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The human ether-à-go-go-related gene (hERG) encodes the pore-forming subunit of the rapidly activating delayed rectifier K(+) current (I(Kr)) important for cardiac repolarization. Dysfunction of the hERG channel causes long QT syndrome (LQTS). Although diverse compounds reduce the hERG current (I(hERG)) by blocking the channel, probucol, a cholesterol-lowering drug that causes LQTS, reduces I(hERG) by decreasing plasma-membrane hERG protein expression. Here, we investigated the mechanisms of probucol effects on hERG expression levels. Our data demonstrate that probucol accelerated the degradation of mature hERG channels, which associated with caveolin-1 (Cav1) in hERG-expressing HEK cells. In human embryonic kidney (HEK) cells without hERG expression, probucol promoted endogenous Cav1 degradation. In hERG-expressing HEK cells, overexpression of Cav1 enhanced, whereas knockdown of Cav1 impeded, probucol-induced reduction of mature hERG channels. Thus, probucol reduces hERG expression through accelerating Cav1 turnover. The effects of probucol on Cav1 and hERG result from probucol's cholesterol-disrupting action, because low-density lipoprotein (LDL), a potent cholesterol carrier, effectively prevented probucol-induced reduction of I(hERG) in hERG-expressing HEK cells and of I(Kr) in neonatal rat cardiomyocytes. Our data provide evidence that targeting hERG-interacting protein caveolin represents a novel mechanism for drugs to decrease hERG expression and cause LQTS.

Proteins enter the nucleus through the nuclear pore complex. Once in the nucleus, some proteins, such as transcriptional regulators, can turn genes on or off, and change the composition of the cell and its function to meet the demands of its environment. This process of protein import into the nucleus is highly controlled and regulated by the expression or function of single cargoes, transport receptors, or the transport channels themselves. Thus, these components of the import process have an impact on transport capacity, which subsequently affects gene expression, signal transduction, and cell growth and development. With such a key position in the process of cell growth, it is reasonable to hypothesize that alterations in nuclear protein transport may play an important role in pathological cell conditions that have abnormal cell growth as a central feature. Indeed, there are now sufficient data to demonstrate that alterations in nuclear protein transport participate in alterations in cell proliferation and hypertrophy. Further study is needed to provide definitive proof that changes in nuclear protein import directly participate in the pathogenesis of diseases such as hypertension, atherosclerosis, cancer, viral infection, and diabetes. However, the data to date have, on select occasions, led to a clear association of alterations in nuclear transport with disease states. Furthermore,this research has led to the important identification of new targets within the process of nuclear protein import that hold therapeutic promise to inhibit viral replication, to improve drug delivery during cancer therapy, and, in general, to modify cell growth and viability during disease conditions.

The intracellular mechanism responsible for the mitogenic effects of lysophosphatidylcholine (LPC) is unclear. Import of proteins from the cytoplasm into the cell nucleus is integral to the regulation of gene expression and cell growth. We hypothesized that LPC exerts its intracellular effects through alterations in nuclear protein import. Rabbit aortic smooth muscle cells incubated with LPC induced a significant increase in cell proliferation in both quiescent cells (63.2+/-6.48% of control) and cells grown in 1% fetal bovine serum (FBS) (28.3+/-7.35% of control). Vascular smooth muscle cells were preincubated with LPC then microinjected with a marker protein for nuclear import. A significant stimulation of nuclear protein transport was observed. Using a conventional nuclear protein import assay in permeabilized cells, a significant stimulation of import (72.3+/-5.2% of control) was again observed when the cytosolic nuclear import cocktail was treated with LPC. This effect was not observed with other lysophosphatidyl species. LPC also activated the extracellular signal-regulated kinase (ERK) 1/2 mitogen-activated protein kinase (MAPK) pathway, and this was blocked by 2'-amino-3'-methoxyflavone (PD98059), which inhibits the activation of ERK 1/2. The stimulation of nuclear import was also blocked by PD98059. LPC-induced MAPK activation augmented GTP hydrolysis by RanGAP, a RanGTPase activating protein and a critical regulatory component of nuclear protein import, and this stimulation was again blocked by PD98059. We conclude that LPC alters gene expression and cell proliferation through striking effects on nuclear protein import via a MAP kinase-induced activation of RanGAP. This may play an important role in cancer and atherosclerosis and other disorders involving accelerated cell growth/proliferation.

Amiloride and its derivatives (benzamil, dichlorobenzamil, 5-(N,N-dimethyl)-amiloride, 5-(N-ethyl-N-isopropyl)-amiloride, (N,N-hexamethylene)- amiloride and 5-(N-methyl-N-isobutyl)-amiloride) are commonly used as selective blockers of Na+/Ca++ exchange or Na+/H+ exchange. Very little information is currently available regarding their effects on cardiac performance. It was observed that addition of amiloride or any of the selected derivatives to the coronary perfusate of the right ventricular wall produced a potent depressive effect on peak developed tension and the rates of tension generation and dissipation. The concentrations at which this occurred are those that are commonly used in ischemia or hypoxia studies. Significantly, the depressive action of the drugs increased with the perfusion duration and never achieved a stable level. An initial, transient positive inotropic effect was observed with some of the drugs. If the drug concentration and perfusion time was limited, the effects were reversible. All of the drugs except amiloride produced extra systoles. The drugs were capable of blocking Ca++ transients in isolated cardiomyocytes but had little effect on intracellular pH. The drugs lengthened the action potential duration and decreased the action potential amplitude and upstroke velocity. Their effects on cardiac performance may involve a complex inhibition of Ca++ influx and K+ efflux in addition to a stimulation of a nonselective cation current. It is concluded that amiloride and its analogs have striking effects on cardiac performance which may be unrelated to their capacity to inhibit Na+/Ca++ or Na+/H+ exchange. In summary, the use of these drugs is not normally recommended in cell or tissue perfusion experiments because of their nonselectivity. However, if the drug concentration and perfusion time is controlled carefully, interpretable data may be obtained in some cases.

The purpose of this study was to determine if chronic administration of verapamil could alter the density of Ca++ channels in the heart as determined by [3H]PN 200-110 binding. Initially, we compared the effects of verapamil given by s.c. injection or via implantable, slow-release verapamil pellets. We found the majority of animals treated with the pellets died within 24 hr. Those that survived exhibited significantly depressed maximal binding and Kd values for PN 200-110 binding to cardiac membranes, but binding to brain membranes was unaffected. Quantitation of the serum levels of verapamil and its metabolites by high-performance liquid chromatography demonstrated that the verapamil pellets did not release the drug evenly over a 3-week period. Most of the drug was released in toxic quantities during the 1st day after implantation. Verapamil administered by injection (2.5-30 mg/kg/day) for up to 16 weeks raised plasma verapamil levels to 25 to 250 ng/ml, but had no effect on Ca++ channel characteristics in cardiac or brain tissue. The maximal binding and Kd values for skeletal muscle PN 200-110 binding were increased only at the highest dosage for 8 weeks duration. Our results demonstrate that implantable pellets are not a reliable administration method for verapamil and cardiac Ca++ channels are highly resistant to change during chronic verapamil administration.

During reperfusion with pH 7.2 media after 55 min of ischemia, the recovery of developed tension (DT) and the maximal resting tension (RT) of the isolated right ventricular wall of rats were 58.8 +/- 6.5 and 201 +/- 20%, respectively, of the preischemic values. DT and RT in the ventricular wall perfused with pH 7.9 media for 6 min during reperfusion were 40 +/- 5.9 and 285 +/- 13%, respectively, of the preischemic values (P less than .05 vs. pH 7.2 group). The Na(+)-H+ exchange inhibitor, 5-(N,N-dimethyl)amiloride (DMA), effectively antagonized the detrimental effect of pH 7.9 media. A pH 6.5 media inhibited DT recovery and the rise in RT in the first 6 min of reperfusion. Subsequent reperfusion with pH 7.2 media resulted in cardiac dysfunction similar to that observed when reperfused at pH 7.2 only. Cellular Na+ and Ca++ were significantly elevated after 6 min of reperfusion at pH 7.2. Na+ and Ca++ levels were increased further if reperfusion was carried out at pH 7.9. Inclusion of 20 microM DMA during reperfusion at 7.9 significantly reduced cellular Na+ and Ca++. Creatine phosphokinase activity in the coronary effluent rose significantly during reperfusion at pH 7.2 and this was exacerbated if the reperfusion pH was 7.9. DMA treatment during reperfusion could significantly inhibit this elevation. The data lend further support for an important role of Na(+)-H+ exchange in the development of ischemia-reperfusion injury.

During reperfusion in the isolated right ventricular wall of the rat after 60 min of ischemia, developed tension and resting tension were 35 +/- 4 and 221 +/- 12%, respectively, of preischemic values. Including 35 microM ouabain in the perfusate before and after ischemia resulted in more severe cardiac dysfunction during reperfusion than in drug-untreated hearts. Introduction of the Na(+)-H+ exchange inhibitor, 5-(N,N-dimethyl)-amiloride (DMA), could effectively protect the right ventricular wall against ischemia-reperfusion dysfunction in the presence or absence of ouabain. The ion content in the right ventricular wall was measured with atomic absorbance spectrophotometry. Before ischemia, Na+,Ca++ and K+ content were 53.4 +/- 6.4, 2.70 +/- 0.22 and 262 +/- 7.7 mumol/g of dry weight tissue, respectively. After 60 min of ischemia and 6 min of reperfusion, Na+,Ca++ and K+ content were 73.4 +/- 7.2, 3.79 +/- 0.31 and 180 +/- 15 mumol/g of dry weight tissue, respectively (P less than .05). Introduction of 20 microM DMA normalized ion content in the muscles which was consistent with the contractile function recovery during reperfusion. The data suggest that a rise in intracellular Na+ in the early stage of reperfusion represents a crucial or primary step for the development of cardiac contractile dysfunction. DMA, which protects against severe reperfusion-induced cardiac contractile dysfunction, appears to act via a normalization of tissue sodium levels. This action is consistent with its proposed role as a blocker of transsarcolemmal Na(+)-H+ exchange.

The influence of the nonsteroidal anti-inflammatory drugs indomethacin (10 microgram/ml), mefenamic acid (10 ng/ml) and acetylsalicylic acid (50 microgram/ml) were investigated for their effects on hypoxically perfused rat hearts. Hypoxia-induced mechanical cardiodepression was attenuated to varying degrees by most drugs, although only mefenamic acid reduced the degree of bradycardia. All the agents reduced in some measure the rise in resting tension produced by hypoxic perfusion and enhanced the functional activity of the myocardium upon reoxygenation of hearts made hypoxic for 10 min only. The degree of contracture after reoxygenation was substantially attenuated by all three drugs. Although coronary flow decreased dramatically during prolonged hypoxia, this phenomenon was significantly prevented by the nonsteroidal anti-inflammatory drugs. Cellular Na content of hypoxic hearts treated with mefenamic acid was elevated, whereas no other significant electrolyte changes were observed. The presence of mefenamic acid results in a reduced extracellular space size, whereas cellular water content was increased by both mefenamic acid and acetylsalicylic acid. Calcium transport of isolated mitochondria from hypoxically or normoxically perfused hearts was not modified by any treatment. Indomethacin, acetylsalicylic acid and mefenamic acid all significantly inhibited prostaglandin release from the heart. These results suggest that various nonsteroidal anti-inflammatory drugs may offer some protective influence on hypoxic rat hearts particularly with respect to coronary artery patency during hypoxic perfusion.