Protective action of doxycycline against diabetic cardiomyopathy in rats.

BACKGROUND AND PURPOSE: Reactive oxygen and nitrogen species play an important role in the development of diabetic cardiomyopathy. They can activate matrix metalloproteinases (MMPs), and MMP-2 in particular is known to mediate early consequences of oxidative stress injury in the heart. Therefore, we investigated the role of MMP-2 and the effect of the MMP inhibitor doxycycline on the changes of heart function caused by diabetes. EXPERIMENTAL APPROACH: Using streptozotocin-induced diabetic rats, we evaluated the effect of doxycycline on both mechanical and electrical function of isolated hearts, papillary muscle and cardiomyocytes. KEY RESULTS: Doxycycline abolished the diabetes-induced depression in left ventricular developed pressure and the rates of changes in developed pressure in isolated hearts and normalized the prolongation of the action potential in papillary muscles. In cardiomyocytes isolated from doxycycline-treated diabetic rats, the altered kinetic parameters of Ca(2+) transients, depressed Ca(2+) loading of sarcoplasmic reticulum and basal intracellular Ca(2+) level, and the spatio-temporal properties of Ca(2+) sparks were significantly restored. Gelatin zymography and western blot data indicated that the diabetes-induced alterations in MMP-2 activity and protein level, level of tissue inhibitor of matrix metalloproteinase-4 and loss of troponin I were restored to control levels with doxycycline. CONCLUSIONS AND IMPLICATIONS: Our data suggest that these beneficial effects of doxycycline on the mechanical, electrical and biochemical properties of the diabetic rat heart appear, at least in part, to be related to inhibition of MMP activity, implying a role for MMPs in the development of diabetic cardiomyopathy.

Ouabain-induced signaling and vascular smooth muscle cell proliferation.

The hypothesis of this study is that the sodium pump complex acts as an intracellular signal-transducing molecule in canine vascular smooth muscle cells through its interaction with other membrane and cytoskeletal proteins. We have demonstrated that 1 nm ouabain induced transactivation of the epidermal growth factor receptor (EGFR), resulting in increased proliferation and bromodeoxyuridine (BrdUrd) uptake. Immunoprecipitation and Western blotting showed that the EGFR and Src were phosphorylated within 5 min of 10(-9) m ouabain stimulation. Both ouabain-induced DNA synthesis (BrdUrd uptake) and MAPK42/44 phosphorylation were inhibited by the Src inhibitor PP2, the EGFR kinase inhibitor AG1478, the tyrosine kinase inhibitor genistein, and the MEK1 inhibitor PD98059. Ouabain concentrations higher than 1 nm had little or no stimulating effect on proliferation or BrdUrd uptake but did minimally activate ERK1/2. Thus, low concentrations of ouabain, which do not inhibit the sodium pump sufficiently to perturb the resting cellular ionic milieu, initiate a transactivational signaling cascade leading to vascular smooth muscle cell proliferation.

Low concentrations of ouabain induce vascular smooth muscle cell proliferation.

Ouabain is a well known inhibitor of the Na+ pump in all mammalian cells. We have demonstrated that ouabain at concentrations below those which inhibit the pump, i.e. 0.1 nM and 1.0 nM, induce proliferation of saphenous vein smooth muscle cells as measured by bromodeoxyuridine (BrdU) uptake. Ouabain at these low concentrations also activated MAPK. Proliferating concentrations of the drug did not increase levels of Ca(i)2+, suggesting no effect of this ion in the process. In addition, incubation of the cells in low levels of K+, which has been shown to inhibit the pump, had no effect on proliferation. These data show that low concentrations of ouabain that do not inhibit the Na+ pump can activate proliferation of vascular smooth muscle cells, suggesting that the pump complex may act as a transducing receptor.

Regulation of Na(+) pump expression by vascular smooth muscle cells.

The Na(+) pump and its regulation is important for maintaining membrane potential and transmembrane Na(+) gradient in all mammalian cells and thus is essential for cell survival and function. Vascular smooth muscle cells (VSMC) have a relatively low number of pump sites on their membrane compared with other cells. We wished to determine the mechanisms for regulating the number of pump sites in these cells. We used canine saphenous vein VSMC cultured in 10% serum and passaged one time. These cells were subcultured in 5% serum media with low K(+) (1 mM vs. control of 5 mM), and their pump expression was assessed. These VSMC upregulated their pump sites as early as 4 h after treatment (measured by [(3)H]ouabain binding). At this early time point, there was no detectable increase in protein expression of either alpha(1)- or beta(1)-subunits of the pump shown by Western blots. When the cells were treated with the phosphoinositide 3-kinase (PI-3-K) inhibitor LY-294002 (which is known to inhibit cytoplasmic transport processes) in low-K(+) media, the pump site upregulation was inhibited. These data suggest that the low-K(+)-induced upregulation of Na(+) pump number can occur by translocation of preformed pumps from intracellular stores.

Expression of plasma membrane calcium ATPases in phenotypically distinct canine vascular smooth muscle cells.

Our laboratory has identified at least two types of vascular smooth muscle cells (VSMCs) that exist in canine arteries and veins: type 1 cells, located in the media express muscle specific proteins but do not proliferate in culture; and type 2 cells, located in both media and adventitia, do not express muscle specific protein but proliferate in culture. Plasma membrane Ca(2+)-ATPases (PMCAs) have been implicated in proliferation control. The present study examines the expression of PMCA isoforms and calmodulin-binding domain splice variants in these two types of canine VSMCs. PMCA protein was found in both type 1 and type 2 cells. Reverse transcriptase-polymerase chain reaction assays were developed for canine PMCA calmodulin-binding domain splice variants. We cloned and sequenced isolates corresponding to PMCA1b, 4a and 4b from canine VSMCs. PMCA 2 and 3 were not detected. Freshly isolated type 1 cells expressed PMCA 1b, 4a and 4b, while freshly isolated type 2 cells expressed PMCA1b and 4b. Upon placement in culture, type 2 cells originating from either carotid artery or saphenous vein demonstrated a time-dependent upregulation of PMCA4a mRNA. Treatment with the phosphoinositide 3-kinase inhibitor wortmannin produced concentration-dependent inhibition of both PMCA4a upregulation and [(3)H]thymidine incorporation. These findings suggest a role for phosphoinositide 3-kinase in regulating PMCA expression, which may be important in the control of Ca(2+)-sensitive VSMC functions.

Effects of intestinal ischemia-reperfusion on major conduit arteries.

Intestinal ischemia-reperfusion (I-R) is a common and serious clinical condition associated with simultaneous remote organ dysfunction. The purpose of this study was to investigate the effects of intestinal I-R on the vasomotor functions of major conduit arteries. Anesthetized rabbits were randomly assigned to one of three groups: sham-operated controls (Group I), and one-hour intestinal ischemia with two-hour reperfusion (Group II) or four-hour reperfusion (Group III). The following mechanisms of vasomotor functions were studied in abdominal aorta, superior mesenteric, renal, pulmonary, and carotid arterial rings: (1) endothelial-dependent vasodilation response to acetylcholine, (2) endothelial-independent vasodilation response to nitroprusside, (3) beta-adrenergic vasodilation response to isoproterenol, and (4) phenylephrine-induced vasoconstriction. Intestinal injury was quantified using malondialdehyde (MDA) concentration and wet-to-dry intestine weight ratio. Intestinal I-R did not affect the maximal responsiveness or the sensitivity to acetylcholine, nitroprusside, and isoproterenol in all the vessels studied. The maximal contractile response to phenylephrine increased significantly in mesenteric artery in Group II, (227.1+/-15.1% vs. 152.8+/-11.7% in controls) (p<0.05). Intestinal MDA concentration, a marker of oxidant injury, increased from 39.87+/-9.41 nmol/g to 67.8+/-8.8 nmol/g in group II (p<0.01), and to 94.8+/-7.56 nmol/g in Group III (p<0.001). Wet-to-dry intestine weight ratio increased from 3.62+/-0.12 to 4.28+/-0.17 in Group II (p<0.01), to 4.62+/-0.14 in Group III (p<0.001). These data indicate that although the intestines of the animals subjected to intestinal I-R are seriously injured, the smooth muscle relaxation of major conduit arteries was not affected.