Oxidative inactivation of the lipid phosphatase phosphatase and tensin homolog on chromosome ten (PTEN) as a novel mechanism of acquired long QT syndrome.

The most common cause of cardiac side effects of pharmaco-therapy is acquired long QT syndrome, which is characterized by abnormal cardiac repolarization and most often caused by direct blockade of the cardiac potassium channel human ether a-go-go-related gene (hERG). However, little is known about therapeutic compounds that target ion channels other than hERG. We have discovered that arsenic trioxide (As(2)O(3)), a very potent antineoplastic compound for the treatment of acute promyelocytic leukemia, is proarrhythmic via two separate mechanisms: a well characterized inhibition of hERG/I(Kr) trafficking and a poorly understood increase of cardiac calcium currents. We have analyzed the latter mechanism in the present study using biochemical and electrophysiological methods. We find that oxidative inactivation of the lipid phosphatase PTEN by As(2)O(3) enhances cardiac calcium currents in the therapeutic concentration range via a PI3Kα-dependent increase in phosphatidylinositol 3,4,5-triphosphate (PIP(3)) production. In guinea pig ventricular myocytes, even a modest reduction in PTEN activity is sufficient to increase cellular PIP(3) levels. Under control conditions, PIP(3) levels are kept low by PTEN and do not affect calcium current amplitudes. Based on pharmacological experiments and intracellular infusion of PIP(3), we propose that in guinea pig ventricular myocytes, PIP(3) regulates calcium currents independently of the protein kinase Akt along a pathway that includes a secondary oxidation-sensitive target. Overall, our report describes a novel form of acquired long QT syndrome where the target modified by As(2)O(3) is an intracellular signaling cascade.

Induction of sensory long-term facilitation in the carotid body by intermittent hypoxia: implications for recurrent apneas.

Reflexes from the carotid body have been implicated in cardiorespiratory disorders associated with chronic intermittent hypoxia (CIH). To investigate whether CIH causes functional and/or structural plasticity in the carotid body, rats were subjected to 10 days of recurrent hypoxia or normoxia. Acute exposures to 10 episodes of hypoxia evoked long-term facilitation (LTF) of carotid body sensory activity in CIH-conditioned but not in control animals. The magnitude of sensory LTF depended on the length of CIH conditioning and was completely reversible and unique to CIH, because conditioning with a comparable duration of sustained hypoxia was ineffective. Histological analysis revealed no differences in carotid body morphology between control and CIH animals. Previous treatment with superoxide anion (O2.-) scavenger prevented sensory LTF. In the CIH-conditioned animals, carotid body aconitase enzyme activity decreased compared with controls. These observations suggest that increased generation of reactive oxygen species contribute to sensory LTF. In CIH animals, carotid body complex I activity of the mitochondrial electron transport is inhibited, suggesting mitochondria as one source of O2.- generation. These observations demonstrate that CIH induces a previously uncharacterized form of reactive oxygen species-dependent, reversible, functional plasticity in carotid body sensory activity. The sensory LTF may contribute to persistent reflex activation of sympathetic nerve activity and blood pressure in recurrent apnea patients experiencing CIH.

CO(2) and pH independently modulate L-type Ca(2+) current in rabbit carotid body glomus cells.

The carotid bodies respond to changes in arterial O(2), CO(2), and pH, and Ca(2+) influx via voltage-gated Ca(2+) channels is an important step in the chemoreception process. The objectives of the present study were as follows: 1) to determine whether hypercapnia modulates Ca(2+) current in glomus cells, and if so, to determine if this modulation is secondary to changes in pH; 2) to examine the mechanism of CO(2) modulation of the Ca(2+) current; and 3) to determine whether the effects of hypercapnia and hypoxia on Ca(2+) channel activity in glomus cells are synergistic. The effects of CO(2) on Ca(2+) current were monitored in glomus cells isolated from rabbit carotid bodies using both perforated and conventional patch-clamp techniques. Raising CO(2) in the extracellular solution from 5 to 10% (hypercapnia) reversibly augmented the whole-cell Ca(2+) current. This augmentation was rapid and increased the whole-cell Ca(2+) current similarly in both the perforated and the conventional patch configurations by 16 +/- 2% (n = 5) and 15 +/- 1% (n = 32), respectively. The following observations suggest that the effects of CO(2) are not secondary to changes in pH: 1) isohydric hypercapnia (pH maintained at 7.4) augmented the Ca(2+) current by 24 +/- 2% (n = 6); 2) decreasing the pH of the extra- or intracellular solutions decreased the Ca(2+) current by 43 +/- 4% (n = 8) and 13 +/- 1% (n = 5), respectively; and 3) hypercapnia did not shift the half-maximal activation voltage (V(1/2)), whereas intracellular and extracellular acidosis alone caused shifts in V(1/2). Furthermore, 100 nM of a membrane-permeable protein kinase A inhibitor prevented the augmentation by CO(2), and 500 microM 8-Br-cAMP mimicked the effect of CO(2) by augmenting the Ca(2+) current by 10 +/- 2% (n = 6). Also, cyclic AMP levels in carotid bodies increased from 1.98 +/- 0.6 to 9.0 +/- 2 pmol/microg protein in response to hypercapnia. In contrast, decreasing pH in the nominal absence of CO(2) did not affect cAMP levels in rabbit carotid bodies. Further, nisoldipine, but not omega-conotoxin MVIIC, prevented augmentation of the Ca(2+) current by CO(2). In addition, when combined, hypercapnia and hypoxia augmented the Ca(2+) current by 26 +/- 4% (n = 7), which is greater than either stimulus alone, suggesting the effects are additive. Taken together, these results indicate that L-type Ca(2+) current is augmented by hypercapnia. The effect of CO(2) is not secondary to changes in pH and seems to be mediated by a protein kinase A-dependent mechanism. Furthermore, hypercapnia and hypoxia act additively in stimulating Ca(2+) current in glomus cells.