Dual action (stimulation, inhibition) of D600 on contractility and calcium channels in guinea-pig and cat heart cells.

1. We examined the effects of D600 (0.2-40 microM, generally 2 microM) on the following (i) developed tension in guinea-pig papillary muscles, (ii) calcium current (Ica) and tension in cat ventricular muscle strands, (iii) Ica in guinea-pig and cat ventricular myocytes, (iv) single Ca2+ channel currents carried by Ba2+ in cell-attached membrane patches of guinea-pig ventricular myocytes, and (v) Ba2+ currents through dihydropyridine (DHP)-binding sites (skeletal muscle) reconstituted into single functional Ca2+ channels in lipid bilayers. 2. In 27 of 140 preparations studied, D600 elicited a transient stimulation that preceded marked inhibition. The stimulation was normally of short duration (less than 5 min) and moderate strength (less than 50% increase). 3. D600 had no effect on the unit conductance of single cardiac Ca2+ channels. Stimulation was characterized by a decrease in the number of records with no openings (blanks) and an increase in the open-state probability of non-blanks (longer open times, shorter closed times). Inhibition began with an increase in the number of blanks and later included a curtailment of open times and a prolongation of closed times. The net effect after 9 min D600 was a 75% reduction in average current amplitude. 4. A similar pattern of changes in channel open and closed times produced enhancement and then depression of time-averaged open-state probability in single reconstituted channels. 5. Single Ca2+ channel current that was stimulated by adrenaline was only slightly depressed after 2 microM-D600 for 30 min. It may be that channel phosphorylation or Gs-protein activation following beta-receptor stimulation reduces channel affinity for D600. 6. Short-lived binding of D600 to a single inhibitory site may enhance association/activation of Gs-protein and thereby cause transient up-regulation prior to increased drug occupancy and inhibition. Alternatively, there may be separate stimulatory and inhibitory sites. One aspect of inhibition, the increased frequency of blanks, is attributed to a stabilization of the inactivated state; the other aspect, changes in fast kinetics, seems to require a different explanation.