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-media ted changes of Ca channel activity were minimized. The most direct act ion of (-) BAY K 8644 is the change of gating kinetics which results i n a strictly voltage-dependent increase of the peak current in the vol tage range between -40 and 0 mV. Peak currents were increased dose dep endently in the concentration range from 1 to 30 nM. Analysis of peak current/voltage relations revealed a linear shift of the current activ ation by approximately 23 mV to more negative membrane potentials, wit hout any change in its voltage dependence and in the current reversal potential or the maximum whole-cell conductance. Measurement of Ca cur rent activation and deactivation time constants suggests that (-) BAY K 8644 prolongs the single-channel open time without affecting the clo sed time. From the shift of the open time function to more negative vo ltages 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 comp onent 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 coeffi cient of 1.25 were determined. Flash-induced competition experiments w ith the Ca antagonist nifedipine allowed the measurement of binding ki netics of (-) BAY K 8644. The association rate constant is estimated t o about 5 X 10(6) mol-1 . s-1 and dissociation time constant is approx imately 50-70 s; both are in close agreement with receptor binding stu dies. Results are discussed in relation to models for drug action of d ihydropyridine-type compounds and to implications for the structure of the Ca channel protein.