THE MOLECULAR-MODE OF ACTION OF THE CA AGONIST (-) BAY K-8644 ON THE CARDIAC CA CHANNEL

The primary drug action of (-) BAY K 8644 on whole-cell Ca current in atrial myocytes was measured under conditions where secondary Ca-mediated changes of Ca channel activity were minimized. The most direct action of (-) BAY K 8644 is the change of gating kinetics which results in a strictly voltage-dependent increase of the peak current in the voltage range between -40 and 0 mV. Peak currents were increased dose dependently in the concentration range from 1 to 30 nM. Analysis of peak current/voltage relations revealed a linear shift of the current activation by approximately 23 mV to more negative membrane potentials, without any change in its voltage dependence and in the current reversal potential or the maximum whole-cell conductance. Measurement of Ca current activation and deactivation time constants suggests that (-) BAY K 8644 prolongs the single-channel open time without affecting the closed time. From the shift of the open time function to more negative voltages by about 50 mV the energy transferred to the gating process is calculated to be 5.4 kJ/mol (1.3 kcal/mol). The drug-induced slow component of tail current has been used to estimate the true dose/response relation for (-) BAY K 8644. A K(D) value of 4.3 nM and a Hill coefficient of 1.25 were determined. Flash-induced competition experiments with the Ca antagonist nifedipine allowed the measurement of binding kinetics of (-) BAY K 8644. The association rate constant is estimated to about 5 X 10(6) mol-1 . s-1 and dissociation time constant is approximately 50-70 s; both are in close agreement with receptor binding studies. Results are discussed in relation to models for drug action of dihydropyridine-type compounds and to implications for the structure of the Ca channel protein.