Effects of the enantiomers of BayK 8644 on the charge movement of L-type Ca channels in guinea-pig ventricular myocytes

The effects of the agonist enantiomer S(-)Bay K 8644 on gating charge of L-type Ca channels were studied in single ventricular myocytes. From a holding potential (V-h) of -40 mV, saturating (250 nm) S(-)Bay K shifted the half-distribution voltage of the activation charge (Q1) vs. V curve -7.5 +/- 0.8 mV, almost identical to the shift produced in the Ba conductance vs. V curve (- 7.7 +/- 2 mV). The maximum Q1 was reduced by 1.7 +/- 0.2 nC/muF, whereas Q2 (charge moved in inactivated channels) was increased in a similar amount (1.4 +/- 0.4 nC/muF). The steady-state availability curves for Q1, Q2, and Ba current showed almost identical negative shifts of - 14.8 +/- 1.7 mV, - 18.6 +/- 5.8 mV, and - 15.2 +/- 2.7 mV, respectively. The effects of the antagonist enantiomer R(+)BayK 8644 were also studied, the Q1 vs. V curve was not significantly shifted, but Q1(max) (V-h = -40 mV) was reduced and the Q1 availability curve shifted by -24.6 +/- 1.2 mV. We concluded that: a) the left shift in the Q1 vs. V activation curve produced by S(-)BayK is a purely agonistic effect; b) S(-)BayK induced a significantly larger negative shift in the availability curve than in the Q1 vs. V relation, consistent with a direct promotion of inactivation; c) as expected for a more potent antagonist, R(+)Bay K induced a significantly larger negative shift in the availability curve than did S(-) Bay K.