Differential effects of calcium antagonists on viability of adult rat ventricular myocytes

This study was designed to determine whether the various classes of Ca²⁺ channel blockers have differential protective effects on isolated adult rat ventricular myocytes exposed to high K⁺ under anoxic (100% N₂) conditions. Calcium-tolerant myocytes were incubated under control (4 mm K⁺) aerobic conditions and then subjected to high K⁺ (75 mm) and N₂. The cells were assessed by morphological criteria (i.e. absence of blebbing, granulation etc.), maintenance of ATP levels, exclusion of trypan blue, and the presence or absence of spontaneous contractile activity. Under control conditions, the cells were quiescent and declined at a rate of ≈10%/h. In the absence of O₂, the rate of cell decline was significantly faster. Verapamil, diltiazem and the dihydropyridines had no significant effects on cell decline under these conditions. Cells exposed to 75 mm K₀⁺ exhibited contractile activity and accelerated rate of decline under anoxic conditions; these effects were independent of lowering Na₀⁺ to 75 mm. Cells in high K₀⁺ and N₂ were significantly protected (i.e. contractile activity and rate of decline were decreased) by verapamil, less so by diltiazem, and not at all by the dihydropyridines. The uptake of ⁴⁵Ca²⁺ into cells in high K₀⁺ was not significantly altered by verapamil or diltiazem. Caffeine induced the immediate cessation of contractile activity of cells incubated in high K₀⁺, but did not affect the accelerated rate of cell decline under anoxic conditions. Verapamil and diltiazem still conferred significant protection in this non-beating cell preparation. Neither verapamil nor diltiazem had any effect on the oscillation frequency of skinned heart cells. The data indicate that, based on their sites and mechanisms of action, the various classes of Ca²⁺ antagonists can differentially affect cells under pathophysiological conditions. Verapamil and diltiazem may act at the sarcolemmal membrane of depolarized cells at sites independent of the Na⁺/Ca²⁺ exchange carrier and the fast inactivating Ca²⁺ channel.