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

Yokoshiki et al. (Yokoshiki, H., Sumii, K., Sperelakis, N., 1996. Inhibition of L-type calcium current in rat ventricular cells by the tyrosine kinase inhibitor, genistein and its inactive analog, daidzein. J. Mel. Cell. Cardiol. 28, 807-814) reported that genistein and daidzein inhibited L-type Ca2+ current (I-Ca(L)) in young rat ventricular cells. Therefore, we investigated the developmental differences in the effect of genistein, an inhibitor of tyrosine kinases, on I-Ca(L) in freshly-isolated neonatal (3-7 days) and adult (2-5 months) rat ventricular myocytes using whole-cell voltage clamp and single-channel recordings (cell-attached configuration). For whole-cell voltage clamp, I-Ca(L) was measured as the peak inward current at a test potential of + 10 mV by applying a 300 ms pulse from a holding potential of -40 mV. To isolate I-Ca(L) the pipette solution was Cs+-rich and the bath solution was Na+-, K+-free. Ca2+ (1.8 mM) was used as charge carrier. Bath application of 100 mu M genistein (sufficient for maximal effect) decreased the basal I-Ca(L) by 43.3% (n = 27) in neonatal cells and by 30.6% (n = 14) in adult cells (P < 0.05). In the current/voltage relationships, the potential of peak I-Ca(L) was shifted to the right by genistein by 8.6 mV in neonatal and by 9.3 mV in adult cells. Genistein produced a shift of the steady-state inactivation curve (to the left) in neonatal cells (from -16.0 +/- 3.9 mV to -26.1 +/- 4.2 mV; P < 0.05) and in adult cells (-15.9 +/- 3.2 mV to -22.9 +/- 3.3 mV; P < 0.05); the slope factor was not affected. For single-channel recordings in cell-attached patches, Ca2+ currents were evoked by applying a 150 ms pulse from a holding potential of -40 mV to a test potential of 0 mV. The pipette solution contained 110 mM Ba2+ las charge carrier), and the bath solution contained 150 mM K+ (to bring resting potential to near zero). Genistein (50 mu M) decreased the open probability of the channels from 2.8% to 0.75% (P < 0.05) in absence of Bay K 8644, and from 24% to 7.9% (P < 0.05) in presence of Bay K 8644; the mean open time and the slope conductance of the currents were not affected. In conclusion, (1) genistein inhibits the basal I-Ca(L) in in rat ventricular cells and (2) the inhibition of I-Ca(L) by genistein is greater in immature cells than in adult cells. (C) 1998 Elsevier Science B.V.