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

Abstract

1. Effects 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 specific tyrosine kinase inhibitor, genistein, inhibited the pinacidil-induced current in a concentration-dependent manner with an IC₅₀ of 5.5 μM. Superfusion of Ca²⁺-free solution did not affect this inhibitory effect of genistein. At higher concentrations, genistein inhibited the voltage-dependent Ba²⁺ and K⁺ currents with IC₅₀ values of >100 μM and 75 μM respectively. Tyrphostin B46 (30 μM), a tyrosine kinase inhibitor, also inhibited the pinacidil-induced current by 70 % of the control.

3. Sodium orthovanadate (100 μM), an inhibitor of tyrosine phosphatase, slightly but significantly enhanced both the pinacidil-induced and delayed rectifier K⁺ currents. Daidzein (100 μM), an inactive analogue of genistein, did not inhibit these currents.

4. Neither herbimycin A (1 μM), lavendustin A (30 μM), tyrphostin 23 (10 μM), which are also tyrosine kinase inhibitors, nor wortmannin (10 μM), a phosphatidylinositol 3-kinase inhibitor, had an effect on either the pinacidil-induced or delayed rectifier K⁺ currents. Epidermal growth factor (EGF; 1 μg ml⁻¹) did not induce an outward current or enhance the pinacidil-induced current.

5. Pinacidil alone, in the cell-attached configuration, or pinacidil with GDP, in the inside-out configuration, activated a 42 pS channel in the smooth muscle cells of the rabbit portal vein. Genistein (30 μM) reduced the channel's open probability without inducing a change in unitary conductance at any holding potential (−30 to +20 mV).

6. In the inside-out configuration, genistein at 30 μM did not change the mean channel open time, but reduced the burst duration. At 100 μM genistein abolished channel opening. The inhibitory potencies with which 30 and 100 μM genistein acted on the unitary current of the ATP-sensitive K⁺ channel were similar to those seen in the whole-cell voltage-clamp configuration.

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.

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