RPTPmu and protein tyrosine phosphorylation regulate K(+) channel mRNA expression in adult cardiac myocytes.

Previously, we reported that cell-cell contact regulates K(+) channel mRNA expression in cultured adult rat cardiac myocytes. Here we show that exposing cardiac myocytes to tyrosine kinase inhibitors (genistein, tyrphostin A25), but not inactive analogs, prevents downregulation of Kv1.5 mRNA and upregulation of Kv4.2 mRNA normally observed when they are cultured under low-density conditions. Furthermore, cardiac myocytes cocultured with cells that endogenously (Mv 1 Lu) or heterologously (Chinese hamster ovary cells) express the receptor-type protein tyrosine phosphatase mu (RPTPmu) display Kv1.5 mRNA levels paralleling that which was observed in myocytes cultured under high-density conditions and in intact tissue. In contrast, myocytes cocultured with control cells failed to produce this response. Finally, it is shown that Kv4.2 mRNA expression is unaffected by RPTPmu. These findings reveal that multiple tyrosine phosphorylation-dependent mechanisms control cardiac myocyte K(+) channel genes. Furthermore, we conclude that RPTPmu specifically regulates cardiac myocyte Kv1.5 mRNA expression. Thus this receptor protein tyrosine phosphatase may be important in responses to pathological conditions associated with the loss of cell-cell interactions in the heart.

Cell-cell contact between adult rat cardiac myocytes regulates Kv1.5 and Kv4.2 K+ channel mRNA expression.

Regulation of voltage-gated K+ channel genes represents an important mechanism for modulating cardiac excitability. Here we demonstrate that expression of two K+ channel mRNAs is reciprocally controlled by cell-cell interactions between adult cardiac myocytes. It is shown that culturing acutely dissociated rat ventricular myocytes for 3 h results in a dramatic downregulation of Kv1.5 mRNA and a modest upregulation of Kv4.2 mRNA. These effects are specific, because similar changes are not detected with other channel mRNAs. Increasing myocyte density promotes maintenance of Kv1.5 gene expression, whereas Kv4.2 mRNA expression was found to be inversely proportional to cell density. Conditioned culture medium did not mimic the effects of high cell density. However, paraformaldehyde-fixed myocytes were comparable to live cells in their ability to influence K+ channel message levels. Thus the reciprocal effects of cell density on the expression of Kv1.5 and Kv4.2 genes are mediated by direct contact between adult cardiac myocytes. These findings reveal for the first time that cardiac myocyte gene expression is influenced by signaling induced by cell-cell contact.

Decreased expression of Kv4.2 and novel Kv4.3 K+ channel subunit mRNAs in ventricles of renovascular hypertensive rats.

Hypertension-induced cardiac hypertrophy is associated with alterations in ventricular action potentials. To understand molecular mechanisms underlying this electrical abnormality, expression of cardiac voltage-gated K+ channel subunit genes was examined in ventricles of renovascular hypertensive rats. While generating a rat Kv4.3 probe, we discovered a previously unreported 19-amino acid insertion in the C-terminal intracellular region of the channel subunit. RNase protection assays indicated that this novel isoform is predominant in rat lung and heart. Effects of renovascular hypertension were then determined by using renal artery clipping models: two-kidney, one clip (2K-1C) rats, a model of high-renin hypertension with a normal plasma volume, and one-kidney, one clip (1K-1C) rats, a model of normal renin with a raised plasma volume. Expression of Kv4.2 and Kv4.3 mRNAs was diminished by > 50% in ventricles of 2K-1C rats; however, no changes in the expression of Kv1.2, Kv1.4, Kv1.5, Kv2.1, or KvLQT1 mRNAs were detected. Similar downregulation of Kv4.2 and Kv4.3 mRNAs was detected in 1K-1C rats. Chronic administration of captopril, an angiotensin-converting enzyme inhibitor, blocked the development of hypertension and the suppression of Kv4 subfamily channel mRNA expression in 2K-1C rats. Furthermore, captopril administration to sham-operated rats significantly increased Kv4.2 mRNA. These results indicate that renovascular hypertension causes specific reductions in Kv4 subfamily channel mRNA expression and that this effect is likely to be mediated primarily by an increase in cardiac afterload.

Dexamethasone and stress upregulate Kv1.5 K+ channel gene expression in rat ventricular myocytes.

Hormones may produce long-term effects on excitability by regulating K+ channel gene expression. Previous studies demonstrated that administration of dexamethasone, a glucocorticoid receptor agonist, to adrenalectomized rats, rapidly induces Kv1.5 K+ channel expression in the ventricle of the hear. Here, RNase protection assays and Northern blots are used to examine the cell type specificity of dexamethasone action and to test whether Kv1.5 gene expression can be regulated by a physiological stimulus. We show that Kv1.5 mRNA expression in the central nervous system is highest in the hypothalamus. However, dexamethasone treatment of adrenalectomized rats fails to affect Kv1.5 mRNA levels in hypothalamus or lung. In contrast, dramatic upregulation of Kv1.5 mRNA is seen in skeletal muscle and pituitary. Increased Kv1.5 message also found in isolated ventricular cardiomyocytes following in vivo treatment with dexamethasone. Finally, it is shown that cold stress of intact rats significantly increases cardiac Kv1.5 mRNA expression. We conclude that dexamethasone induction of Kv1.5 gene is tissue-specific. Furthermore, our results suggest that stress may act via glucocorticoids to increase Kv1.5 gene expression in ventricular cardiomyocytes. Hence, K+ channel gene expression can be influenced by physiological and pharmacological stimuli.

GABAB antagonists diminish the inhibitory gating of auditory response in the rat hippocampus.

Auditory evoked responses recorded from the CA3 region of the rat hippocampus show diminished response to repeated stimuli, suggesting the activity of an inhibitory gating mechanism. The effects on this putative gating mechanism of two GABAB receptor antagonists, CGP35348 and CGP46381, were characterized in a conditioning testing paradigm. Both compounds, administered intracerebroventricularly, antagonized the suppression of response to the test stimulus. The results are consistent with the hypothesis that this inhibitory gating of response involves GABAB receptors, which may control the release of glutamate from excitatory pathways in the hippocampus.

Adaptive supersensitivity and the Na+/K+ pump in the guinea pig vas deferens: time course of the decline in the alpha 2 subunit.

Adaptive supersensitivity in the guinea pig vas deferens has been shown previously to be associated with decreases in transmembrane potential, Na+/K+-ATPase activity, [3H]ouabain binding sites, and density of the alpha 2 subunit of the pump. One of several procedures that induce adaptive supersensitivity in the guinea pig vas deferens is neurotransmitter depletion by chronic administration of reserpine. Guinea pigs were treated with reserpine (1.0 mg/kg/day, intraperitoneally) for 2, 5, or 8 days. Tissues were homogenized and the concentration of the alpha 2 subunit was quantified by use of the selective antibody McB2, slot blot analysis, enhanced chemiluminescence, and densitometric analysis. As reported previously, the concentration of the alpha 2 protein was reduced 41% after 5 days of pretreatment. The reduction was maintained at 8 days (37%). However, there was no change from control after 2 days of pretreatment with reserpine. Thus, the time course of the decline in the alpha 2 subunit is similar to that of the appearance of supersensitivity, depolarization, and the declines in Na+/K+-ATPase and [3H]ouabain binding established earlier. Based upon results in the literature for several different tissues and species, membrane depolarization and decreases in Na+/K+ pump sites may represent widely occurring adaptive mechanisms.

Adaptive supersensitivity in the guinea pig vas deferens is associated with a reduction in the abundance of the alpha 2 subunit isoform of Na+/K(+)-ATPase.

Adaptive supersensitivity has been demonstrated previously in the guinea pig vas deferens after chronic treatment with reserpine, postganglionic denervation, or preganglionic denervation. The magnitude of the change in sensitivity was similar regardless of the method of induction; the underlying mechanism was identified as a partial depolarization secondary to reduced activity of the Na+/K+ pump. Experiments were conducted to quantitatively determine whether the identified losses in Na+/K(+)-ATPase activity and [3H]ouabain binding were due to reductions in the levels of specific protein subunits of the sodium pump. Electrophoretic separation and quantification of the abundance of alpha subunit isoforms were accomplished using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and slot blot analysis. Supersensitivity was induced in the guinea pig vas deferens through pretreatment with reserpine (1.0 mg/kg/day x 5 days). The abundance of the alpha 2 subunit isoform was reduced by 41% in tissue homogenates obtained from animals treated with reserpine, compared with untreated controls. In contrast, there was no significant alteration in the alpha 1 subunit isoform (a protein similar in size to that previously identified in vascular smooth muscle as a "truncated" form of the protein). These data suggest that the adaptation of the guinea pig vas deferens after a chronic reduction in net stimulus is mediated through a change in a specific cellular protein. This evidence supports the assignment of the alpha 2 subunit isoform as the specific protein responsible for the development of nonspecific adaptive supersensitivity in the guinea pig vas deferens.