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The effects of Cobra venom cardiotoxin (CTX) on the cellular morphology, twitch amplitude and intracellular calcium ([Ca2+]i) of the ventricular myocytes were studied. [Ca2+]i and twitch amplitude were determined with a fluorometric ratio method using Fura-2/AM and Calcium Green-1 as calcium indicators, and a videomicroscopic technique, respectively. Addition of 0.001-1 microM CTX led to a time-dependent loss of rod shaped cells, beginning at 1 min, and remaining stable by 20 min. CTX 1 microM initially caused a transient augmentation in amplitude of the electrically induced-[Ca2+]i transient and twitch amplitude in the single cardiac myocyte. This was followed by a prolongation in duration of [Ca2+]i. Eventually, cells became inexcitable and abruptly underwent contracture, and [Ca2+]i remained elevated. In the absence of electrical stimulation, 1 microM CTX induced a Ca2+ spike followed by a sustained elevation of [Ca2+]i, an effect different from that of 40 mm KCl or 10 mm caffeine, which caused a transient elevation in [Ca2+]i. Digital imaging microscopy of Calcium Green-1 fluorescence revealed that the increase in [Ca2+]i was accompanied by changes in cell shape without leakage of fluorescence dye in the early stage after administration of the toxin. In the absence of [Ca2+]o, the initial [Ca2+]i spike was reduced, but the second phase of elevation of [Ca2+]i still occurred. In addition, experiments using Mn2+ quench technique suggested that Ca2+-influx was induced by CTX, and that both ryanodine and thapsigargin, known to deplete Ca2+ from its intracellular pool, abolished the second phase of the elevation of [Ca2+]i. The effects of cardiotoxin were abolished by 10 mM Ni2+ and 10 mM -Ca2+-o, but not by 5 microM verapamil. In conclusion, the observations indicate that CTX causes an initial increase followed by a second sustained elevation in [Ca2+]i, which is accompanied by changes in cell shape-from rod to round-and hypercontracture. The initial [Ca2+]i spikes were attributed to the extracellular Ca2+ influx, while the second [Ca2+]i elevation was related to internal Ca2+ release. The high [Ca2+]i may be responsible for hypercontracture and cell death. Further studies are needed to verify it.