Inhibitory effects of genistein on ATP‐sensitive K⁺ channels in rabbit portal vein smooth muscle

Abstract

1 E ects on the pinacidil-induced outward current of inhibitors of tyrosine kinases and phosphatases were investigated by use of a patch-clamp method in smooth muscle cells of the rabbit portal vein. 2 A speci®c tyrosine kinase inhibitor, genistein, inhibited the pinacidil-induced current in a concentration-dependent manner with an IC 50 of 5.5 mM. Superfusion of Ca 2+ -free solution did not a ect this inhibitory e ect of genistein. At higher concentrations, genistein inhibited the voltagedependent Ba 2+ and K + currents with IC 50 values of 4100 mM and 75 mM respectively. Tyrphostin B46 (30 mM), a tyrosine kinase inhibitor, also inhibited the pinacidil-induced current by 70 % of the control. 3 Sodium orthovanadate (100 mM), an inhibitor of tyrosine phosphatase, slightly but signi®cantly enhanced both the pinacidil-induced and delayed recti®er K + currents. Daidzein (100 mM), an inactive analogue of genistein, did not inhibit these currents. 4 Neither herbimycin A (1 mM), lavendustin A (30 mM), tyrphostin 23 (10 mM), which are also tyrosine kinase inhibitors, nor wortmannin (10 mM), a phosphatidylinositol 3-kinase inhibitor, had an e ect on either the pinacidil-induced or delayed recti®er K + currents. Epidermal growth factor (EGF; 1 mg ml 71 ) did not induce an outward current or enhance the pinacidil-induced current. 5 Pinacidil alone, in the cell-attached con®guration, or pinacidil with GDP, in the inside-out con®guration, activated a 42 pS channel in the smooth muscle cells of the rabbit portal vein. Genistein (30 mM) reduced the channel's open probability without inducing a change in unitary conductance at any holding potential (730 to +20 mV). 6 In the inside-out con®guration, genistein at 30 mM did not change the mean channel open time, but reduced the burst duration. At 100 mM genistein abolished channel opening. The inhibitory potencies with which 30 and 100 mM genistein acted on the unitary current of the ATP-sensitive K + channel were similar to those seen in the whole-cell voltage-clamp con®guration. 7 Although direct inhibitory actions of genistein on the ATP-sensitive K + channels are not ruled out, our results suggest that a protein tyrosine kinase may play a role in the regulation of ATP-sensitive K + channel activity in the rabbit portal vein.

Genistein, a Soybean Isoflavone, Inhibits Inward Rectifier K+ Channels in Rat Osteoclasts.

“…PTK inhibitors targeting the substrate binding site of PTKs (e.g., tyrphostins) have been frequently used to assess the modulating effects of tyrosine phosphorylation on ionic channels. In those studies, concentration ranges of 10 to 100 M have been used (for tyrphostin B46 [10]; for tyrphostin B42 [12]). At similar concentrations, tyrphostins were capable of inhibiting I Kir (Fig.…”

“…Genistein has also been shown to modulate ion channel activity in various cells through mechanisms dependent on [10][11][12] or independent of [13][14][15][16] PTK. Inward rectifier K ϩ channels expressed in osteoclasts have been suggested to play important roles in maintaining the activity of bone resorption [17].…”

Cardiovascular Ion Channels as a Molecular Target of Flavonoids

“…muscle in a dose-dependent manner [41]. They state that current inhibition might be due to a combination of direct channel inhibition together with tyrosine kinase activation [41]. Jin et al…”

“…Taken together, activation of calcium activated potassium channels seems to be a key mechanism in flavonoid induced vasorelaxation and might account for a good portion of the observed beneficial effect of flavonoids in CVD. muscle in a dose-dependent manner [41]. They state that current inhibition might be due to a combination of direct channel inhibition together with tyrosine kinase activation [41].…”

Activation of Background Membrane Conductance by the Tyrosine Kinase Inhibitor Tyrphostin A23 and Its Inactive Analog Tyrphostin A1 in Guinea Pig Ventricular Myocytes

“…We deem that the observed effects of tyrphostins are non-specific ones that are independent of TK inhibition. In this regard, tyrphostins are relatively low molecular weight compounds that compete with TK substrates and ATP (1, 2) and can directly act on other cellular targets (7,9,12,15). Thus, it is possible to speculate that the stimulatory action of tyrphostins is due to opportunistic binding of tyrphostin molecules to the regulatory region of the exchanger protein and subsequent modulation of the exchanger activity.…”