipocyte is significant for the peripheral metabolism of components of the renin-angiotensin system.

Transforming growth factor alpha and atrial natriuretic peptide in white adipose tissue depots in rats.

To detect the presence in adipose tissue of peptides known to affect tissue growth and to investigate potential regional differences, epididymal and perirenal adipose tissue depots from male Sprague-Dawley rats were separated into adipocyte and stroma-vascular fractions by collagenase digestion, sequential centrifugation and filtration. Identity and integrity of the fractions were demonstrated by light and electron microscopy, while dose-response curves for angiotensin-converting enzyme (ACE) were performed, revealing maintained functional capacity of the stroma-vascular fraction. ACE, atrial natriuretic peptide (ANP), and transforming growth factor-alpha (TGF-alpha) concentrations were significantly greater in epididymal than perirenal stroma-vascular tissue. Adipocyte fractions from both depots contained significant concentrations of ANP and TGF-alpha. There was no detectable ACE in the adipocyte fractions, indicating that no contaminating stromal-vascular cells were present in these fractions. These data show significant concentrations of peptides with effects on growth in subfractions of adipose tissue and demonstrate regional differences in concentrations between fat depots.