Inhibition of Ca2+ current in neonatal and adult rat ventricular myocytes by the tyrosine kinase inhibitor, genistein

A role for focal adhesion kinase in facilitating the contractile responses of murine gastric fundus smooth muscles

KEY POINTS

Activation of focal adhesion kinase (FAK) by integrin signalling facilitates smooth muscle contraction by transmitting the force generated by myofilament activation to the extracellular matrix and throughout the smooth muscle tissue. Here we report that electrical field stimulation (EFS) of cholinergic motor neurons activates FAK in gastric fundus smooth muscles, and that FAK activation by EFS is atropine-sensitive but nicardipine-insensitive. PDBu and calyculin A contracted gastric fundus muscles Ca²⁺ -independently and also activated FAK. Inhibition of FAK activation inhibits the contractile responses evoked by EFS, and inhibits CPI-17 phosphorylation at T38. This study indicates that mechanical force or tension is sufficient to activate FAK, and that FAK appears to be involved in the activation of the protein kinase C-CPI-17 Ca²⁺ sensitization pathway in gastric fundus smooth muscles. These results reveal a novel role for FAK in gastric fundus smooth muscle contraction by facilitating CPI-17 phosphorylation.

ABSTRACT

Smooth muscle contraction involves regulating myosin light chain phosphorylation and dephosphorylation by myosin light chain kinase and myosin light chain phosphatase. C-kinase potentiated protein phosphatase-1 inhibitor of 17 kDa (CPI-17) and myosin phosphatase targeting subunit of myosin light-chain phosphatase (MYPT1) are crucial for regulating gastrointestinal smooth muscle contraction by inhibiting myosin light chain phosphatase. Integrin signalling involves the dynamic recruitment of several proteins, including focal adhesion kinase (FAK), to focal adhesions. FAK tyrosine kinase activation is involved in cell adhesion to the extracellular matrix via integrin signalling. FAK participates in linking the force generated by myofilament activation to the extracellular matrix and throughout the smooth muscle tissue. Here, we show that cholinergic stimulation activates FAK in gastric fundus smooth muscles. Electrical field stimulation in the presence of N^ω -nitro-l-arginine methyl ester and MRS2500 contracted gastric fundus smooth muscle strips and increased FAK Y397 phosphorylation (pY397). Atropine blocked the contractions and prevented the increase in pY397. The FAK inhibitor PF-431396 inhibited the contractions and the increase in pY397. PF-431396 also inhibited the electrical field stimulation-induced increase in CPI-17 T38 phosphorylation, and reduced MYPT1 T696 and T853, and myosin light chain S19 phosphorylation. Ca²⁺ influx was unaffected by PF-431396. Nicardipine inhibited the contractions but had no effect on the increase in pY397. Phorbol 12,13-dibutyrate or calyculin A contracted gastric fundus smooth muscle strips Ca²⁺ independently and increased pY397. Our findings suggest that FAK is activated by mechanical forces during contraction and reveal a novel role of FAK in the regulation of CPI-17 phosphorylation.

Cardiovascular ion channels as a molecular target of flavonoids

Flavonoids are a class of naturally occurring polyphenols abundant in edibles and beverages of plant origin. Epidemiological studies consistently associate high flavonoid intake with a reduced risk for the development of cardiovascular diseases. So far these beneficial effects have been mainly attributed to nonspecific antioxidant and antiinflammatory properties. However, there is an increasing body of evidence that flavonoids specifically target molecular structures including cardiovascular ion channels. Playing a pivotal role in the regulation of vascular tone and cardiac electric activity, ion channels represent a major target for the induction of antihypertensive and cardioprotective effects. Thus, pharmacological properties of flavonoids on cardiovascular ion channels, ion currents and tissue preparations are being increasingly addressed in experimental studies. Whereas it has become clear that cardiovascular ion channels represent an important molecular target of flavonoids, the published data have not yet been systematically reviewed.

Pharmacological evidence for altered src kinase regulation of I (Ca,L) in patients with chronic atrial fibrillation

A reduction in L-type Ca(2+) current (I (Ca,L)) contributes to electrical remodeling in chronic atrial fibrillation (AF). Whether the decrease in I (Ca,L) is solely due to a reduction in channel proteins remains controversial. Protein tyrosine kinases (PTK) have been described as potent modulators of I (Ca,L) in cardiomyocytes. We studied alpha(1C) L-type Ca(2+) channel subunit expression and the regulation of I (Ca,L) by PTK in chronic AF using PTK inhibitors: genistein, a nonselective inhibitor of PTK, and 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo-3,4-d-pyrimidine (PP1), a selective inhibitor of src kinases. Furthermore, type-1 and type-2A protein phosphatase activity was measured with phosphorylase as substrate in whole-cell lysates derived from atrial tissue of AF patients. Right atrial appendages were obtained from patients undergoing open-heart surgery. Protein levels of alpha(1C) L-type Ca(2+) channel subunit were determined using Western blot analysis and normalized to the protein amounts of calsequestrin as internal control. The protein concentrations of alpha(1C) did not differ between AF and sinus rhythm (SR; alpha(1C)/calsequestrin: 1.0 +/- 0.1 and 1.2 +/- 0.2, respectively, n = 8 patients). In cardiomyocytes from patients in SR (n = 20 patients), genistein and PP1 both evoked similar increases in I (Ca,L) from 3.0 +/- 0.3 to 6.1 +/- 0.8 pA/pF and from 2.8 +/- 0.4 to 6.1 +/- 0.6 pA/pF, respectively. In cells from AF patients (n = 10 patients), basal I (Ca,L) was significantly lower. In this case, genistein lead to the same relative increase in I (Ca,L) as in SR cells (from 1.46 +/- 0.30 to 3.2 +/- 1.0 pA/pF), whereas no increase was elicited by PP1 suggesting impaired regulation of I (Ca,L) by src kinases in AF. Total and type 1 and type 2A-related phosphatase activities were higher in tissue from patients with chronic AF compared to SR (4.8 +/- 0.4, 2.1 +/- 0.2, and 2.7 +/- 0.4 nmol/mg/min and 3.6 +/- 0.4, 1.3 +/- 0.2, and 2.4 +/- 0.3 nmol/mg/min, respectively, n = 7 patients per group). Downregulation of I (Ca,L) in AF is not due to a reduction in L-type Ca(2+) channel protein expression. Indirect evidence for an impaired src kinase regulation of I (Ca,L) together with an increased phosphatase activity suggests that a complex alteration in the kinase/phosphatase balance leads to I (Ca,L) dysregulation in chronic AF.

Lipid products of phosphoinositide 3-kinase abrogate genistein-induced fusion inhibition in myoblasts

Genistein (4',5,7-trihydroxyisoflavone) is a tyrosine kinase inhibitor. Although the agent has shown to inhibit myoblast differentiation, neither intracellular target(s) as a tyrosine kinase inhibitor nor action mechanism of the agent is well known. Here we studied the effect of genistein on the differentiation of myoblasts. Genistein strongly but reversibly blocked both myoblast fusion and synthesis of the muscle-specific proteins. The agent also reversibly reduced the phosphorylation level of focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase, and its interaction with p85, the regulatory subunit of phosphoinositide 3-kinase (PI3-kinase). In addition, genistein indirectly inhibited PI3-kinase activity and blocked calcium influx which is required for myoblast fusion. However, both genistein-induced inhibition of cell fusion and calcium influx were abrogated by the lipid products of PI3-kinase. These results demonstrate that genistein can exert their effect on the signaling pathway from FAK to calcium influx via PI3-kinase in the differentiation of myoblasts.

Direct inhibition of the pacemaker (If) current in rabbit sinoatrial node cells by genistein

Genistein is a tyrosine kinase inhibitor which interferes with the activity of several ionic channels either by altering modulatory phosphorylating processes or by direct binding. In whole-cell conditions, genistein induces a partial inhibition of the pacemaker (I(f)) current recorded in cardiac sinoatrial and ventricular myocytes. We investigated the mechanism of action of genistein (50 microM) on the I(f) current in whole-cell, cell-attached, and inside-out configurations, and the measured fractional inhibitions were similar: 26.6, 27.2, and 33.6%, respectively. When ATP was removed from the whole-cell pipette solution no differences were revealed in the effect of the drug when compared to metabolically active cells. Genistein fully maintained its blocking ability even when herbimycin, a tyrosine kinase inhibitor, was added to the whole-cell ATP-free pipette solution. Genistein-induced block was independent of the gating state of the channel and did not display voltage or current dependence; this independence distinguishes genistein from all other f-channel blockers. When inside-out experiments were performed to test for a direct interaction with the channel, genistein, superfused on the intracellular side of the membrane, decreased the maximal I(f) conductance, and slightly shifted the current-activation curve to the left. Furthermore, the effect of genistein was independent of cAMP modulation. We conclude that, in addition to its tyrosine kinase-inhibitory properties, genistein also blocks I(f) by directly interacting with the channel, and thus cannot be considered a valuable pharmacological tool to investigate phosphorylation-dependent modulatory pathways of the I(f) current and of cardiac rhythm.

Src family tyrosine kinases inhibit single L-type: Ca2+ channel activity in human atrial myocytes

OBJECTIVE

Tyrosine kinases (TKs) are important regulators of the L-type Ca(2+) channel (LTCC) current in various cell types. However, there are no data addressing the role of TKs in the control of single LTCC activity in human atrial cardiac myocytes, where changes in LTCC gating properties have been described in a number of disease states.

METHODS AND RESULTS

Single LTCC activity was recorded in isolated human atrial myocytes. The broad-spectrum TK inhibitor genistein and the Src family-selective TK inhibitor PP1 significantly enhanced single LTCC ensemble average current, availability, and open probability; the latter was due to significant increases of mean open time and mode 2 gating. Conversely, the tyrosine phosphatase inhibitor bisperoxo-phenanthroline-vanadate inhibited single LTCC activity, indicating that LTCC gating properties in human atrial myocytes are controlled by TKs and tyrosine phosphatases in a reciprocal fashion. The effects of genistein on single LTCC activity were not affected by stimulation (8Br-cAMP) or inhibition (Rp-8-CPT-cAMPS) of protein kinase A (PKA) or by inhibition of serine/threonine phosphatases types I and IIa (okadaic acid), indicating that TKs inhibit LTCC gating in human atrial myocytes independent of PKA and phosphatases types I and IIa. However, inhibition of protein kinase C (PKC) by staurosporine or bisindolylmaleimide reversed the stimulatory effects of genistein on single LTCC gating properties, indicating that PKC is required for the inhibitory effect of TKs on single LTCC activity.

CONCLUSION

Src family TKs inhibit single LTCC activity in human atrial myocytes via PKC-dependent, but PKA and phosphatase types I and IIa-independent, molecular pathways.

Genistein directly blocks glycine receptors of rat neurons freshly isolated from the ventral tegmental area

The effects of tyrosine kinase inhibitors on the glycine-induced current (I(Gly)) were studied in rat neurons freshly isolated from the ventral tegmental area (VTA). Genistein reversibly and concentration-dependently depressed I(Gly), with an IC(50) of 13 microM. Preincubation with genistein had no effect on I(Gly), indicating that genistein is effective only when glycine is bound to the receptor and channels are most likely open. Genistein depressed maximum I(Gly) without significantly changing the EC(50) for glycine. Genistein-induced inhibition of I(Gly) was sensitive to membrane voltage, being greater at positive membrane potentials. A kinetic analysis indicated that genistein lengthens the time constant of I(Gly) activation, but has no effect on deactivation or desensitization. When genistein was rapidly washed out, a transient rebound current probably reflected a faster dissociation of genistein, with respect to glycine. Results of competition experiments suggest that genistein acts on the same region of the glycine receptor as picrotoxin. Daidzein, an analog of genistein that does not act on protein kinases, also inhibited I(Gly). Co-application of lavendustin A, a specific inhibitor of tyrosine kinase, had no effect on I(Gly). Our results extend to neurons isolated from the VTA, the previous finding that genistein directly inhibits glycine receptors of hypothalamic brain slices.

Inhibition of fast sodium current in rabbit ventricular myocytes by protein tyrosine kinase inhibitors

The present study investigated the effects of protein tyrosine kinase inhibitors on the fast sodium current ( I(Na)) in rabbit ventricular myocytes. Single rabbit ventricular myocytes were isolated enzymatically using Langendorff perfusion. I(Na) was recorded using the whole-cell patch-clamp technique at room temperature. The protein tyrosine kinase inhibitors genistein, AG957, ST638, and PP2 reversibly inhibited I(Na) in a concentration-dependent manner. At a test pulse potential of -30 mV, genistein (n=7) inhibited I(Na) by 37.7+/-3.2%, 53.4+/-2.5%, and 71.8+/-2.7% at concentrations of 15, 50, and 100 microM, respectively, without changing the voltage dependence of activation, while 100 microM AG957, 100 microM ST638, and 30 microM PP2 inhibited I(Na) by 38.7+/-2.4, 35.8+/-3.4, and 21.1+/-3.9%, respectively. Genistein (100 microM) and AG957 (100 microM) shifted the voltage for half-maximal inactivation of I(Na) from -76.7+/-2.0 mV (n=10) in control to -88.37+/-2.6 mV (n=6, P<0.05), and -82.9+/-1.7 (n=4, P<0.05), respectively, without changing the slope factor. Genistein and AG957 also significantly prolonged the time course of I(Na) recovery from inactivation. Daidzein and PP3, inactive analogs of genistein and PP2, respectively, did not inhibit I(Na) significantly. We conclude that protein tyrosine kinase signaling pathways may play an important role in regulation of I(Na) in cardiac myocytes.

Genistein inhibits cardiac L-type Ca(2+) channel activity by a tyrosine kinase-independent mechanism

It has been suggested that protein tyrosine kinase (PTK) activity can directly regulate cardiac L-type Ca(2+) channels. This conclusion is based to a large extent on the observation that the PTK inhibitor genistein can inhibit the cardiac L-type Ca(2+) current. The purpose of the present study was to determine whether the ability of genistein to inhibit cardiac L-type Ca(2+) channel activity is due to inhibition of PTK activity. Genistein significantly reduced the magnitude of the L-type Ca(2+) current in guinea pig ventricular myocytes recorded using the whole-cell patch-clamp technique. However, this effect was associated with extracellular, not intracellular, application of the drug. Peroxovanadate (PVN), a potent protein tyrosine phosphatase inhibitor, had no effect on the basal Ca(2+) current. PVN was also ineffective in preventing the inhibitory effect of genistein. Internal perfusion of cells with a pipette solution containing ATPgammaS was used to prevent reversibility of phosphorylation-dependent processes. This treatment did not alter the inhibitory effect of genistein, although it did result in irreversible protein kinase A-dependent regulation of the Ca(2+) current. Bath application of lavendustin A, a PTK inhibitor that is structurally unrelated to genistein, did not affect the Ca(2+) current amplitude. The inhibitory effect of genistein was also associated with a hyperpolarizing shift in the voltage dependence of Ca(2+) channel inactivation. These results are consistent with the conclusion that the cardiac L-type Ca(2+) current is not directly regulated by PTK activity and that the inhibitory effect of genistein is due to direct non-catalytic blockade of the channels.

Differential MAP kinase activation and Na(+)/H(+) exchanger phosphorylation by H(2)O(2) in rat cardiac myocytes

Bursts in reactive oxygen species production are important mediators of contractile dysfunction during ischemia-reperfusion injury. Cellular mechanisms that mediate reactive oxygen species-induced changes in cardiac myocyte function have not been fully characterized. In the present study, H(2)O(2) (50 microM) decreased contractility of adult rat ventricular myocytes. H(2)O(2) caused a concentration- and time-dependent activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), p38, and c-Jun NH(2)-terminal kinase (JNK) mitogen-activated protein (MAP) kinases in adult rat ventricular myocytes. H(2)O(2) (50 microM) caused transient activation of ERK1/2 and p38 MAP kinase that was detected as early as 5 min, was maximal at 20 min (9.6 +/- 1.2- and 9.0 +/- 1.6-fold, respectively, vs. control), and returned to baseline at 60 min. JNK activation occurred more slowly (1.6 +/- 0.2-fold vs. control at 60 min) but was sustained at 3.5 h. The protein kinase C inhibitor chelerythrine completely blocked JNK activation and reduced ERK1/2 and p38 activation. The tyrosine kinase inhibitors genistein and PP-2 blocked JNK, but not ERK1/2 and p38, activation. H(2)O(2)-induced Na(+)/H(+) exchanger phosphorylation was blocked by the MAP kinase kinase inhibitor U-0126 (5 microM). These results demonstrate that H(2)O(2)-induced activation of MAP kinases may contribute to cardiac myocyte dysfunction during ischemia-reperfusion.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

Effects of dietary phytoestrogen on global myocardial ischemia-reperfusion injury in isolated female rat hearts

We investigated the effects of phytoestrogen on global myocardial ischemia-reperfusion injury in five groups of female rats. A high-phytoestrogen group (HPE) was ovariectomized (Ovx) and fed a diet containing soybean protein and a high-isoflavone soy extract. Another Ovx group of rats was fed the same diet as the HPE group but treated with the estrogen receptor blocker ICI-182,780 (HPE + ICI). A third group of Ovx rats was fed a diet containing soybean protein alone (low-phytoestrogen content; LPE). A fourth Ovx group was fed a diet free of phytoestrogen (Ovx). The fifth group of rats was sham ovariectomized (sham). Hearts from all rats were subjected to 30 min of global, hypothermic (4 degrees C), cardioplegic ischemia and 120 min of normothermic (37 degrees C) reperfusion with oxygenated Krebs-Henseleit buffer. Compared with either the sham or the HPE group, the Ovx and HPE + ICI groups had significantly decreased first derivative of left ventricular pressure (dP/dt), coronary flow rate (CFR), nitrite production and mitochondrial respiratory function and significantly increased Ca2+ accumulation and myocardial histological and ultrastructural injury. The CFR of the LPE group was significantly different from that of either Ovx or HPE + ICI group but the dP/dt, nitrite production, Ca2+ accumulation, and mitochondrial function were not. Our results indicate that diets containing phytoestrogen extract play a cardioprotective role in global myocardial ischemia-reperfusion in female rats.

Involvement of tyrosine kinase activity in 1alpha,25(OH)2-vitamin D3 signal transduction in skeletal muscle cells

In cultured chick skeletal muscle cells loaded with Fura-2, the tyrosine kinase inhibitors herbimycin A and genistein abolished both the fast inositol 1,4,5-trisphosphatedependent Ca(2+) release from internal stores and extracellular Ca(2+) influx induced by 1alpha, 25(OH)(2)-vitamin D(3) (1alpha,25(OH)(2)D(3)). Daidzein, an inactive analog of genistein, was without effects. Tyrosine phosphatase inhibition by orthovanadate increased cytosolic Ca(2+). Anti-phosphotyrosine immunoblot analysis revealed that 1alpha, 25(OH)(2)D(3) rapidly (0.5-10 min) stimulates in a concentrationdependent fashion (0.1-10 nm) tyrosine phosphorylation of several myoblast proteins, among which the major targets of the hormone could be immunochemically identified as phospholipase Cgamma (127 kDa), which mediates intracellular store Ca(2+) mobilization and external Ca(2+) influx, and the growth-related proteins mitogen-activated protein (MAP) kinase (42/44 kDa) and c-myc (65 kDa). Genistein suppressed the increase in phosphorylation and concomitant elevation of MAPK activity elicited by the sterol. Both genistein and the MAPK kinase (MEK) inhibitor PD98059 abolished stimulation of DNA synthesis by 1alpha,25(OH)(2)D(3). The sterol-induced increase in tyrosine phosphorylation of c-myc, a finding not reported before for cell growth regulators, was totally suppressed by the specific Src inhibitor PP1. These results demonstrate that tyrosine phosphorylation is a previously unrecognized mechanism involved in 1alpha,25(OH)(2)D(3) regulation of Ca(2+) homeostasis in hormone target cells. In addition, the data involve tyrosine kinase cascades in the mitogenic effects of 1alpha, 25(OH)(2)D(3) on skeletal muscle cells.

Phosphatidic acid increases inositol-1,4,5,-trisphosphate and [Ca2+]i levels in neonatal rat cardiomyocytes

Phosphatidic acid (PA), which can be synthesized de novo, or as a product of phosphatidylcholine hydrolysis and/or phosphorylation of 1,2-diacylglycerol (DAG), mediates diverse cellular functions in various cell types, including cardiomyocytes. We set out to characterize the effect of PA on intracellular free calcium ([Ca2+]i) and inositol-1,4,5-trisphosphate (IP(3)) levels in primary cultures of neonatal rat cardiomyocytes. Addition of PA led to rapid, concentration and time dependent increases in both IP(3) and [Ca2+]i levels in adherent cells. There was strong correlation in the concentration-response relationships between IP(3) and [Ca2+]i increases evoked by PA. Incubation with the sarcoplasmic reticulum (SR) Ca2+ pump inhibitor, cyclopiazonic acid (CPA), significantly attenuated the PA evoked [Ca2+]i increase but had no significant effect on IP(3) accumulation. The phospholipase C (PLC) inhibitor, D-609, attenuated both IP(3) and [Ca2+]i elevations evoked by PA whereas staurosporine (STS), a potent and non-selective PKC inhibitor, had no significant effect on either. Another PLC inhibitor, U73122, but not its inactive analog, U73343, also inhibited PA evoked increases in [Ca2+]i. Depletion of extracellular calcium attenuated both basal and PA evoked increases in [Ca2+]i. The PLA(2) inhibitors, bromophenylacyl-bromide (BPB) and CDP-choline, had no effect on PA evoked [Ca2+]i responses. Neither the DAG analog, dioctanoylglycerol, nor the DAG kinase inhibitor, R59949, affected PA evoked changes in [Ca2+]i. Taken together, these data indicate that PA, in a manner independent of PKC, DAG, or PLA(2), may enhance Ca2+ release from IP(3) sensitive SR Ca(2+) stores via activation of PLC in neonatal rat cardiomyocytes.

L-type Ca2+ current in guinea pig ventricular myocytes treated with modulators of tyrosine phosphorylation

Guinea pig ventricular myocytes in whole cell configuration were treated with tyrosine kinase (TK) inhibitors [genistein (Gst), tyrphostin A23 (T23), and tyrphostin A25 (T25)] and with inactive analogs [daidzein, genistin, and tyrphostin A1 (T1)] to measure effects on L-type Ca2+ current (ICa,L). Gst inhibited ICa,L (IC50 = 47 microM) without affecting its time course or shifting the ICa, L-voltage relationship. At the highest concentration of isoflavone tested (200 microM), ICa,L was inhibited by 66 +/- 7% (Gst), 22 +/- 2% (daidzein), and 1 +/- 3% (genistin). Inhibition of ICa,L by the active tyrphostins was significantly larger than inhibition by T1; at 200 microM the inhibitions were 72 +/- 6% (T23), 71 +/- 6% (T25), and 27 +/- 6% (T1). The phosphotyrosine phosphatase inhibitor orthovanadate (1 mM) had a small stimulatory effect (6 +/- 2%) on basal ICa,L and blocked the inhibition of ICa,L by TK inhibitors. The data suggest a role for the TK-phosphotyrosine phosphatase system in the regulation of cardiac Ca2+ channels.

Tyrosine kinase-dependent modulation by interferon-alpha of the ATP-sensitive K+ current in rabbit ventricular myocytes

We examined the effects of interferon-alpha on the ATP-sensitive K+ current (IK,ATP) in rabbit ventricular cells using the patch-clamp technique. IK,ATP was induced by NaCN. Whole-cell experiments indicated that interferon-alpha (5 x 10(2) - 2.4 x 10(4) U/ml) inhibited IK,ATP in a concentration-dependent manner (60.7+/-7.5% with 2.4 x 10(4) U/ml). In cell-attached configuration, interferon-alpha (2.4 x 10(4) U/ml) applied to the external solution also inhibited the activity of the single ATP-sensitive K+ (KATP) channel by 56.0+/-5.8% without affecting the single channel conductance. The inhibitory effect of IK,ATP by interferon-alpha was blocked by genistein and herbimycin A, tyrosine kinase inhibitors, but was not affected by N-(2-metylpiperazyl)-5-isoquinolinesulfoamide (H-7), an inhibitor of protein kinase C and cAMP-dependent protein kinase. These findings suggest that interferon-alpha inhibits the cardiac KATP channel through the activation of tyrosine kinase. The tyrosine kinase-mediated inhibition of IK,ATP by cytokines may aggravate cell damage during myocardial ischemia.